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  • richardmitnick 1:51 pm on December 11, 2017 Permalink | Reply
    Tags: , , , Binary neutron stars, , , , , NGC 4993   

    From DES: “What the galaxy that hosted the gravitational wave event GW170817 can teach us about binary neutron stars” 

    Dark Energy Icon

    The Dark Energy Survey

    November 22, 2017 [Just now in social media.]
    Antonella Palmese
    Sunayana Bhargava

    Astronomers know many facts about galaxies. For example, we know that their colours tell us about the stars inside them and how old they are. We also know that their shapes can tell us about how they formed. Past and current large-scale surveys such as the Dark Energy Survey (DES) observe millions of galaxies at different distances, and therefore at different stages of their evolution. These galaxies can be catalogued and characterized in a number of different ways. However, one type of star system we know little about are binary neutron stars (BNS). The handful of confirmed binary neutron stars found have all been within our own galaxy.

    The optical counterpart to GW170817 was observed by the Dark Energy Camera (DECam) and other instruments to have come from a galaxy named NGC 4993, which is 130 million light years away from us. This event was likely produced by a binary neutron star merger. Antonella Palmese, together with other galaxy evolution and gravitational wave experts (Will Hartley, Marcelle Soares-Santos, Jim Annis, Huan Lin, Christopher Conselice, Federica Tarsitano and more) asked the question: what can we learn about the stars in NGC 4993? How did this binary system emerge in the overall history of the galaxy? Although we only have one snapshot of this galaxy, which is precisely 130 million years old, we can make use of other properties to infer how this galaxy evolved over cosmic time.

    At first glance, NGC 4993 looks like a normal, old massive elliptical galaxy (left panel in Figure 1), known by astronomers as an “early type galaxy”. But if we examine it more closely, we see that it contains shell structures: arcs of brighter stellar densities around the center of the galaxy. If we consider the profile of a typical, early type galaxy (see middle panel in Figure 1) and subtract it from the profile of NGC 4993, we notice key differences that help us characterize the kind of environment needed for binary neutron stars to form.

    Figure 1. Left panel: DECam image of NCG 4993. Shell structures indicative of a recent galaxy merger are clearly visible. Middle panel: r-band residuals after subtraction of a Sérsic light profile. Right panel: F606W-band HST ACS image with a 3 component galaxy model subtracted. Dust lanes crossing the centre of the galaxy are evident after this subtraction. The green lines show the position of the transient.

    A number of papers starting from the 1980s have supported, with simulations and observations, the idea that these kind of shell structures are the debris of a recent merger between two galaxies (see a simulation example: http://hubblesite.org/video/558/news/4-galaxies ). During the merger, the stars from the smaller galaxy that passed close to NGC4993 millions of years ago were stripped away. As a result, many stars are concentrated in these arc-like regions. From the innermost shell position and the velocity of stars, we estimate that the shells in this galaxy should be visible for ~200 million years before dispersing. This means, if we still see them, the galaxy merger must have happened up to 200 million years before the BNS coalescence (see Figure 2 for a timeline). Could the dynamics of this galaxy merger be involved in the formation of the GW progenitor?

    Figure 2. Timeline of NGC 4993

    DES only observes in optical photometric bands so we added information from infrared and spectroscopic surveys to study this galaxy in greater detail. We find more evidence for a recent galaxy merger (e.g. dust lanes, right panel of Figure 1, and two different stellar populations). We also find that the age of most of the stars in this galaxy is ~11 billion years old – only a few billion years younger than the Universe! This means that during its ‘recent’ stages, this galaxy has not been forming stars.

    Most of the current models for the formation of BNS suggest that they begin as a binary of two massive stars from a star formation event. During the evolution of the massive star binary, both stars will become supernova. If the gravitational force between the stars is strong enough to keep them bound against the force of the supernovae explosions, they become neutron stars in orbit until they coalesce. Simulations show us that neutron stars usually orbit around each other for ~500 million years before they merge, but it can take up to some billion years. Their lifetime before becoming neutron stars is much shorter than that.

    So if there was no recent star formation in NGC 4993, where did these massive stars, which go on to become neutron stars, come from? If they formed 11 billion years ago with other stars in the galaxy, why did they only merge now? Our work shows that it is unlikely that the BNS was formed ordinarily. We do not expect this BNS to be so old given the current knowledge of their expected lifetime from simulations. We instead suggest that the formation of the BNS was not through traditional channels. Instead, dynamical interactions between stars due to the galaxy merger might have caused the two neutron stars to form a binary or to coalesce. The plan for the future is to discover many more of these BNS systems inside their galactic hosts. With more data, it will be possible to determine how galaxies are able to produce the right conditions for this energetic dance of dense bodies to occur, creating ripples of energy (gravitational waves) that teach us about the Universe.

    See the full article here .

    Please help promote STEM in your local schools.

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    DECam, built at FNAL
    DECam, built at FNAL
    CTIO Victor M Blanco 4m Telescope
    CTIO Victor M Blanco 4m Telescope interior
    CTIO Victor M Blanco Telescope at Cerro Tololo which houses the DECAm

    The Dark Energy Survey (DES) is designed to probe the origin of the accelerating universe and help uncover the nature of dark energy by measuring the 14-billion-year history of cosmic expansion with high precision. More than 120 scientists from 23 institutions in the United States, Spain, the United Kingdom, Brazil, and Germany are working on the project. This collaboration [has built] an extremely sensitive 570-Megapixel digital camera, DECam, and [has mounted] it on the Blanco 4-meter telescope at Cerro Tololo Inter-American Observatory high in the Chilean Andes. Started in Sept. 2012 and continuing for five years, DES will survey a large swath of the southern sky out to vast distances in order to provide new clues to this most fundamental of questions.

  • richardmitnick 10:46 am on October 16, 2017 Permalink | Reply
    Tags: , , , , , , , , NGC 4993,   

    From Hubble: “NASA Missions Catch First Light From a Gravitational-Wave Event” 

    NASA Hubble Banner

    NASA/ESA Hubble Telescope

    NASA/ESA Hubble Telescope

    Oct 16, 2017

    Christine Pulliam
    Space Telescope Science Institute, Baltimore, Maryland

    Ray Villard
    Space Telescope Science Institute, Baltimore, Maryland

    Felicia Chou
    NASA Headquarters, Washington, D.C.

    Dewayne Washington
    Goddard Space Flight Center, Greenbelt, Maryland

    Neutron Star Collision Cooks Up Exotic Elements, Gravitational Waves

    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.

    NASA/Fermi Telescope

    NASA/Fermi LAT

    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.


    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

    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)

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

    NASA/Chandra Telescope

    NASA/Spitzer Infrared Telescope

    PanSTARRS telescope, U Hawaii, Mauna Kea, Hawaii, USA

    “This is extremely exciting science,” said Paul Hertz, director of NASA’s Astrophysics Division at the agency’s headquarters in Washington. “Now, for the first time, we’ve seen light and gravitational waves produced by the same event. The detection of a gravitational-wave source’s light has revealed details of the event that cannot be determined from gravitational waves alone. The multiplier effect of study with many observatories is incredible.”

    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.”

    “This is the one we’ve all been waiting for,” said David Reitze, executive director of the LIGO Laboratory at Caltech in Pasadena, California. “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.”

    “The favored explanation for short gamma-ray bursts is that they’re caused by a jet of debris moving near the speed of light produced in the merger of neutron stars or a neutron star and a black hole,” said Eric Burns, a member of Fermi’s Gamma-ray Burst Monitor team at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “LIGO tells us there was a merger of compact objects, and Fermi tells us there was a short gamma-ray burst. Together, we know that what we observed was the merging of two neutron stars, dramatically confirming the relationship.”

    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

    When Swift turned to the galaxy shortly after Fermi’s gamma-ray burst detection, it found a bright and quickly fading ultraviolet (UV) source.

    “We did not expect a kilonova to produce bright UV emission,” said Goddard’s S. Bradley Cenko, principal investigator for Swift. “We think this was produced by the short-lived disk of debris that powered the gamma-ray burst.”

    Over time, material hurled out by the jet slows and widens as it sweeps up and heats interstellar material, producing so-called afterglow emission that includes X-rays. But the spacecraft saw no X-rays — a surprise for an event that produced higher-energy gamma rays.

    NASA’s Chandra X-ray Observatory clearly detected X-rays nine days after the source was discovered. Scientists think the delay was a result of our viewing angle, and it took time for the jet directed toward Earth to expand into our line of sight.

    “The detection of X-rays demonstrates that neutron star mergers can form powerful jets streaming out at near light speed,” said Goddard’s Eleonora Troja, who led one of the Chandra teams and found the X-ray emission. “We had to wait for nine days to detect it because we viewed it from the side, unlike anything we had seen before.”

    On Aug. 22, NASA’s Hubble Space Telescope began imaging the kilonova and capturing its near-infrared spectrum, which revealed the motion and chemical composition of the expanding debris.

    “The spectrum looked exactly like how theoretical physicists had predicted the outcome of the merger of two neutron stars would appear,” said Andrew Levan at the University of Warwick in Coventry, England, who led one of the proposals for Hubble spectral observations. “It tied this object to the gravitational wave source beyond all reasonable doubt.”

    Astronomers think a kilonova’s visible and infrared light primarily arises through heating from the decay of radioactive elements formed in the neutron-rich debris. Crashing neutron stars may be the universe’s dominant source for many of the heaviest elements, including platinum and gold.

    Because of its Earth-trailing orbit, Spitzer was uniquely situated to observe the kilonova long after the Sun moved too close to the galaxy on the sky for other telescopes to see it. Spitzer’s Sept. 30 observation captured the longest-wavelength infrared light from the kilonova, which unveils the quantity of heavy elements forged.

    “Spitzer was the last to join the party, but it will have the final word on how much gold was forged,” says Mansi Kasliwal, Caltech assistant professor and principal investigator of the Spitzer observing program.

    Numerous scientific papers describing and interpreting these observations have been published in Science, Nature, Physical Review Letters, and The Astrophysical Journal.

    Gravitational waves were directly detected for the first time in 2015 by LIGO, whose architects were awarded the 2017 Nobel Prize in physics for the discovery.

    NASA’s Hubble Studies Source of Gravitational Waves

    On August 17, 2017, weak ripples in the fabric of space-time known as gravitational waves washed over Earth. Unlike previously detected gravitational waves, these were accompanied by light, allowing astronomers to pinpoint the source. NASA’s Hubble Space Telescope turned its powerful gaze onto the new beacon, obtaining both images and spectra. The resulting data will help reveal details of the titanic collision that created the gravitational waves, and its aftermath.

    The Laser Interferometer Gravitational-Wave Observatory (LIGO) detected gravitational waves at 8:41 a.m. EDT on August 17. Two seconds later, NASA’s Fermi Gamma-ray Space Telescope measured a short pulse of gamma rays known as a gamma-ray burst. Many observatories, including space telescopes, probed the suspected location of the source, and within about 12 hours several spotted their quarry.

    In a distant galaxy called NGC 4993, about 130 million light-years from Earth, a point of light shone where nothing had been before. It was about a thousand times brighter than a variety of stellar flare called a nova, putting it in a class of objects astronomers call “kilonovae.” It also faded noticeably over 6 days of Hubble observations.

    “This appears to be the trifecta for which the astronomical community has been waiting: Gravitational waves, a gamma-ray burst, and a kilonova all happening together,” said Ori Fox of the Space Telescope Science Institute, Baltimore, Maryland.

    The source of all three was the collision of two neutron stars, the aged remains of a binary star system. A neutron star forms when the core of a dying massive star collapses, a process so violent that it crushes protons and electrons together to form subatomic particles called neutrons. The result is like a giant atomic nucleus, cramming several Suns’ worth of material into a ball just a few miles across.

    In NGC 4993, two neutron stars once spiraled around each other at blinding speed. As they drew closer together, they whirled even faster, spinning as fast as a blender near the end. Powerful tidal forces ripped off huge chunks while the remainder collided and merged, forming a larger neutron star or perhaps a black hole. Leftovers spewed out into space. Freed from the crushing pressure, neutrons turned back into protons and electrons, forming a variety of chemical elements heavier than iron.

    “We think neutron star collisions are a source of all kinds of heavy elements, from the gold in our jewelry to the plutonium that powers spacecraft, power plants, and bombs,” said Andy Fruchter of the Space Telescope Science Institute.

    Several teams of scientists are using Hubble’s suite of cameras and spectrographs to study the gravitational wave source. Fruchter, Fox, and their colleagues used Hubble to obtain a spectrum of the object in infrared light. By splitting the light of the source into a rainbow spectrum, astronomers can probe the chemical elements that are present. The spectrum showed several broad bumps and wiggles that signal the formation of some of the heaviest elements in nature.

    “The spectrum looked exactly like how theoretical physicists had predicted the outcome of the merger of two neutron stars would appear. It tied this object to the gravitational wave source beyond all reasonable doubt,” said Andrew Levan at the University of Warwick in Coventry, England, who led one of the proposals for Hubble spectral observations. Additional spectral observations were led by Nial Tanvir of the University of Leicester, England.

    Spectral lines can be used as fingerprints to identify individual elements. However, this spectrum is proving a challenge to interpret.

    “Beyond the fact that two neutron stars flung a lot of matter out into space, we’re not yet sure what else the spectrum is telling us,” explained Fruchter. “Because the material is moving so fast, the spectral lines are smeared out. Also, there are all kinds of unusual isotopes, many of which are short-lived and undergo radioactive decay. The good news is that it’s an exquisite spectrum, so we have a lot of data to work with and analyze.”

    Hubble also picked up visible light from the event that gradually faded over the course of several days. Astronomers believe that this light came from a powerful “wind” of material speeding outward. These observations hint that astronomers viewed the collision from above the orbital plane of the neutron stars. If seen from the side (along the orbital plane), matter ejected during the merger would have obscured the visible light and only infrared light would be visible.

    “What we see from a kilonova might depend on our viewing angle. The same type of event would appear different depending on whether we’re looking at it face-on or edge-on, which came as a total surprise to us,” said Eleonora Troja of the University of Maryland, College Park, Maryland, and NASA’s Goddard Space Flight Center, Greenbelt, Maryland. Troja is also a principal investigator of a team using Hubble observations to study the object.

    The gravitational wave source now is too close to the Sun on the sky for Hubble and other observatories to study. It will come back into view in November. Until then, astronomers will be working diligently to learn all they can about this unique event.

    The launch of NASA’s James Webb Space Telescope also will offer an opportunity to examine the infrared light from the source, should that glow remain detectable in the months and years to come.

    NASA/ESA/CSA Webb Telescope annotated

    Related Links
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    The science paper by N.R. Tanvir et al. (Astrophysical Journal Letters)
    The science paper by A.J. Levan et al. (Astrophysical Journal Letters)
    NASA’s Hubble Portal
    NASA’s Fermi Portal
    NASA’s Swift Portal
    NASA’s Chandra Portal
    NASA’s Spitzer Portal
    LIGO Scientific Collaboration
    European Gravitational Observatory
    Hubble Europe’s Press Release
    The science paper by E. Troja et al (Nature)

    See the full article here .

    Please help promote STEM in your local schools.

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    The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency. NASA’s Goddard Space Flight Center manages the telescope. The Space Telescope Science Institute (STScI), is a free-standing science center, located on the campus of The Johns Hopkins University and operated by the Association of Universities for Research in Astronomy (AURA) for NASA, conducts Hubble science operations.

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