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  • richardmitnick 4:06 pm on November 17, 2017 Permalink | Reply
    Tags: , , , , , , gravitational wave science, GW170608   

    From AAS NOVA: “LIGO Finds Lightest Black-Hole Binary” 

    AASNOVA

    AAS NOVA

    1
    Cornell SXS, the Simulating eXtreme Spacetimes (SXS) project

    Wednesday evening the Laser Interferometer Gravitational-wave Observatory (LIGO) collaboration quietly mentioned that they’d found gravitational waves from yet another black-hole binary back in June. This casual announcement reveals what is so far the lightest pair of black holes we’ve watched merge — opening the door for comparisons to the black holes we’ve detected by electromagnetic means.

    A Routine Detection

    2
    The chirp signal of GW170608 detected by LIGO Hanford and LIGO Livingston. [LIGO collaboration 2017]

    After the fanfare of the previous four black-hole-binary merger announcements over the past year and a half — as well as the announcement of the one neutron-star binary merger in August — GW170608 marks our entry into the era in which gravitational-wave detections are officially “routine”.

    GW170608, a gravitational-wave signal from the merger of two black holes roughly a billion light-years away, was detected in June of this year. This detection occurred after we’d already found gravitational waves from several black-hole binaries with the two LIGO detectors in the U.S., but before the Virgo interferometer came online in Europe and increased the joint ability of the detectors to localize sources.

    3
    Mass estimates for the two components of GW170608 using different models. [LIGO collaboration 2017]

    Overall, GW170608 is fairly unremarkable: it was detected by both LIGO Hanford and LIGO Livingston some 7 ms apart, and the signal looks not unlike those of the previous LIGO detections. But because we’re still in the early days of gravitational-wave astronomy, every discovery is still remarkable in some way! GW170608 stands out as being the lightest pair of black holes we’ve yet to see merge, with component masses before the merger estimated at ~12 and ~7 times the mass of the Sun.

    Why Size Matters

    With the exception of GW151226, the gravitational-wave signal discovered on Boxing Day last year, all of the black holes that have been discovered by LIGO/Virgo have been quite large: the masses of the components have all been estimated at 20 solar masses or more. This has made it difficult to compare these black holes to those detected by electromagnetic means — which are mostly under 10 solar masses in size.

    4
    GW170608 is the lowest-mass of the LIGO/Virgo black-hole mergers shown in blue. The primary mass is comparable to the masses of black holes we have measured by electromagnetic means (purple detections). [LIGO-Virgo/Frank Elavsky/Northwestern]

    One type of electromagnetically detected black hole are those in low-mass X-ray binaries (LMXBs). LMXBs consist of a black hole and a non-compact companion: a low-mass donor star that overflows its Roche lobe, feeding material onto the black hole. It is thought that these black holes form without significant spin, and are later spun up as a result of the mass accretion. Before LIGO, however, we didn’t have any non-accreting black holes of this size to observe for comparison.

    Now, detections like GW170608 and the Boxing Day event (which was also on the low end of the mass scale) are allowing us to start exploring spin distributions of non-accreting black holes to determine if we’re right in our understanding of black-hole spins. We don’t yet have a large enough comparison sample to make a definitive statement, but GW170608 is indicative of a wealth of more discoveries we can hope to find in LIGO’s next observing run, after a series of further design upgrades scheduled to conclude in 2018. The future of gravitational wave astronomy continues to look promising!

    Citation

    LIGO collaboration, submitted to ApJL. https://arxiv.org/abs/1711.05578

    See the full article here .

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    AAS Mission and Vision Statement

    The mission of the American Astronomical Society is to enhance and share humanity’s scientific understanding of the Universe.

    The Society, through its publications, disseminates and archives the results of astronomical research. The Society also communicates and explains our understanding of the universe to the public.
    The Society facilitates and strengthens the interactions among members through professional meetings and other means. The Society supports member divisions representing specialized research and astronomical interests.
    The Society represents the goals of its community of members to the nation and the world. The Society also works with other scientific and educational societies to promote the advancement of science.
    The Society, through its members, trains, mentors and supports the next generation of astronomers. The Society supports and promotes increased participation of historically underrepresented groups in astronomy.
    The Society assists its members to develop their skills in the fields of education and public outreach at all levels. The Society promotes broad interest in astronomy, which enhances science literacy and leads many to careers in science and engineering.

    Adopted June 7, 2009

     
  • richardmitnick 12:45 pm on November 4, 2017 Permalink | Reply
    Tags: , gravitational wave science, Kelly Holley-Bockelmann, ,   

    From Vanderbilt University: Women in STEM – “VU astronomer, Kelly Holley-Bockelmann, heads U.S. study team for space-based gravitational wave detector” 

    Vanderbilt U Bloc

    Vanderbilt University

    Nov. 3, 2017
    David Salisbury
    david.salisbury@vanderbilt.edu

    ESA/LISA Pathfinder

    ESA/eLISA space based the future of gravitational wave research

    1
    Illustration of one of the three satellites that will form the Laser Interferometer Space Antenna (NASA)

    Kelly Holley-Bockelmann, associate professor of astrophysics at Vanderbilt University, has been appointed by NASA’s Astrophysics Directorate to be chair of the U.S. Laser Interferometer Space Antenna (LISA) Study Team, a group of 18 scientists who will advise NASA on science issues related to the proposed space observatory.

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    Kelly Holley-Bockelmann

    LISA, which is designed to take the fledgling field of gravitational wave astronomy to the next level, is an international scientific effort led by the European Space Agency in collaboration with NASA. The $1 billion-plus project consists of three satellites linked by laser beams, all orbiting the sun in an equilateral triangle 2.5 million kilometers on a side, tentatively scheduled for launch in 2030.

    Only two years ago, a land-based gravitational wave observatory confirmed Einstein’s prediction that gravitational fluctuations from moving matter excite infinitesimal ripples in space—this first detection of gravitational waves earned the 2017 Nobel Prize in Physics. Just last month, the collision of a pair of neutron stars was observed in both light and gravity through a joint effort involving thousands of astronomers on every continent in the world. These achievements demonstrated that gravitational waves open a new window on the cosmos, one that can provide important new insights into the nature of some of the most violent phenomena in the universe.


    VIRGO Gravitational Wave interferometer, near Pisa, Italy

    Caltech/MIT Advanced aLigo Hanford, WA, USA installation


    Caltech/MIT Advanced aLigo detector installation Livingston, LA, USA

    Cornell SXS, the Simulating eXtreme Spacetimes (SXS) project

    Gravitational waves. Credit: MPI for Gravitational Physics/W.Benger-Zib

    ESA/eLISA the future of gravitational wave research

    1
    Skymap showing how adding Virgo to LIGO helps in reducing the size of the source-likely region in the sky. (Credit: Giuseppe Greco (Virgo Urbino group)

    Ground-based gravitational wave detectors, such as the Laser Interferometer Gravitational Wave Observatory in the U.S. and the European Gravitational Observatory in Italy, are tuned to detect higher frequency, shorter wavelength ripples in space produced by rotating neutron stars, mergers between neutron stars and stellar mass black holes and stellar explosions. LISA is tuned to detect lower frequencies and longer wavelengths produced by mergers between black holes millions of times more massive than the sun. The space-based system is also designed to track neutron stars and stellar mass black holes in orbit around the massive black hole at heart of the Milky Way, and will map tens of millions of tightly bound binary star systems throughout the galaxy.

    As chair of the study team, Holley-Bockelmann will also represent U.S. interests within the international LISA Consortium.

    “Our team will lead the U.S. effort to build the new field of gravitational wave astronomy,” said Holley-Bockelmann. “The choices we make will help dictate the pace of discovery, the health and the culture of this new field. Taking a step back, this is the first time we’ve had a chance to build an entirely new field in physics since quantum mechanics about 100 years ago. It’s an incredible time to be an astrophysicist.”

    In her new position, Holley-Bockelmann’s goals are to help develop the case for LISA science for the National Academy of Science’s 2020 Decadal Survey, which contains the academy’s recommendations for astronomical research in the next decade. She also intends to act as a representative and advocate for LISA science through invited talks, workshops, town hall meetings, social media and other communication channels.

    In addition, Holley-Bockelmann—co-director of the Fisk-Vanderbilt Masters-to-PhD Bridge Program, which assists under-represented minorities obtain doctoral degrees in science, math and engineering—intends to help prepare traditional astronomers and gravitational wave scientists to join forces and combine data from gravitational and conventional astronomical observatories. This “multimessenger astronomy” promises a more comprehensive picture of the titanic collisions, explosions and other cosmic events that generate powerful ripples in space time.

    “Oddly enough, I think my work with the Bridge program will be useful here. I know what techniques can help people transition from one type of expertise to another, and hope to implement some of these practices to build a bridge between electromagnetic and gravitational wave astronomers,” she observed.

    See the full article here .

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    Commodore Cornelius Vanderbilt was in his 79th year when he decided to make the gift that founded Vanderbilt University in the spring of 1873.

    The $1 million that he gave to endow and build the university was the commodore’s only major philanthropy. Methodist Bishop Holland N. McTyeire of Nashville, husband of Amelia Townsend who was a cousin of the commodore’s young second wife Frank Crawford, went to New York for medical treatment early in 1873 and spent time recovering in the Vanderbilt mansion. He won the commodore’s admiration and support for the project of building a university in the South that would “contribute to strengthening the ties which should exist between all sections of our common country.”

    McTyeire chose the site for the campus, supervised the construction of buildings and personally planted many of the trees that today make Vanderbilt a national arboretum. At the outset, the university consisted of one Main Building (now Kirkland Hall), an astronomical observatory and houses for professors. Landon C. Garland was Vanderbilt’s first chancellor, serving from 1875 to 1893. He advised McTyeire in selecting the faculty, arranged the curriculum and set the policies of the university.

    For the first 40 years of its existence, Vanderbilt was under the auspices of the Methodist Episcopal Church, South. The Vanderbilt Board of Trust severed its ties with the church in June 1914 as a result of a dispute with the bishops over who would appoint university trustees.

    kirkland hallFrom the outset, Vanderbilt met two definitions of a university: It offered work in the liberal arts and sciences beyond the baccalaureate degree and it embraced several professional schools in addition to its college. James H. Kirkland, the longest serving chancellor in university history (1893-1937), followed Chancellor Garland. He guided Vanderbilt to rebuild after a fire in 1905 that consumed the main building, which was renamed in Kirkland’s honor, and all its contents. He also navigated the university through the separation from the Methodist Church. Notable advances in graduate studies were made under the third chancellor, Oliver Cromwell Carmichael (1937-46). He also created the Joint University Library, brought about by a coalition of Vanderbilt, Peabody College and Scarritt College.

    Remarkable continuity has characterized the government of Vanderbilt. The original charter, issued in 1872, was amended in 1873 to make the legal name of the corporation “The Vanderbilt University.” The charter has not been altered since.

    The university is self-governing under a Board of Trust that, since the beginning, has elected its own members and officers. The university’s general government is vested in the Board of Trust. The immediate government of the university is committed to the chancellor, who is elected by the Board of Trust.

    The original Vanderbilt campus consisted of 75 acres. By 1960, the campus had spread to about 260 acres of land. When George Peabody College for Teachers merged with Vanderbilt in 1979, about 53 acres were added.

    wyatt centerVanderbilt’s student enrollment tended to double itself each 25 years during the first century of the university’s history: 307 in the fall of 1875; 754 in 1900; 1,377 in 1925; 3,529 in 1950; 7,034 in 1975. In the fall of 1999 the enrollment was 10,127.

    In the planning of Vanderbilt, the assumption seemed to be that it would be an all-male institution. Yet the board never enacted rules prohibiting women. At least one woman attended Vanderbilt classes every year from 1875 on. Most came to classes by courtesy of professors or as special or irregular (non-degree) students. From 1892 to 1901 women at Vanderbilt gained full legal equality except in one respect — access to dorms. In 1894 the faculty and board allowed women to compete for academic prizes. By 1897, four or five women entered with each freshman class. By 1913 the student body contained 78 women, or just more than 20 percent of the academic enrollment.

    National recognition of the university’s status came in 1949 with election of Vanderbilt to membership in the select Association of American Universities. In the 1950s Vanderbilt began to outgrow its provincial roots and to measure its achievements by national standards under the leadership of Chancellor Harvie Branscomb. By its 90th anniversary in 1963, Vanderbilt for the first time ranked in the top 20 private universities in the United States.

    Vanderbilt continued to excel in research, and the number of university buildings more than doubled under the leadership of Chancellors Alexander Heard (1963-1982) and Joe B. Wyatt (1982-2000), only the fifth and sixth chancellors in Vanderbilt’s long and distinguished history. Heard added three schools (Blair, the Owen Graduate School of Management and Peabody College) to the seven already existing and constructed three dozen buildings. During Wyatt’s tenure, Vanderbilt acquired or built one-third of the campus buildings and made great strides in diversity, volunteerism and technology.

    The university grew and changed significantly under its seventh chancellor, Gordon Gee, who served from 2000 to 2007. Vanderbilt led the country in the rate of growth for academic research funding, which increased to more than $450 million and became one of the most selective undergraduate institutions in the country.

    On March 1, 2008, Nicholas S. Zeppos was named Vanderbilt’s eighth chancellor after serving as interim chancellor beginning Aug. 1, 2007. Prior to that, he spent 2002-2008 as Vanderbilt’s provost, overseeing undergraduate, graduate and professional education programs as well as development, alumni relations and research efforts in liberal arts and sciences, engineering, music, education, business, law and divinity. He first came to Vanderbilt in 1987 as an assistant professor in the law school. In his first five years, Zeppos led the university through the most challenging economic times since the Great Depression, while continuing to attract the best students and faculty from across the country and around the world. Vanderbilt got through the economic crisis notably less scathed than many of its peers and began and remained committed to its much-praised enhanced financial aid policy for all undergraduates during the same timespan. The Martha Rivers Ingram Commons for first-year students opened in 2008 and College Halls, the next phase in the residential education system at Vanderbilt, is on track to open in the fall of 2014. During Zeppos’ first five years, Vanderbilt has drawn robust support from federal funding agencies, and the Medical Center entered into agreements with regional hospitals and health care systems in middle and east Tennessee that will bring Vanderbilt care to patients across the state.

    studentsToday, Vanderbilt University is a private research university of about 6,500 undergraduates and 5,300 graduate and professional students. The university comprises 10 schools, a public policy center and The Freedom Forum First Amendment Center. Vanderbilt offers undergraduate programs in the liberal arts and sciences, engineering, music, education and human development as well as a full range of graduate and professional degrees. The university is consistently ranked as one of the nation’s top 20 universities by publications such as U.S. News & World Report, with several programs and disciplines ranking in the top 10.

    Cutting-edge research and liberal arts, combined with strong ties to a distinguished medical center, creates an invigorating atmosphere where students tailor their education to meet their goals and researchers collaborate to solve complex questions affecting our health, culture and society.

    Vanderbilt, an independent, privately supported university, and the separate, non-profit Vanderbilt University Medical Center share a respected name and enjoy close collaboration through education and research. Together, the number of people employed by these two organizations exceeds that of the largest private employer in the Middle Tennessee region.
    Related links

     
  • richardmitnick 7:32 am on October 29, 2017 Permalink | Reply
    Tags: , , , , , gravitational wave science,   

    From ScienceNews: “What detecting gravitational waves means for the expansion of the universe” 

    ScienceNews bloc

    ScienceNews

    October 24, 2017
    Lisa Grossman

    Speed of spacetime ripples rules out some alternatives to dark energy.

    1
    BANG, FLASH Light waves and gravitational waves from a pair of colliding neutron stars reached Earth at almost the same time, ruling out theories about the universe based on predictions that the two kinds of waves might travel at different speeds. Illustration by Robin Dienel courtesy of the Carnegie Institution for Science.

    Ripples in spacetime travel at the speed of light. That fact, confirmed by the recent detection of a pair of colliding stellar corpses, kills a whole category of theories that mess with the laws of gravity to explain why the universe is expanding as fast as it is.

    On October 16, physicists announced that the Advanced Laser Interferometer Gravitational-Wave Observatory, LIGO, had detected gravitational waves from a neutron star merger (SN Online: 10/16/17).


    VIRGO Gravitational Wave interferometer, near Pisa, Italy

    Caltech/MIT Advanced aLigo Hanford, WA, USA installation


    Caltech/MIT Advanced aLigo detector installation Livingston, LA, USA

    Cornell SXS, the Simulating eXtreme Spacetimes (SXS) project

    Gravitational waves. Credit: MPI for Gravitational Physics/W.Benger-Zib

    ESA/eLISA the future of gravitational wave research

    1
    Skymap showing how adding Virgo to LIGO helps in reducing the size of the source-likely region in the sky. (Credit: Giuseppe Greco (Virgo Urbino group)

    Also, the neutron stars emitted high-energy light shortly after merging. The Fermi space telescope spotted that light coming from the same region of the sky 1.7 seconds after the gravitational wave detection.

    NASA/Fermi Telescope


    NASA/Fermi LAT

    That observation showed for the first time that gravitational waves, the shivers in spacetime set off when massive bodies move, travel at the speed of light to within a tenth of a trillionth of a percent.

    Within a day, five papers were posted at arXiv.org mourning hundreds of expanding universe theories that predicted gravitational waves should travel faster than light — an impossibility without changes to Einstein’s laws of gravity. These theories “are very, very dead,” says the coauthor of one of the papers, cosmologist Miguel Zumalacárregui of the Nordic Institute for Theoretical Physics, or NORDITA, in Stockholm. “We need to go back to our blackboards and start thinking of other alternatives.”

    In the 1990s, observations of exploding stars showed that more distant explosions were dimmer than existing theories predicted. That suggested that the universe is expanding at an ever-increasing rate (SN: 10/22/11, p. 13). Cosmologists have struggled ever since to explain why.

    The most popular explanation for the speedup is that spacetime is filled with a peculiar entity dubbed dark energy. “You can think of it like a mysterious fluid that pushes everything apart and counteracts gravity,” says cosmologist Jeremy Sakstein of the University of Pennsylvania, coauthor of another new paper.

    Lambda-Cold Dark Matter, Accelerated Expansion of the Universe, Big Bang-Inflation (timeline of the universe) Date 2010 Credit: Alex MittelmannColdcreation

    In the simplest version of this theory, the density of this dark energy has not changed over the history of the universe, so physicists call it a cosmological constant. This doesn’t require any changes to gravity — which is good, because gravity has been well-tested inside the solar system.

    The cosmological constant idea matches observations of the wider universe, but it has some theoretical difficulties. Dark energy is about 120 orders of magnitude weaker than theorists calculate it should be (SN Online: 11/18/13), a mismatch that makes scientists uncomfortable.

    Also, different methods for measuring the rate of expansion come up with slightly different numbers (SN: 8/6/16, p. 10). Measurements based on exploding stars suggest that distant galaxies are speeding away from each other at 73 kilometers per second for each megaparsec (about 3.3 million light-years) of space between them. But observations based on the cosmic microwave background, ancient light that encodes information about the conditions of the early universe, found that the expansion rate is 67 km/s per megaparsec. The disagreement suggests that either one of the measurements is wrong, or the theory behind dark energy needs a tweak.

    So instead of invoking a substance to counteract gravity, theorists tried to explain the expanding universe by weakening gravity itself. Any modifications to gravity need to leave the solar system intact. “It’s quite hard to build a theory that accelerates the universe and also doesn’t mess up the solar system,” says cosmologist Tessa Baker of the University of Oxford, coauthor of still another paper.

    These theories take hundreds of forms. “This field of modified gravity theories is a zoo,” says Baker. Some suggest that gravity leaks out into extra dimensions of space and time. Many others account for the universe’s speedy spreading by adding a different mysterious entity — some unknown particle perhaps — that drains gravity’s strength as the universe evolves.

    But the new entity would have another crucial effect: It could slow the speed of light waves, similar to the way light travels more slowly through water than through air. That means that the best alternatives to dark energy required gravitational waves to travel faster than light — which they don’t.

    Justin Khoury, a theoretical physicist at the University of Pennsylvania who has worked on several of the alternative gravity theories but was not involved in the new papers, was surprised that one gravitational-wave observation ruled out so many theories at once. He’s hardly disappointed, though.

    “The fact that we’re learning something about dark energy because of this measurement is incredibly exciting,” he says.

    Observing gravitational waves and light waves at the same time offers a third, independent way to measure how fast the universe is expanding. For now, that rate lies frustratingly right between the two clashing measurements scientists already had, at 70 km/s per megaparsec. But it’s still imprecise. Once LIGO and other observatories have seen 10 or 20 more neutron star collisions, researchers should be able to tell which measurement is correct and figure out whether dark energy needs an update, Zumalacárregui says.

    “Gravitational waves may kill these models, but eventually they have the potential to tell us if this discrepancy is for real,” he says. “That’s something that is in itself very beautiful.”

    See the full article here .

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  • richardmitnick 6:05 am on October 24, 2017 Permalink | Reply
    Tags: , , , , , gravitational wave science, , Production of gamma rays from merging neutron stars   

    From Princeton: “Steven S. Gubser: Thunder and Lightning from Neutron Star mergers” 

    Princeton University
    Princeton University

    October 18, 2017
    Steven S. Gubser

    As of late 2015, we have a new way of probing the cosmos: gravitational radiation. Thanks to LIGO (the Laser Interferometer Gravitational-wave Observatory) and its new sibling Virgo (a similar interferometer in Italy), we can now “hear” the thumps and chirps of colliding massive objects in the universe.


    VIRGO Gravitational Wave interferometer, near Pisa, Italy

    Caltech/MIT Advanced aLigo Hanford, WA, USA installation


    Caltech/MIT Advanced aLigo detector installation Livingston, LA, USA

    Cornell SXS, the Simulating eXtreme Spacetimes (SXS) project

    Gravitational waves. Credit: MPI for Gravitational Physics/W.Benger-Zib

    ESA/eLISA the future of gravitational wave research

    1
    Skymap showing how adding Virgo to LIGO helps in reducing the size of the source-likely region in the sky. (Credit: Giuseppe Greco (Virgo Urbino group)

    Not for nothing has this soundtrack been described by LIGO scientists as “the music of the cosmos.” This music is at a frequency easily discerned by human hearing, from somewhat under a hundred hertz to several hundred hertz. Moreover, gravitational radiation, like sound, is wholly different from light. It is possible for heavy dark objects like black holes to produce mighty gravitational thumps without at the same time emitting any significant amount of light. Indeed, the first observations of gravitational waves came from black hole merger events whose total power briefly exceeded the light from all stars in the known universe. But we didn’t observe any light from these events at all, because almost all their power went into gravitational radiation.

    In August 2017, LIGO and Virgo observed a collision of neutron stars which did produce observable light, notably in the form of gamma rays. Think of it as cosmic thunder and lightning, where the thunder is the gravitational waves and the lightning is the gamma rays. When we see a flash of ordinary lightning, we can count a few seconds until we hear the thunder. Knowing that sound travels one mile in about five seconds, we can reckon how distant the event is. The reason this method works is that light travels much faster than sound, so we can think of the transmission of light as instantaneous for purposes of our estimate.

    Things are very different for the neutron star collision, in that the event took place about 130 million light years away, but the thunder and lightning arrived on earth pretty much simultaneously. To be precise, the thunder was first: LIGO and Virgo heard a basso rumble rising to a characteristic “whoop,” and just 1.7 seconds later, the Fermi and INTEGRAL experiments observed gamma ray bursts from a source whose location was consistent with the LIGO and Virgo observations.

    NASA/Fermi Telescope

    NASA/Fermi LAT

    ESA/Integral

    The production of gamma rays from merging neutron stars is not a simple process, so it’s not clear to me whether we can pin that 1.7 seconds down as a delay precisely due to the astrophysical production mechanisms; but at least we can say with some confidence that the propagation time of light and gravity waves are the same to within a few seconds over 130 million light years. From a certain point of view, that amounts to one of the most precise measurements in physics: the ratio of the speed of light to the speed of gravity equals 1, correct to about 14 decimal places or better.

    The whole story adds up much more easily when we remember that gravitational waves are not sound at all. In fact, they’re nothing like ordinary sound, which is a longitudinal wave in air, where individual air molecules are swept forward and backward just a little as the sound waves pass them by. Gravitational waves instead involve transverse disturbances of spacetime, where space is stretched in one direction and squeezed in another—but both of those stretch-squeeze directions are at right angles to the direction of the wave. Light has a similar transverse quality: It is made up of electric and magnetic fields, again in directions that are at right angles to the direction in which the light travels. It turns out that a deep principle underlying both Maxwell’s electromagnetism and Einstein’s general relativity forces light and gravitational waves to be transverse. This principle is called gauge symmetry, and it also guarantees that photons and gravitons are massless, which implies in turn that they travel at the same speed regardless of wavelength.

    It’s possible to have transverse sound waves: For instance, shearing waves in crystals are a form of sound. They typically travel at a different speed from longitudinal sound waves. No principle of gauge symmetry forbids longitudinal sound waves, and indeed they can be directly observed, along with their transverse cousins, in ordinary materials like metals. The gauge symmetries that forbid longitudinal light waves and longitudinal gravity waves are abstract, but a useful first cut at the idea is that there is extra information in electromagnetism and in gravity, kind of like an error-correcting code. A much more modest form of symmetry is enough to characterize the behavior of ordinary sound waves: It suffices to note that air (at macroscopic scales) is a uniform medium, so that nothing changes in a volume of air if we displace all of it by a constant distance.

    In short, Maxwell’s and Einstein’s theories have a feeling of being overbuilt to guarantee a constant speed of propagation. And they cannot coexist peacefully as theories unless these speeds are identical. As we continue Einstein’s hunt for a unified theory combining electromagnetism and gravity, this highly symmetrical, overbuilt quality is one of our biggest clues.

    The transverse nature of gravitational waves is immediately relevant to the latest LIGO / Virgo detection. It is responsible for the existence of blind spots in each of the three detectors (LIGO Hanford, LIGO Livingston, and Virgo). It seems like blind spots would be bad, but they actually turned out to be pretty convenient: The signal at Virgo was relatively weak, indicating that the direction of the source was close to one of its blind spots. This helped localize the event, and localizing the event helped astronomers home in on it with telescopes. Gamma rays were just the first non-gravitational signal observed: the subsequent light-show from the death throes of the merging neutron stars promises to challenge and improve our understanding of the complex astrophysical processes involved. And the combination of gravitational and electromagnetic observations will surely be a driver of new discoveries in years and decades to come.

    See the full article here .

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    Princeton University Campus

    About Princeton: Overview

    Princeton University is a vibrant community of scholarship and learning that stands in the nation’s service and in the service of all nations. Chartered in 1746, Princeton is the fourth-oldest college in the United States. Princeton is an independent, coeducational, nondenominational institution that provides undergraduate and graduate instruction in the humanities, social sciences, natural sciences and engineering.

    As a world-renowned research university, Princeton seeks to achieve the highest levels of distinction in the discovery and transmission of knowledge and understanding. At the same time, Princeton is distinctive among research universities in its commitment to undergraduate teaching.

    Today, more than 1,100 faculty members instruct approximately 5,200 undergraduate students and 2,600 graduate students. The University’s generous financial aid program ensures that talented students from all economic backgrounds can afford a Princeton education.

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  • richardmitnick 9:58 am on October 19, 2017 Permalink | Reply
    Tags: , , , , , gravitational wave science, Proof that standard sirens work!   

    From astrobites: “Proof that standard sirens work!” 

    Astrobites bloc

    astrobites

    Oct 19, 2017
    Kelly Malone.

    Title: A gravitational-wave standard siren measurement of the Hubble constant
    Authors: The LIGO Scientific Collaboration, the Virgo Collaboration, The 1M2H Collaboration, The Dark Energy Camera GW-EM Collaboration, The DES Collaboration, The DLT40 Collaboration, The Las Cumbres Observatory Collaboration, The VINROUGE Collaboration, and the MASTER Collaboration



    1M2H


    Status: Accepted by Nature (in press) [open access]

    By now, you’ve almost certainly heard about this week’s big scientific announcement: a binary neutron star merger resulting in the observation of both gravitational waves and counterparts all over the electromagnetic spectrum (you can read Astrobites’ coverage here). Astronomers are very excited about this event; as the first of its type to be observed, it ushers in a new era of multi-messenger astronomy as well as confirms that neutron star mergers are a source of short gamma-ray bursts.

    However, there is a lot of other interesting science we can get out of this event as well, which implications for many subfields of astrophysics. Just scan this list of 81 (at last count) papers about the event for an idea of the magnitude of physics we can do here. Today’s paper goes all the way into the field of cosmology and uses the event to derive the Hubble Constant completely independently of existing calculations.

    See the full article here .

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    What do we do?

    Astrobites is a daily astrophysical literature journal written by graduate students in astronomy. Our goal is to present one interesting paper per day in a brief format that is accessible to undergraduate students in the physical sciences who are interested in active research.
    Why read Astrobites?

    Reading a technical paper from an unfamiliar subfield is intimidating. It may not be obvious how the techniques used by the researchers really work or what role the new research plays in answering the bigger questions motivating that field, not to mention the obscure jargon! For most people, it takes years for scientific papers to become meaningful.
    Our goal is to solve this problem, one paper at a time. In 5 minutes a day reading Astrobites, you should not only learn about one interesting piece of current work, but also get a peek at the broader picture of research in a new area of astronomy.

     
  • richardmitnick 12:57 pm on October 17, 2017 Permalink | Reply
    Tags: , , , , , , , gravitational wave science   

    From Ethan Siegel: “Why Neutron Stars, Not Black Holes, Show The Future Of Gravitational Wave Astronomy” 

    From Ethan Siegel

    Oct 17, 2017

    2
    In the final moments of merging, two neutron stars don’t merely emit gravitational waves, but a catastrophic explosion that echoes across the electromagnetic spectrum. University of Warwick / Mark Garlick

    On August 17, the signals from two merging neutron stars reached Earth after a journey of 130 million light years. After an 11 billion year dance, these remnants of once-massive, blue stars that died in supernovae so long ago spiraled into one another after emitting enough gravitational radiation to see their orbits decay. As each one moves through the changing spacetime created by the gravitational field and motion of the other, its momentum changes, causing the two masses to orbit one another more closely over time. Eventually, they meet, and when they do, they undergo a catastrophic reaction: a kilonova. For the first time, we’ve recorded the inspiral and merger in the gravitational wave sky, noticing it in all three detectors (LIGO Livingston, LIGO Hanford, and Virgo), as well as in the electromagnetic sky, from gamma rays all the way through the optical and into the radio. At last, gravitational wave astronomy is now a part of astronomy.


    VIRGO Gravitational Wave interferometer, near Pisa, Italy

    Caltech/MIT Advanced aLigo Hanford, WA, USA installation


    Caltech/MIT Advanced aLigo detector installation Livingston, LA, USA

    Cornell SXS, the Simulating eXtreme Spacetimes (SXS) project

    Gravitational waves. Credit: MPI for Gravitational Physics/W.Benger-Zib

    ESA/eLISA the future of gravitational wave research

    1
    Skymap showing how adding Virgo to LIGO helps in reducing the size of the source-likely region in the sky. (Credit: Giuseppe Greco (Virgo Urbino group)

    3
    From the very first binary neutron star system ever discovered, we knew that gravitational radiation was carrying energy away. It was only a matter of time before we found a system in the final stages of inspiral and merger. NASA (L), Max Planck Institute for Radio Astronomy / Michael Kramer

    We knew this had to happen eventually. Neutron stars have very large masses, estimated at over the mass of the Sun each, and very small sizes. Imagine an atomic nucleus that didn’t contain a handful, a few dozen, or even a few hundred protons and neutrons inside, but rather a star’s worth: 1057 of them. These incredible objects swoop through space, faster and faster, as the fabric of space itself bends and radiates due to their mutual presence. Pulsars in binary systems coalesce, and in the very final stages of inspiral, the strain they impose on a detector even a hundred million light years away can be detectable. We’ve seen the indirect evidence for decades: the decay of their mutual orbits. But the direct evidence, now available, changes everything.

    4
    The strain on the detectors, from the inspiral of the two neutron stars, can be clearly seen even visibly from the twin LIGO detectors. The less-sensitive Virgo detector provides incredibly accurate location information as well. B.P. Abbott et al., PRL 119, 161101 (2017)

    Each time these waves pass through your detector, they cause a slight expanding-and-contracting of the laser arms. Because the neutron star system is so thoroughly predictable, decaying at the rate predicted by Einstein’s equations, we know exactly how the frequency and amplitude of the inspiral ought to behave. Unlike black hole systems of higher masses, the frequency of these low-mass systems falls in the detectable range of the LIGO and Virgo detectors for much longer time periods. While the overwhelming majority of black hole-black hole mergers registered in the LIGO detectors for only a fraction of a second, these neutron stars, even at a distance of over 100 million light years, had their signals detected for almost half a minute!

    5
    This figure shows reconstructions of the four confident and one candidate (LVT151012) gravitational wave signals detected by LIGO and Virgo to date, including the most recent black hole detection GW170814 (which was observed in all three detectors). LIGO/Virgo/B. Farr (University of Oregon)

    This time, the Fermi gamma-ray satellite detected a transient burst, consistent with previously seen kilonovae, just 1.7 seconds after the arrival of the final “chirp” of the gravitational wave signal.

    NASA/Fermi Telescope


    NASA/Fermi LAT

    By time 11 hours had passed, the LIGO/Virgo team had pinpointed an area on the sky just 28 square degrees in size: the smallest localized region ever seen. Even though the neutron star signal was so much less intense in magnitude than the black hole signals were, the fact that the detectors had caught so many orbits gave the team the strongest signal to date: a signal-to-noise ratio of more than 32!

    6
    By adding in the data from the Virgo detector, even though the signal-to-noise ratio was low, we were able to make the greatest-precision detection of a gravitational wave source of all time. B.P. Abbott et al., PRL 119, 161101 (2017)

    By knowing where this signal was, we could then train our greatest optical, infrared, and radio telescopes on this site in the sky, where the galaxy NGC 4993 was located (at the correct distance). Over the next two weeks, we saw an electromagnetic counterpart to the gravitational wave source, and the afterglow of the gamma-ray burst that Fermi saw. For the first time, we had observed a neutron star merger in gravitational waves and across the light spectrum, confirming what theorists had suspected in spectacular fashion: that this is where the majority of the heaviest elements in the Universe originate.

    7
    Just hours after the gravitational wave signal arrived, optical telescopes were able to hone in on the galaxy home to the merger, watching the site of the blast brighten and fade in practically real-time.

    Dark Energy Survey


    Dark Energy Camera [DECam], built at FNAL


    NOAO/CTIO Victor M Blanco 4m Telescope which houses the DECam at Cerro Tololo, Chile, housing DECam

    But also encoded in this merger are a few incredible facts that you may not realize; facts that point the way to the future of gravitational wave astronomy.

    1.) Binary neutron stars barely spin at all! In isolation, neutron stars can be some of the most rapidly spinning objects in the Universe, up to a significant percentage of the speed of light. The fastest rotate over 700 times per second… but not in a binary system! The close presence of another large mass means that tidal forces are large, and hence the friction of one rotating body on another causes them both to slow down. By time they merge, neither one can be rotating at any appreciable speed, allowing us to constrain the orbital parameters from the gravitational wave signal extremely tightly.

    7
    Some of the most important parameters of the merging gravitational wave system were reported quite precisely, owing to the non-rotating nature of the neutron star-neutron star system. B.P. Abbott et al., PRL 119, 161101 (2017)

    2.) At least 28 Jupiter masses’ worth of material was converted into energy via E = mc2. We’ve never seen neutron star-neutron star mergers in gravitational waves before. In black hole-black hole systems of equivalent mass, up to 5% of the total mass gets converted into energy. In neutron star systems, its expected to be less, because the collision occurs between nuclei, not between singularities; the two masses can’t get as close. Still, at least 1% of the total mass was converted into pure energy via Einstein’s mass-energy equivalence, a very impressive and large amount of energy!

    8
    All massless particles travel at the speed of light, including the photon, gluon and gravitational waves, which carry the electromagnetic, strong nuclear and gravitational interactions, respectively. NASA/Sonoma State University/Aurore Simonnet

    3.) Gravitational waves move at exactly the speed of light! Before this detection, we never had a gravitational wave and a light signal simultaneously identifiable to compare with one another. After a journey of 130 million light years, the first electromagnetic signal from this detection arrived just 1.7 seconds after the peak of the gravitational wave signal. That means, at most, the difference between the speed of gravity and the speed of light is about 0.12 microns-per-second, or 0.00000000000004%. It’s anticipated that these two speeds are exactly equal, and the delay of the light signal comes from the fact that the light-producing reactions in the neutron star take a second or two to reach the surface.

    8
    The galaxy NGC 4993, located 130 million light years away, had been imaged many times before. But just after the August 17, 2017 detection of gravitational waves, a new transient source of light was seen: the optical counterpart of a neutron star-neutron star merger. P.K. Blanchard / E. Berger / Pan-STARRS / DECam

    Pann-STARS telescope, U Hawaii, Mauna Kea, Hawaii, USA, 4,207 m (13,802 ft) above sea level

    4.) A faster response time is possible! By time we first located the three-dimensional place on the sky where the electromagnetic signal was, twelve hours had passed. Sure, we were able to observe the optical counterpart immediately, but it would’ve been better to get in on the ground floor. As automated analysis improves, as well as the synchronization of all three detectors, the better we’ll do. Over the coming years, LIGO will get slightly more sensitive, Virgo will do better, and two additional LIGO-like detectors, KAGRA in Japan and LIGO-India, will come online. Instead of half a day, we may be soon talking about response times in a matter of minutes or even seconds.

    KAGRA gravitational wave detector, Kamioka mine in Kamioka-cho, Hida-city, Gifu-prefecture, Japan

    LIGO-India in the Hingoli district in western India

    9
    On the ground, a noise ‘glitch’ in the LIGO Livingston detector meant that the automated software failed to extract the signal, requiring manual intervention. B.P. Abbott et al., PRL 119, 161101 (2017)

    5.) Going to space will be the ultimate in gravitational wave observing. Here on the ground, part of the reason it took so long to find the location was that in Livingston, LA, there was a “noise” glitch: something caused the detector on the ground to vibrate. As a result, the automated software couldn’t extract the true signal, and manual intervention was required. The LIGO-Virgo team did an amazing job, but were these detectors in space, this wouldn’t even have been an issue in the first place. There is no seismic noise in the abyss of interplanetary space.

    ESA/eLISA space based the future of gravitational wave research

    10
    Neutron stars, when they merge, can exhibit gravitational wave and electromagnetic signals simultaneously, unlike black holes.
    Dana Berry / Skyworks Digital, Inc.

    Unlike merging black holes, inspiraling and merging neutron stars:

    Can be seen for a much longer time, due to their low masses,
    Will emit electromagnetic counterparts, allowing for the gravitational and electromagnetic skies to be unified,
    Are far more numerous, with the only reason we’ve seen more black holes is due to the increased range for them,
    And can be used to learn information about the Universe, such as the speed of gravity, that black holes cannot teach us.
    The delay of around 11 hours from the merger to the first optical and infrared signatures isn’t due to physics, but due to our own instrumental limitations here. As our analysis techniques improve, and more events are discovered, we’ll learn exactly how long it takes before visible light signatures are created by neutron star-neutron star mergers.

    At last, the origin of the heavy elements are confirmed; the speed of gravity is definitively known; and the gravitational wave and electromagnetic skies are one. Any doubters of LIGO now have the independent confirmation they’ve been clamoring for, and there is no ambiguity left. The future of astronomy includes gravitational waves, and that future is here, today. Congratulations, one and all. Today, all of Earth is the beneficiary of this incredible knowledge.

    See the full article here .

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    “Starts With A Bang! is a blog/video blog about cosmology, physics, astronomy, and anything else I find interesting enough to write about. I am a firm believer that the highest good in life is learning, and the greatest evil is willful ignorance. The goal of everything on this site is to help inform you about our world, how we came to be here, and to understand how it all works. As I write these pages for you, I hope to not only explain to you what we know, think, and believe, but how we know it, and why we draw the conclusions we do. It is my hope that you find this interesting, informative, and accessible,” says Ethan

     
  • richardmitnick 8:03 am on October 17, 2017 Permalink | Reply
    Tags: , , , , , gravitational wave science,   

    From Swinburne: “Colliding stars reveal their gravitational wave secrets” 

    Swinburne U bloc

    Swinburne University

    1
    Artists impression of two merging neutron stars: National Science Foundation/LIGO/Sonoma State University/A. Simonnet

    17 October 2017
    Katherine Towers
    +61 3 9214 5789
    ktowers@swin.edu.au

    Australian scientists have announced an “unparalleled” astrophysical discovery revealing that they have detected gravitational waves from the death spiral of two neutron stars.

    Less than a month after three US professors were awarded the Nobel Prize in Physics for the 2015 discovery of gravitational waves, a team of Australian astrophysicists, including Swinburne researchers, have announced a new international discovery.

    It is the first time a cosmic event has been observed and measured in both light and gravitational waves.

    Swinburne hosts the $31.3 million Australian Research Council’s Centre of Excellence for Gravitational Wave Discovery (OzGrav) which was established last year to capitalise on the original discovery of gravitational waves.

    Swinburne’s Professor Matthew Bailes, Director of OzGrav, says the new discovery has enabled scientists to pinpoint the origin of gravitational waves and to actually see “the colossal event” that accompanied the gravitational waves.

    “This was the first time that any cosmic event was observed through both the light it emitted and the gravitational ripples it caused in the fabric of space and time,” Professor Bailes says.

    “The subsequent avalanche of science was virtually unparalleled in modern astrophysics.”

    OzGrav is a collaboration between six universities in Australia – Swinburne University of Technology, Australian National University, the University of Western Australia, Monash University, the University of Melbourne, and the University of Adelaide.

    Blinkers off

    Swinburne’s Associate Professor, Jeffrey Cooke, a Chief Investigator with OzGrav, says the event will go down in history as the dawn of a new era of gravitational wave multi-messenger astronomy.

    “Before this event, it was like we were sitting in an IMAX theatre with blindfolds on,” he says.

    “The gravitational wave detectors let us “hear” the movies of black hole collisions but we couldn’t see anything.

    “This event lifted the blindfolds and, wow, what an amazing show.”

    OzGrav Chief Investigator, Professor Ju Li from the University of Western Australia says: “It is extraordinary that with one faint sound, the faintest sound ever detected, we have created one giant leap in our understanding of the universe.”

    Gravitational waves were first predicted by Albert Einstein about 100 years ago in his Theory of General Relativity. Einstein’s theory described how gravity warps and distorts space-time.

    His mathematics showed that massive accelerating objects – such as neutron stars or black holes – that orbit each other distort both space and time and emit a type of radiation known as gravitational waves.

    The resultant gravitational waves are ripples in space and time.

    Einstein did not believe gravitational waves would ever be detected.

    In September 2015, the California-based observatory specifically established to search for gravitational waves first sensed gravitational waves. The observatory is called aLIGO, short for the Advanced Laser Interferometer Gravitational-Wave Observatory.


    VIRGO Gravitational Wave interferometer, near Pisa, Italy

    Caltech/MIT Advanced aLigo Hanford, WA, USA installation


    Caltech/MIT Advanced aLigo detector installation Livingston, LA, USA

    Cornell SXS, the Simulating eXtreme Spacetimes (SXS) project

    Gravitational waves. Credit: MPI for Gravitational Physics/W.Benger-Zib

    ESA/eLISA the future of gravitational wave research

    1
    Skymap showing how adding Virgo to LIGO helps in reducing the size of the source-likely region in the sky. (Credit: Giuseppe Greco (Virgo Urbino group)

    Shortly after being switched on in 2015, aLIGO sensed distortions in space-time. These distortions were caused by passing gravitational waves that were generated by colliding black holes around 1.3 billion years ago.

    Professors Rainer Weiss, Barry C Barish and Kip S Thorne were awarded the 2017 Nobel Prize for Physics for the discovery.

    This latest event has enabled researchers to further measure gravitational waves but also see from where they originated.

    See the full article here .

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    Swinburne U Campus

    Swinburne is a large and culturally diverse organisation. A desire to innovate and bring about positive change motivates our students and staff. The result is in an institution that grows and evolves each year.

     
  • richardmitnick 6:04 am on October 17, 2017 Permalink | Reply
    Tags: , , , , , gravitational wave science, SSS17a, ,   

    From UCSC: “First observations of merging neutron stars mark a new era in astronomy” 

    UC Santa Cruz

    UC Santa Cruz

    October 16, 2017
    Tim Stephens
    stephens@ucsc.edu

    UC Santa Cruz team made the first ever observations of a visible event linked to the detection of gravitational waves, using the small Swope Telescope in Chile.

    Carnegie Institution Swope telescope at Las Campanas, Chile, 100 kilometres (62 mi) northeast of the city of La Serena. near the north end of a 7 km (4.3 mi) long mountain ridge. Cerro Las Campanas, near the southern end and over 2,500 m (8,200 ft) high, at Las Campanas, Chile

    [/caption]

    1
    The merger of two neutron stars generated a bright kilonova observed by UC Santa Cruz astronomers, as depicted in this artist’s illustration. (Credit: Illustration by Robin Dienel courtesy of the Carnegie Institution for Science)

    Two months ago, the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) notified astronomers around the world of the possible detection of gravitational waves from the merger of two neutron stars.


    VIRGO Gravitational Wave interferometer, near Pisa, Italy

    Caltech/MIT Advanced aLigo Hanford, WA, USA installation


    Caltech/MIT Advanced aLigo detector installation Livingston, LA, USA

    Cornell SXS, the Simulating eXtreme Spacetimes (SXS) project

    Gravitational waves. Credit: MPI for Gravitational Physics/W.Benger-Zib

    ESA/eLISA the future of gravitational wave research

    1
    Skymap showing how adding Virgo to LIGO helps in reducing the size of the source-likely region in the sky. (Credit: Giuseppe Greco (Virgo Urbino group)

    From that moment on August 17, the race was on to detect a visible counterpart, because unlike the colliding black holes responsible for LIGO’s four previous detections of gravitational waves, this event was expected to produce a brilliant explosion of visible light and other types of radiation.

    A small team led by Ryan Foley, an assistant professor of astronomy and astrophysics at UC Santa Cruz, was the first to find the source of the gravitational waves, located in a galaxy 130 million light-years away called NGC 4993. Foley’s team captured the first images of the event with the 1-meter Swope Telescope at the Carnegie Institution’s Las Campanas Observatory in Chile.

    “This is a huge discovery,” Foley said. “We’re finally connecting these two different ways of looking at the universe, observing the same thing in light and gravitational waves, and for that alone this is a landmark event. It’s like being able to see and hear something at the same time.”

    3
    The first optical image of a gravitational wave source was taken by a team led by UCSC astronomer Ryan Foley. This image of Swope Supernova Survey 2017a (SSS17a, indicated by arrow) shows the light emitted from the cataclysmic merger of two neutron stars. (Image credit: 1M2H Team/UC Santa Cruz & Carnegie Observatories/Ryan Foley)

    Carnegie Las Campanas Observatory in the southern Atacama,Desert of Chile in the Atacama Region approximately 100 kilometres (62 mi) northeast of the city of La Serena,near the southern end and over 2,500 m (8,200 ft) high,

    4
    The first optical image of a gravitational wave source was taken by a team led by UCSC astronomer Ryan Foley. This image of Swope Supernova Survey 2017a (SSS17a, indicated by arrow) shows the light emitted from the cataclysmic merger of two neutron stars. (Image credit: 1M2H Team/UC Santa Cruz & Carnegie Observatories/Ryan Foley)

    5
    The UCSC team found SSS17a by comparing a new image of the galaxy N4993 (right) with images taken four months earlier by the Hubble Space Telescope (left). (Image credits: Left, Hubble/STScI; Right, 1M2H Team/UC Santa Cruz & Carnegie Observatories/Ryan Foley)

    6
    The violent merger of two neutron stars is thought to involve three main energy-transfer processes, shown in this diagram, that give rise to the different types of radiation seen by astronomers, including a gamma-ray burst and a kilonova explosion seen in visible light. As the two stars spiral toward one another and merge to form a “hyper-massive” neutron star, a small fraction of the matter is ejected in a tidal tail (labeled “red component” in the diagram). The merger generates a short gamma-ray burst resulting from twin jets of material moving out from the rotational poles of the merger at close to the speed of light, likely triggered after the collapse of the remnant onto a black hole. In addition, an intense outflow of neutrinos from the hyper-massive neutron star drives a wind of material moving at about one-tenth the speed of light (labeled “blue component” in the diagram). The blue and red wavelengths that dominate the light from the kilonova at different stages result from different elements in the ejected material, which is heated by radioactive decay processes. (Image credit: Murguia-Berthier et al., Science)

    New window

    Theoretical astrophysicist Enrico Ramirez-Ruiz, professor and chair of astronomy and astrophysics at UC Santa Cruz and a member of Foley’s team, said the observations have opened a new window into understanding the physics of neutron star mergers. Among other things, the results could resolve a hotly debated question about the origins of gold and other heavy elements in the universe, which Ramirez-Ruiz has been studying for years.

    “I think this can prove our idea that most of these elements are made in neutron star mergers,” he said. “We are seeing the heavy elements like gold and platinum being made in real time.”

    Foley’s team is publishing four papers October 16 in Science based on their observations and analysis, as well as three papers in The Astrophysical Journal Letters, and they are coauthors of several more papers in Nature and other journals, including two major papers led by the LIGO collaboration.

    For science papers, see https://sciencesprings.wordpress.com/2017/10/16/from-hubble-nasa-missions-catch-first-light-from-a-gravitational-wave-event/ ]

    The key Science papers include one presenting the discovery of the first optical counterpart to a gravitational wave source, led by UCSC graduate student David Coulter, and another, led by postdoctoral fellow Charles Kilpatrick, presenting a state-of-the-art comparison of the observations with theoretical models to confirm that it was a neutron-star merger. Two other Science papers were led by Foley’s collaborators at the Carnegie Institution for Science.

    By coincidence, the LIGO detection came on the final day of a scientific workshop on “Astrophysics with gravitational wave detections,” which Ramirez-Ruiz had organized at the Niels Bohr Institute in Copenhagen and where Foley had just given a talk. “I wish we had filmed Ryan’s talk, because he was so gloomy about our chances to observe a neutron star merger,” Ramirez-Ruiz said. “But then he went on to outline his strategy, and it was that strategy that enabled his team to find it before anyone else.”

    Strategy

    Foley’s strategy involved prioritizing the galaxies within the search field indicated by the LIGO team, targeting those most likely to harbor binary pairs of neutron stars, and getting as many of those galaxies as possible into each field of view. Other teams covered the search field more methodically, “like mowing the lawn,” Foley said. His team found the source in the ninth field they observed, after waiting 10 hours for the sun to set in Chile.

    “As soon as the sun went down, we started looking,” Foley said. “By finding it as quickly as we did, we were able to build up a really nice data set.”

    He noted that the source was bright enough to have been seen by amateur astronomers, and it likely would have been visible from Africa hours before it was visible in Chile. Gamma rays emitted by the neutron star merger were detected by the Fermi Gamma-ray Space Telescope at nearly the same time as the gravitational waves, but the Fermi data gave no better information about the location of the source than LIGO did.

    Foley’s team took the first image of the optical source 11 hours after the LIGO detection and, after confirming their discovery, announced it to the astronomy community an hour later. Dozens of other teams quickly followed up with observations from other telescopes. Foley’s team also obtained the first spectra of the source with the Magellan Telescopes at Carnegie’s Las Campanas Observatory.

    Rich data set

    The gravitational wave source was named GW170817, and the optical source was named Swope Supernova Survey 2017a (SSS17a). By about seven days later, the source had faded and could no longer be detected in visible light. While it was visible, however, astronomers were able to gather a treasure trove of data on this extraordinary astrophysical phenomenon.

    “It’s such a rich data set, the amount of science to come from this one thing is incredible,” Ramirez-Ruiz said.

    Neutron stars are among the most exotic forms of matter in the universe, consisting almost entirely of neutrons and so dense that a sugar cube of neutron star material would weigh about a billion tons. The violent merger of two neutron stars ejects a huge amount of this neutron-rich material, powering the synthesis of heavy elements in a process called rapid neutron capture, or the “r-process.”

    The radiation this emits looks nothing like an ordinary supernova or exploding star. Astrophysicists like Ramirez-Ruiz have developed numerical models to predict what such an event, called a kilonova, would look like, but this is the first time one has actually been observed in such detail. Kilpatrick said the data fit remarkably well with the predictions of theoretical models.

    “It doesn’t look like anything we’ve ever seen before,” he said. “It got very bright very quickly, then started fading rapidly, changing from blue to red as it cooled down. It’s completely unprecedented.”

    A theoretical synthesis of data from across the spectrum, from radio waves to gamma rays, was led by Ariadna Murguia-Berthier, a graduate student working with Ramirez-Ruiz, and published in The Astrophysical Journal Letters, providing a coherent theoretical framework for understanding the full range of observations. Their analysis indicates, for example, that the merger triggered a relativistic jet (material moving at near the speed of light) that generated the gamma-ray burst, while matter torn from the merger system and ejected at lower speeds drove the r-process and the kilonova emissions at ultraviolet, optical, and infrared wavelengths.

    Ramirez-Ruiz has calculated that a single neutron-star merger can generate an amount of gold equal to the mass of Jupiter. The team’s calculations of heavy element production by SSS17a suggest that neutron star mergers can account for about half of all the elements heavier than iron in the universe.

    Not a joke

    The detection came just one week before the end of LIGO’s second observing run, which had begun in November 2016. Foley was in Copenhagen, taking advantage of his one afternoon off to visit Tivoli Gardens with his partner, when he got a text from Coulter alerting him to the LIGO detection. At first, he thought it was a joke, but soon he was pedaling his bicycle madly back to the University of Copenhagen to begin working with his team on a detailed search plan.

    “It was crazy. We barely got it done, but our team was incredible and it all came together,” Foley said. “We got lucky, but luck favors the prepared, and we were ready.”

    Foley’s team at UC Santa Cruz includes Ramirez-Ruiz, Coulter, Kilpatrick, Murguia-Berthier, professor of astronomy and astrophysics J. Xavier Prochaska, postdoctoral researcher Yen-Chen Pan, and graduate students Matthew Siebert, Cesar Rojas-Bravo, and Enia Xhakaj. Other team members include Maria Drout, Ben Shappee, and Tony Piro at the Observatories of the Carnegie Institution for Science; UC Berkeley astronomer Daniel Kasen; and Armin Rest at the Space Telescope Science Institute.

    Their team is called the One-Meter, Two-Hemisphere (1M2H) Collaboration because they use two one-meter telescopes, one in each hemisphere: the Nickel Telescope at UC’s Lick Observatory and Carnegie’s Swope Telescope in Chile. The UCSC group is supported in part by the National Science Foundation, Gordon and Betty Moore Foundation, Heising-Simons Foundation, and Kavli Foundation; fellowships for Foley and Ramirez-Ruiz from the David and Lucile Packard Foundation and for Foley from the Alfred P. Sloan Foundation; a Niels Bohr Professorship for Ramirez-Ruiz from the Danish National Research Foundation; and the UC Institute for Mexico and the United States (UC MEXUS).

    See the full article here .

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    UCO Lick Shane Telescope
    UCO Lick Shane Telescope interior
    Shane Telescope at UCO Lick Observatory, UCSC

    Lick Automated Planet Finder telescope, Mount Hamilton, CA, USA

    Lick Automated Planet Finder telescope, Mount Hamilton, CA, USA

    UC Santa Cruz campus
    The University of California, Santa Cruz, opened in 1965 and grew, one college at a time, to its current (2008-09) enrollment of more than 16,000 students. Undergraduates pursue more than 60 majors supervised by divisional deans of humanities, physical & biological sciences, social sciences, and arts. Graduate students work toward graduate certificates, master’s degrees, or doctoral degrees in more than 30 academic fields under the supervision of the divisional and graduate deans. The dean of the Jack Baskin School of Engineering oversees the campus’s undergraduate and graduate engineering programs.

    UCSC is the home base for the Lick Observatory.

    Lick Observatory's Great Lick 91-centimeter (36-inch) telescope housed in the South (large) Dome of main building
    Lick Observatory’s Great Lick 91-centimeter (36-inch) telescope housed in the South (large) Dome of main building

    Search for extraterrestrial intelligence expands at Lick Observatory
    New instrument scans the sky for pulses of infrared light
    March 23, 2015
    By Hilary Lebow
    1
    The NIROSETI instrument saw first light on the Nickel 1-meter Telescope at Lick Observatory on March 15, 2015. (Photo by Laurie Hatch) UCSC Lick Nickel telescope

    Astronomers are expanding the search for extraterrestrial intelligence into a new realm with detectors tuned to infrared light at UC’s Lick Observatory. A new instrument, called NIROSETI, will soon scour the sky for messages from other worlds.

    “Infrared light would be an excellent means of interstellar communication,” said Shelley Wright, an assistant professor of physics at UC San Diego who led the development of the new instrument while at the University of Toronto’s Dunlap Institute for Astronomy & Astrophysics.

    Wright worked on an earlier SETI project at Lick Observatory as a UC Santa Cruz undergraduate, when she built an optical instrument designed by UC Berkeley researchers. The infrared project takes advantage of new technology not available for that first optical search.

    Infrared light would be a good way for extraterrestrials to get our attention here on Earth, since pulses from a powerful infrared laser could outshine a star, if only for a billionth of a second. Interstellar gas and dust is almost transparent to near infrared, so these signals can be seen from great distances. It also takes less energy to send information using infrared signals than with visible light.

    5
    UCSC alumna Shelley Wright, now an assistant professor of physics at UC San Diego, discusses the dichroic filter of the NIROSETI instrument. (Photo by Laurie Hatch)

    Frank Drake, professor emeritus of astronomy and astrophysics at UC Santa Cruz and director emeritus of the SETI Institute, said there are several additional advantages to a search in the infrared realm.

    “The signals are so strong that we only need a small telescope to receive them. Smaller telescopes can offer more observational time, and that is good because we need to search many stars for a chance of success,” said Drake.

    The only downside is that extraterrestrials would need to be transmitting their signals in our direction, Drake said, though he sees this as a positive side to that limitation. “If we get a signal from someone who’s aiming for us, it could mean there’s altruism in the universe. I like that idea. If they want to be friendly, that’s who we will find.”

    Scientists have searched the skies for radio signals for more than 50 years and expanded their search into the optical realm more than a decade ago. The idea of searching in the infrared is not a new one, but instruments capable of capturing pulses of infrared light only recently became available.

    “We had to wait,” Wright said. “I spent eight years waiting and watching as new technology emerged.”

    Now that technology has caught up, the search will extend to stars thousands of light years away, rather than just hundreds. NIROSETI, or Near-Infrared Optical Search for Extraterrestrial Intelligence, could also uncover new information about the physical universe.

    “This is the first time Earthlings have looked at the universe at infrared wavelengths with nanosecond time scales,” said Dan Werthimer, UC Berkeley SETI Project Director. “The instrument could discover new astrophysical phenomena, or perhaps answer the question of whether we are alone.”

    NIROSETI will also gather more information than previous optical detectors by recording levels of light over time so that patterns can be analyzed for potential signs of other civilizations.

    “Searching for intelligent life in the universe is both thrilling and somewhat unorthodox,” said Claire Max, director of UC Observatories and professor of astronomy and astrophysics at UC Santa Cruz. “Lick Observatory has already been the site of several previous SETI searches, so this is a very exciting addition to the current research taking place.”

    NIROSETI will be fully operational by early summer and will scan the skies several times a week on the Nickel 1-meter telescope at Lick Observatory, located on Mt. Hamilton east of San Jose.

    The NIROSETI team also includes Geoffrey Marcy and Andrew Siemion from UC Berkeley; Patrick Dorval, a Dunlap undergraduate, and Elliot Meyer, a Dunlap graduate student; and Richard Treffers of Starman Systems. Funding for the project comes from the generous support of Bill and Susan Bloomfield.

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    UCSC is the home base for the Lick Observatory.

     
  • richardmitnick 4:34 am on October 17, 2017 Permalink | Reply
    Tags: , , , , , , gravitational wave science,   

    From ESA: “Integral sees blast travelling with gravitational waves” 

    ESA Space For Europe Banner

    European Space Agency

    16 October 2017
    Erik Kuulkers
    ESA Integral Project Scientist
    European Space Agency
    Tel: +31 71 565 8470
    Mob: +31 6 30249526
    Email: Erik.Kuulkers@esa.int

    Volodymyr Savchenko
    Integral Science Data Centre
    University of Geneva, Switzerland
    Email: Volodymyr.Savchenko@unige.ch

    Carlo Ferrigno
    Integral Science Data Centre
    University of Geneva, Switzerland
    Tel: +41 7979 67782
    Email: Carlo.Ferrigno@unige.ch

    Paul McNamara
    LISA Study Scientist
    European Space Agency
    Tel: +31 71 565 8239
    Email: paul.mcnamara@esa.int

    Markus Bauer








    ESA Science Communication Officer









    Tel: +31 71 565 6799









    Mob: +31 61 594 3 954









    Email: markus.bauer@esa.int

    1
    Colliding neutron stars. No image credit [and not the best I have ever seen, but their choice, not mine].

    ESA’s Integral satellite recently played a crucial role in discovering the flash of gamma rays linked to the gravitational waves released by the collision of two neutron stars.

    1
    Integral gamma-ray observatory. No image credit

    On 17 August, a burst of gamma rays lit up in space for almost two seconds. It was promptly recorded by Integral and NASA’s Fermi satellite.

    NASA/Fermi Telescope


    NASA/Fermi LAT

    Such short gamma-ray bursts are not uncommon: Integral catches about 20 every year. But this one was special: just seconds before the two satellites saw the blast, an entirely different instrument was triggered on Earth.

    One of the two detectors of the Laser Interferometer Gravitational-wave Observatory (LIGO) experiment, in the USA, recorded the passage of gravitational waves – fluctuations in the fabric of spacetime caused by powerful cosmic events.


    VIRGO Gravitational Wave interferometer, near Pisa, Italy

    Caltech/MIT Advanced aLigo Hanford, WA, USA installation


    Caltech/MIT Advanced aLigo detector installation Livingston, LA, USA

    Cornell SXS, the Simulating eXtreme Spacetimes (SXS) project

    Gravitational waves. Credit: MPI for Gravitational Physics/W.Benger-Zib

    ESA/eLISA the future of gravitational wave research

    1
    Skymap showing how adding Virgo to LIGO helps in reducing the size of the source-likely region in the sky. (Credit: Giuseppe Greco (Virgo Urbino group)

    “This is a ground-breaking discovery, revealing for the first time gravitational waves and highly energetic light released by the same cosmic source,” says Erik Kuulkers, Integral project scientist at ESA.

    Before this finding, gravitational waves had been confirmed on four occasions: in all cases, they were traced back to pairs of merging black holes as they spiralled towards each other.

    The two LIGO detectors had seen the first in September 2015, followed by two more in late 2015 and early 2017. Recently, on 14 August, the fourth observation of gravitational waves also involved Europe’s Virgo instrument in Italy.

    These detections won the LIGO founding scientists the Nobel Prize in physics earlier this month.

    Gravitational waves are the only ‘messenger’ expected when black holes collide. Following these four measurements, scientists across the world began searching with ground and space telescopes for possible luminous bursts linked to the gravitational waves.

    “We had contributed to these earlier searches with Integral, looking for gamma- or X-ray emission and finding none, as expected from the vast majority of theories,” says Volodymyr Savchenko from the Integral Science Data Centre in Geneva, Switzerland.

    This time, however, the story took a different turn.

    Other cosmic clashes are suspected to release not only gravitational waves but also light across the electromagnetic spectrum. This can happen, for example, when the collision involves one or more neutron stars – like black holes, they are compact remnants of what were once massive stars.

    Merging neutron stars have also been thought to be the long-sought sources of short gamma-ray bursts, though no observational proof had yet been found.

    Until August.

    “We realised that we were witnessing something historic when we saw the notification of Fermi’s and LIGO’s detections appear on our internal network almost at the same time, and soon after we saw the confirmation in the data from Integral’s SPI instrument, too,” says Carlo Ferrigno, from the Integral Science Data Centre.

    “Nothing like this had happened before: it was clearly the signature of a neutron star merger,” adds Volodymyr.

    3
    Gamma-ray burst after gravitational waves. No image credit.

    Ordinarily, an alert from only one of the three gravitational-wave detectors would not awaken curiosity so suddenly, but the coincidence with the gamma-ray blast detected from space prompted the LIGO/Virgo scientists to look again.

    It later appeared that both LIGO detectors had recorded the gravitational waves. Owing to its lower sensitivity and different orientation, Virgo produced a smaller response, but combining all three sets of measurements was crucial to locating the source.

    The data pointed to a 28 square degree patch in the sky, equivalent to a square spanning roughly 10 times the diameter of the full Moon on each side. The gravitational wave signal indicated that the source lies only about 130 millions light-years away.

    Without further delay, a large number of ground and space telescopes turned to this portion of the sky.

    About half a day after the detections, scientists at various optical observatories, including the European Southern Observatory’s telescopes in Chile, spotted something new near the core of galaxy NGC 4993. Sitting at just the distance indicated by LIGO/Virgo, it was just what you would expect to see in visible light as neutron stars merged.

    “This is the closest short gamma-ray burst detected among the ones for which we’ve measured the distance, and by far the dimmest one – nearly a million times less bright than average,” says Volodymyr.

    “We think that the unusual properties of this source indicate that the powerful jets that arise in the cosmic clash of the neutron stars are not pointing straight towards us, as happens in the majority of gamma-ray bursts detected.”

    With the position of the source known, a large number of observatories and other sensors continued looking at it for several days and, in some cases, weeks, searching for light and particles emitted in the aftermath of the collision. Many are still observing it.

    After the initial detection of the blast, Integral observed it for five and a half days. No further gamma rays were detected, an important fact in understanding how the neutron stars merged.

    The extensive follow-up campaign revealed signals across the spectrum, first in the ultraviolet, visible and infrared bands, then in X-rays and, eventually, radio wavelengths.

    “What we are witnessing is clearly a kilonova: the neutron-rich material released in the merger is impacting its surroundings, forging a wealth of heavy elements in the process,” explains Carlo.

    “This amazing discovery was made possible by a terrific collaboration of thousands of people working in different observatories and experiments worldwide,” says Erik.

    “We are thrilled that Integral could provide a crucial contribution to confirming the nature of such a rare phenomenon that scientists have been seeking for decades.”

    With high sensitivity to gamma rays and almost full-sky coverage for brief events, Integral is amongst the best astronomical facilities for keeping an eye on gamma-ray bursts.

    When the LIGO/Virgo sensors start their observations again, with improved sensitivity, in late 2018, it is crucial that as many gamma-ray satellites as possible are active to check on the gravitational wave detections.

    Meanwhile, ESA is working on the next generation of gravitational-wave experiments, taking the quest to space with LISA, the Laser Interferometer Space Antenna.

    Planned for launch in 2034, LISA will be sensitive to gravitational waves of lower frequency than those detected with terrestrial instruments. These are released by the clashes of even more exotic cosmic objects: supermassive black holes, which sit at the centre of galaxies and have masses millions to billions of times larger than that of the stellar-mass black holes detected by LIGO and Virgo.

    “LISA will broaden the study of gravitational waves much like the first observations at infrared and radio wavelengths have revolutionised astronomy,” says Paul McNamara, LISA study scientist at ESA.

    “Until then, we are excited that ESA’s high-energy satellites are contributing to the growing field of gravitational-wave astronomy.”

    From ESA

    Published on Oct 16, 2017

    This artist’s impression video shows how two tiny but very dense neutron stars merge and explode as a kilonova. Such a very rare event is expected to produce both gravitational waves and a short gamma-ray burst, both of which were observed on 17 August 2017 by LIGO–Virgo and Fermi/INTEGRAL respectively. Subsequent detailed observations with the NASA/ESA Hubble Space Telescope and other telescopes all over the world have confirmed that this object, seen in the galaxy NGC 4993 about 130 million light-years from the Earth, is indeed a kilonova. These objects are the main source of very heavy chemical elements, such as gold and platinum, in the Universe.
    Credit: ESO/L. Calçada. Music: Johan B. Monell (www.johanmonell.com)

    From NASA
    Published on Oct 16, 2017

    For the first time, NASA scientists have detected light tied to a gravitational-wave event, thanks to two merging neutron stars in the galaxy NGC 4993, located about 130 million light-years from Earth in the constellation Hydra.

    Shortly after 8:41 a.m. EDT on Aug. 17, NASA’s Fermi Gamma-ray Space Telescope picked up a pulse of high-energy light from a powerful explosion, which was immediately reported to astronomers around the globe as a short gamma-ray burst. The scientists at the National Science Foundation’s Laser Interferometer Gravitational-wave Observatory (LIGO) detected gravitational waves dubbed GW170817 from a pair of smashing stars tied to the gamma-ray burst, encouraging astronomers to look for the aftermath of the explosion. Shortly thereafter, the burst was detected as part of a follow-up analysis by ESA’s (European Space Agency’s) INTEGRAL satellite.

    NASA’s Swift, Hubble, Chandra and Spitzer missions, along with dozens of ground-based observatories, including the NASA-funded Pan-STARRS survey, later captured the fading glow of the blast’s expanding debris.

    NASA/SWIFT Telescope

    NASA/ESA Hubble Telescope

    NASA/Chandra Telescope

    NASA/Spitzer Infrared Telescope

    Neutron stars are the crushed, leftover cores of massive stars that previously exploded as supernovas long ago. The merging stars likely had masses between 10 and 60 percent greater than that of our Sun, but they were no wider than Washington, D.C. The pair whirled around each other hundreds of times a second, producing gravitational waves at the same frequency. As they drew closer and orbited faster, the stars eventually broke apart and merged, producing both a gamma-ray burst and a rarely seen flare-up called a “kilonova.”

    Neutron star mergers produce a wide variety of light because the objects form a maelstrom of hot debris when they collide. Merging black holes — the types of events LIGO and its European counterpart, Virgo, have previously seen — very likely consume any matter around them long before they crash, so we don’t expect the same kind of light show.

    Within hours of the initial Fermi detection, LIGO and the Virgo detector at the European Gravitational Observatory near Pisa, Italy, greatly refined the event’s position in the sky with additional analysis of gravitational wave data. Ground-based observatories then quickly located a new optical and infrared source — the kilonova — in NGC 4993.

    To Fermi, this appeared to be a typical short gamma-ray burst, but it occurred less than one-tenth as far away as any other short burst with a known distance, making it among the faintest known. Astronomers are still trying to figure out why this burst is so odd, and how this event relates to the more luminous gamma-ray bursts seen at much greater distances.

    NASA’s Swift, Hubble and Spitzer missions followed the evolution of the kilonova to better understand the composition of this slower-moving material, while Chandra searched for X-rays associated with the remains of the ultra-fast jet.

    NASA/SWIFT Telescope

    This animation captures phenomena observed over the course of nine days following the neutron star merger known as GW170817. They include gravitational waves (pale arcs), a near-light-speed jet that produced gamma rays (magenta), expanding debris from a kilonova that produced ultraviolet (violet), optical and infrared (blue-white to red) emission, and, once the jet directed toward us expanded into our view from Earth, X-rays (blue).
    Credit: NASA’s Goddard Space Flight Center/CI Lab

    See the full article here .

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    The European Space Agency (ESA), established in 1975, is an intergovernmental organization dedicated to the exploration of space, currently with 19 member states. Headquartered in Paris, ESA has a staff of more than 2,000. ESA’s space flight program includes human spaceflight, mainly through the participation in the International Space Station program, the launch and operations of unmanned exploration missions to other planets and the Moon, Earth observation, science, telecommunication as well as maintaining a major spaceport, the Guiana Space Centre at Kourou, French Guiana, and designing launch vehicles. ESA science missions are based at ESTEC in Noordwijk, Netherlands, Earth Observation missions at ESRIN in Frascati, Italy, ESA Mission Control (ESOC) is in Darmstadt, Germany, the European Astronaut Centre (EAC) that trains astronauts for future missions is situated in Cologne, Germany, and the European Space Astronomy Centre is located in Villanueva de la Cañada, Spain.

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  • richardmitnick 4:10 pm on October 16, 2017 Permalink | Reply
    Tags: , , , , gravitational wave science,   

    From University of Illinois Physics: “Early theoretical work at Illinois foreshadowed today’s LIGO/Virgo announcement” 

    U Illinois bloc

    University of Illinois Physics

    U Illinois Physics bloc

    10/16/2017
    Siv Schwink

    1
    U of I Professor of Physics and Astronomy Stuart Shapiro

    Today’s historic joint announcement by the U.S.-based Laser Interferometer Gravitational-Wave Observatory (LIGO) and the Europe-based Virgo detector of the first detection of gravitational waves produced by colliding neutron stars is doubly noteworthy.


    VIRGO Gravitational Wave interferometer, near Pisa, Italy

    Caltech/MIT Advanced aLigo Hanford, WA, USA installation


    Caltech/MIT Advanced aLigo detector installation Livingston, LA, USA

    Cornell SXS, the Simulating eXtreme Spacetimes (SXS) project

    Gravitational waves. Credit: MPI for Gravitational Physics/W.Benger-Zib

    ESA/eLISA the future of gravitational wave research

    1
    Skymap showing how adding Virgo to LIGO helps in reducing the size of the source-likely region in the sky. (Credit: Giuseppe Greco (Virgo Urbino group)

    It’s also the first cosmic event observed in both gravitational waves and light—some 70 ground- and space-based observatories observed the colliding neutron stars. This is arguably the biggest moment to date in “multi-messenger astronomy.”

    In a press release issued by LIGO and Virgo collaborations, National Science Foundation Director France A. Córdova comments, “It is tremendously exciting to experience a rare event that transforms our understanding of the workings of the universe. This discovery realizes a long-standing goal many of us have had, that is, to simultaneously observe rare cosmic events using both traditional as well as gravitational-wave observatories. Only through NSF’s four-decade investment in gravitational-wave observatories, coupled with telescopes that observe from radio to gamma-ray wavelengths, are we able to expand our opportunities to detect new cosmic phenomena and piece together a fresh narrative of the physics of stars in their death throes.”

    Well before the development today’s innovative technologies supporting this simultaneous gravitational-wave and optical observation, early research in numerical relativity at the University of Illinois at Urbana-Champaign helped to lay the theoretical foundation for it. In fact, many features of the discovery had been predicted in the early computational simulations of Professor of Physics and Astronomy Stuart Shapiro and his group.

    “This discovery of merging binary neutron stars with counterpart electromagnetic radiation is particularly rewarding to me and to my University of Illinois colleagues. It means that some of the cosmic phenomena that we have been studying and simulating on supercomputers for many years actually occurs in nature, much in the way we predicted.”

    When Shapiro joined the faculty at Illinois in 1996, bringing with him his then-postdoc Thomas Baumgarte, Shapiro was immediately invited to join NCSA as a research scientist by then-Director Larry Smarr.

    “Physics Illinois was a hotbed of research on the properties of neutron stars, and NCSA was a leading center for numerical relativity,” remembers Shapiro.

    At that time, Shapiro was already an established expert in the physics of compact objects—including black holes, neutron stars, and white dwarfs—and was a co-developer of one of the very first computer codes that could calculate the structure of a spinning neutron star according to Einstein’s theory of general relativity. With Baumgarte and several other collaborators, Shapiro was also one of the builders of the first code in general relativity that modeled binary neutron stars moving in a close circular orbit about each other.

    “While binary neutron stars had been observed at that time, all were in widely separated orbits,” Shapiro recalls. “But these orbits were known to be shrinking at the exact rate predicted by general relativity, due to their emission of gravitational waves.”

    “Of course, gravitational waves from widely separated stars are too weak to be detected directly on Earth,” he adds. “Only after these orbits would decay, after millions or billions of years, and the stars collide and merge, could the waves be detected by LIGO/VIRGO.”

    In 1999, using the computer tools they’d devised, Shapiro and Baumgarte, along with another postdoc in the group, Masaru Shibata, constructed models of a single rotating neutron star with mass greatly exceeding the maximum mass of a non-spinning neutron star. They proposed that such a star was a possible short-lived remnant of a binary neutron star merger and coined the name “hypermassive neutron star” for this transient object. Their model suggested that it would be stable over many rotation periods, but predicted it would eventually collapse to form a spinning black hole.

    The team predicted this delayed collapse would be attributable to gravitational radiation loss, turbulent viscosity, and/or internal magnetic fields, which would radiate away or transport outwards angular momentum (spin), the hypermassive neutron star’s principal source of support against gravitational collapse.

    But solving Einstein’s equations of general relativity to simulate the full evolution of a binary neutron star, beginning with a binary in circular orbit and through its late inspiral, merger, and eventual collapse, was proving to be quite a challenge for numerical relativists. The same was true for the inspiral and merger of binary black holes.

    So in 2000, Baumgarte and Shapiro, building on earlier work by Shibata and his graduate advisor Takashi Nakamura, both of Kyoto University, designed a new, robust numerical algorithm that enabled simulations of the inspiral and merger of these compact binary stars to be performed in a stable fashion according to general relativity. The scheme they designed is now referred to as the Baumgarte-Shapiro-Shibata-Nakamura (BSSN) numerical relativity scheme and is the most widely implemented approach to solving Einstein’s equations on a supercomputer.

    Shapiro and his group spent part of the next several years studying exactly how a hypermassive neutron star eventually collapses to a spinning black hole, finding in 2006 that the black hole typically becomes embedded in a low-mass disk of magnetized gas of neutron-star tidal debris that accretes slowly onto the black hole.

    “This accretion fuels a central engine for generating outward electromagnetic power,” shares Shapiro. “But a central question remained: could the merger of a binary neutron star, or a binary black hole-neutron star, actually be the engine that generates the powerful short gamma-ray bursts that have been observed by satellites since the 1970s?”

    “Such an identification had been hypothesized by theorists, but a firm indication that this might be possible in general relativity was still lacking,” he recounts.

    Shapiro and his group spent the next several years simulating the merger of binary neutron stars and binary black hole-neutron star s with magnetized neutron stars and spinning black holes, becoming one of the very few groups worldwide capable of simulating such an event according to the laws of general relativistic magnetohydrodynamics.

    The key breakthrough came in 2015 when Shapiro and his postdocs Vasilis Paschalidis and Milton Ruiz were able to evolve a magnetized binary black hole-neutron star from its late inspiral, through tidal disruption of the neutron star and merger, to the formation of a magnetized gaseous disk orbiting the remnant black hole.

    They found that in addition to the burst of gravitational waves emitted during this event, a jet of plasma was eventually launched above the poles of the black hole, and this jet also shot a collimated beam of electromagnetic radiation outward into space.

    “Such a jet, confined by the walls of a tightly wound, twisted, helical magnetic field—picture field lines twisted as in a barber-pole—above the black hole poles, was long believed to be a crucial ingredient in generating a short gamma-ray burst. The jet produced in our simulations had the expected duration and typical luminosity of observed short gamma-ray bursts. This finding was a real first and generated a lot of excitement in the field,” Shapiro comments.

    Next the group simulated the merger of magnetized binary neutron stars and, in 2016, reported that those systems that formed a transient hypermassive neutron star following merger and then underwent delayed collapse to a black hole also produced a jet.

    “We determined that this model represented a good candidate for the short gamma-ray burst central engine and a counterpart to the gravitational wave burst,” Shapiro states. “This too was all very exciting.”

    Most recently, in 2017, the team performed simulations for more massive neutron star binaries that had a total mass above a critical threshold that forced them to undergo prompt collapse following merger.

    “These systems do not have time to form a hypermassive neutron star and, as a result, do not launch a jet. Thus, they are apparently not good short gamma-ray burst candidates,” Shapiro continues. “Determining whether or not a binary neutron star can become a short gamma-ray burst may thus depend on its total mass, as well as the nuclear equation of state that provides pressure support in a neutron star and determines the critical threshold mass below which delayed collapse can occur.”

    According to Shapiro, details of the discovery announced today will dictate many further questions and future simulations. Shapiro and his U of I group hope to continue to play a significant role in this very exciting and ongoing scientific investigation.

    See the full article here .

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    U Illinois campus

    The University of Illinois at Urbana-Champaign community of students, scholars, and alumni is changing the world.

    With our land-grant heritage as a foundation, we pioneer innovative research that tackles global problems and expands the human experience. Our transformative learning experiences, in and out of the classroom, are designed to produce alumni who desire to make a significant, societal impact.

     
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