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  • richardmitnick 8:49 pm on February 3, 2016 Permalink | Reply
    Tags: , , Gamma Rays, , ,   

    From phys.org: “Galactic center’s gamma rays unlikely to originate from dark matter, evidence shows” 

    physdotorg
    phys.org

    February 3, 2016
    No writer credit found

    Gamma rays not from dark matter
    Studies by two independent groups from the US and the Netherlands indicate that the observed excess of gamma rays from the inner galaxy likely comes from a new source rather than from dark matter. The best candidates are rapidly rotating neutron stars, which will be prime targets for future searches. The Princeton/MIT group and the Netherlands-based group used two different techniques, non-Poissonian noise and wavelet transformation, respectively, to independently determine that the gamma ray signals were not due to dark matter annihilation. Credit: Christoph Weniger

    Bursts of gamma rays from the center of our galaxy are not likely to be signals of dark matter but rather other astrophysical phenomena such as fast-rotating stars called millisecond pulsars, according to two new studies, one from a team based at Princeton University and the Massachusetts Institute of Technology and another based in the Netherlands.

    Previous studies suggested that gamma rays coming from the dense region of space in the inner Milky Way galaxy could be caused when invisible dark matter particles collide. But using new statistical analysis methods, the two research teams independently found that the gamma ray signals are uncharacteristic of those expected from dark matter. Both teams reported the finding in the journal Physical Review Letters this week.

    “Our analysis suggests that what we are seeing is evidence for a new astrophysical source of gamma rays at the center of the galaxy,” said Mariangela Lisanti, an assistant professor of physics at Princeton. “This is a very complicated region of the sky and there are other astrophysical signals that could be confused with dark matter signals.”

    The center of the Milky Way galaxy is thought to contain dark matter because it is home to a dense concentration of mass, including dense clusters of stars and a black hole. A conclusive finding of dark matter collisions in the galactic center would be a major step forward in confirming our understanding of our universe. “Finding direct evidence for these collisions would be interesting because it would help us understand the relationship between dark matter and ordinary matter,” said Benjamin Safdi, a postdoctoral researcher at MIT who earned his Ph.D. in 2014 at Princeton.

    To tell whether the signals were from dark matter versus other sources, the Princeton/MIT research team turned to image-processing techniques. They looked at what the gamma rays should look like if they indeed come from the collision of hypothesized dark matter particles known as weakly interacting massive particles, or WIMPs. For the analysis, Lisanti, Safdi and Samuel Lee, a former postdoctoral research fellow at Princeton who is now at the Broad Institute, along with colleagues Wei Xue and Tracy Slatyer at MIT, studied images of gamma rays captured by NASA’s Fermi Gamma-ray Space Telescope, which has been mapping the rays since 2008.

    NASA Fermi Telescope
    NASA’s Fermi Gamma-ray Space Telescope

    Dark matter particles are thought to make up about 85 percent of the mass in the universe but have never been directly detected. The collision of two WIMPs, according to a widely accepted model of dark matter, causes them to annihilate each other to produce gamma rays, which are the highest-energy form of light in the universe.

    According to this model, the high-energy particles of light, or photons, should be smoothly distributed among the pixels in the images captured by the Fermi telescope. In contrast, other sources, such as rotating stars known as pulsars, release bursts of light that show up as isolated, bright pixels.

    The researchers applied their statistical analysis method to images collected by the Fermi telescope and found that the distribution of photons was clumpy rather than smooth, indicating that the gamma rays were unlikely to be caused by dark matter particle collisions.

    Exactly what these new sources are is unknown, Lisanti said, but one possibility is that they are very old, rapidly rotating stars known as millisecond pulsars. She said it would be possible to explore the source of the gamma rays using other types of sky surveys involving telescopes that detect radio frequencies.

    Douglas Finkbeiner, a professor of astronomy and physics at Harvard University who was not directly involved in the current study, said that although the finding complicates the search for dark matter, it leads to other areas of discovery. “Our job as astrophysicists is to characterize what we see in the universe, not get some predetermined, wished-for outcome. Of course it would be great to find dark matter, but just figuring out what is going on and making new discoveries is very exciting.”

    According to Christoph Weniger from the University of Amsterdam and lead author of the Netherlands-based study, the finding is a win-win situation: “Either we find hundreds or thousands of millisecond pulsars in the upcoming decade, shedding light on the history of the Milky Way, or we find nothing. In the latter case, a dark matter explanation for the gamma ray excess will become much more obvious.”

    See the full article here.

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    About Phys.org in 100 Words

    Phys.org™ (formerly Physorg.com) is a leading web-based science, research and technology news service which covers a full range of topics. These include physics, earth science, medicine, nanotechnology, electronics, space, biology, chemistry, computer sciences, engineering, mathematics and other sciences and technologies. Launched in 2004, Phys.org’s readership has grown steadily to include 1.75 million scientists, researchers, and engineers every month. Phys.org publishes approximately 100 quality articles every day, offering some of the most comprehensive coverage of sci-tech developments world-wide. Quancast 2009 includes Phys.org in its list of the Global Top 2,000 Websites. Phys.org community members enjoy access to many personalized features such as social networking, a personal home page set-up, RSS/XML feeds, article comments and ranking, the ability to save favorite articles, a daily newsletter, and other options.

     
  • richardmitnick 12:49 pm on February 3, 2016 Permalink | Reply
    Tags: , , Gamma Rays,   

    From AAS NOVA: “Upgrading Fermi Without Traveling to Space” 

    AASNOVA

    Amercan Astronomical Society

    3 February 2016
    Susanna Kohler

    NASA Fermi Telescope
    NASA Fermi

    Fermi Lat sky image
    This image, constructed from 6+ years of observations by NASA’s Fermi Gamma-ray Space Telescope, is the first to show how the entire sky appears at energies between 50 GeV and 2 TeV. A recent improvement to Fermi-LAT’s data analysis software has significantly increased the instrument’s sensitivity, resulting in this spectacular high-energy sky map. [NASA/DOE/Fermi LAT Collaboration]

    The Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope has received an upgrade that increased its sensitivity by a whopping 40% — and nobody had to travel to space to make it happen!

    NASA Fermi LAT
    Fermi LAT

    The difference instead stems from remarkable improvement to the software used to analyze Fermi-LAT’s data, and it has resulted in a new high-energy map of our sky.

    Pass 8

    Fermi-LAT has been surveying the whole sky since August 2008. It detects gamma-ray photons by converting them into electron-positron pairs and tracking the paths of these charged particles. But differentiating this signal from the charged cosmic rays that also pass through the detector — with a flux that can be 10,000 times larger! — is a challenging process. Making this distinction and rebuilding the path of the original gamma ray relies on complex analysis software.

    Pass 8” is a complete reprocessing of all data collected by Fermi-LAT. The software has gone through many revisions before now, but this is the first revision that has taken into account all of the experience that the Fermi team has gained operating the LAT in its orbital environment.

    The improvements made in Pass 8 include better background rejection of misclassified charged particles, improvements to the point spread function and effective area of the detector, and an extension of the effective energy range from below 100 MeV to beyond a few hundred GeV. The changes made in Pass 8 have increased the sensitivity of Fermi-LAT by an astonishing 40%.

    Map of the High-Energy Sky

    The first result from the improvements of Pass 8 is 2FHL, the second catalog of high-energy Fermi sources, constructed from 80 months of data from Fermi-LAT. The 2FHL catalog contains 360 sources from across the sky in the 50 GeV–2TeV range. Here are just a few details:

    47 of the sources are new — they have not previously been detected by Fermi or ground-based gamma-ray detectors.
    86% of the sources can be associated with counterparts at other wavelengths. This includes
    75% that are active galactic nuclei, and
    11% that originate in our galaxy, the majority of which are associated with objects at the final stage of stellar evolution, such as pulsar wind nebulae. and supernova remnants.

    Because the quality of Fermi-LAT’s observations is limited by the number of photons collected, longer observing time will only serve to improve the detections in this catalog. And since only 22% of the 2FHL sources have been observed by ground-based gamma-ray detectors (which have much more limited fields of view), this catalog provides an excellent list of candidates that these detectors can now follow up at very high energies.

    Bonus

    Want to learn more about Pass 8? Check out this video, created by the Fermi team. [NASA’s Goddard Space Flight Center]


    download mp4 video here .

    Citation

    M. Ackermann et al 2016 ApJS 222 5. doi:10.3847/0067-0049/222/1/5

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  • richardmitnick 3:22 pm on January 11, 2016 Permalink | Reply
    Tags: , Gamma Rays, Millisecond pulsars   

    From Astrobites: “Are millisecond pulsars causing excess gamma-rays?” 

    Astrobites bloc

    Astrobites

    Jan 11, 2016
    Kelly Malone

    Paper Title: Title: The Gamma-Ray Luminosity Function of Millisecond Pulsars and Implications for the GeV Excess

    Authors: Dan Hooper – FNAL
    Gopolang Mohlabeng

    1
    Illustration of a millisecond pulsar emitting beams of radiation. As the pulsar rotates, this beam is periodically pointed toward the Earth. Credit: NASA

    The Galactic Center is an exciting area. In addition to the well-known central black hole and the thousands of stars, there is a curious excess of GeV gamma rays. The origin of these gamma rays is currently unknown. This excess gets a fair amount of attention because it has roughly the same characteristics that we would expect from annihilating dark matter (see this past astrobite for more details). One of the other competing explanations is a population of unresolved millisecond pulsars (MSPs), which would be expected to have roughly the same spectral shape as the GeV excess. Millisecond pulsars are rotating neutron stars that emit a beam of radiation with a periods on a millisecond time scale. They probably form when an old neutron star is spun up via accretion of matter onto it from a companion star.

    Previous studies have looked at the millisecond pulsar explanation and concluded that if it were true, the inner portion of the Milky Way should have many more bright MSPs than the Fermi Gamma-Ray Space Telescope has already detected.

    NASA Fermi Telescope
    NASA/Fermi

    However, this conclusion may suffer from a systematic problem with the measurement of distances to many millisecond pulsars. This is because the distances are often estimated using radio dispersion measurements, which in turn rely on models of how electrons are distributed in interstellar space. If you propagate all the resulting uncertainties through, they can end up being fairly significant! If this potential mismeasurement turns out to be a problem, Fermi might not be sensitive to all of the MSPs responsible for the excess at the Galactic Center.

    Hooper and Mohlabeng used a different method to describe the characteristics of the Milky Way MSP population which does not use distances based on the potentially problematic radio dispersion measurements. Instead, they determined the luminosity function (or number of stars as a function of brightness) of the MSPs using the best fit to a model describing the MSP population in the Milky Way that they constructed and observations about gamma-ray emission that come from groups of stars orbiting the Milky Way’s center. The parameters in the model were constrained using the few MSPs that have distances obtained via the motion of the stars over a long period of time, which is more accurate. They also looked at three variables that effect the luminosity of a given MSP: the magnetic field, the period of rotation, and the gamma-ray efficiency.

    Temp 2
    This figure shows how the number of millisecond pulsars changes as a function of luminosity. The black line is the luminosity function from this paper. The red line is from a previous study that uses the uncertain distance calculation. They disagree greatly about the number of very bright MSPs, but mostly agree at lower luminosities. (Source: the paper)

    The authors used their resulting luminosity function and the probability that a given MSP would be detected by Fermi to estimate how many MSP candidates Fermi should have already detected, and came to the conclusion that Fermi still should have detected significantly more MSPs than it has. However, they are not ruling out the millisecond pulsar theory completely. It is quite possible that MSPs near the Galactic Center are less luminous than those found elsewhere, such as in the Galactic Plane or in globular clusters. One theory has MSPs accumulating in the central stellar cluster/Galactic bulge as the result of the groups of stars getting too close to the central black hole. This would lead to older MSPs in the Galactic center than elsewhere. Since pulsars lose rotational kinetic energy with age, this would lead to lower luminosity MSPs. More studies are needed to get to the bottom of this mystery. Further constraints on the MSP population would give weight to the annihilating dark matter explanation for the GeV excess.

    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 6:08 pm on January 7, 2016 Permalink | Reply
    Tags: , , , Gamma Rays,   

    From NASA Fermi: “NASA’s Fermi Satellite Kicks Off a Blazar-detecting Bonanza” 


    Fermi

    Dec. 15, 2015
    Francis Reddy
    NASA’s Goddard Space Flight Center, Greenbelt, Maryland

    A long time ago in a galaxy half the universe away, a flood of high-energy gamma rays began its journey to Earth. When they arrived in April, NASA’s Fermi Gamma-ray Space Telescope caught the outburst, which helped two ground-based gamma-ray observatories detect some of the highest-energy light ever seen from a galaxy so distant. The observations provide a surprising look into the environment near a supermassive black hole at the galaxy’s center and offer a glimpse into the state of the cosmos 7 billion years ago.


    download mp4 video here .
    Explore how gamma-ray telescopes in space and on Earth captured an outburst of high-energy light from PKS 1441+25, a black-hole-powered galaxy more than halfway across the universe. Credits: NASA’s Goddard Space Flight Center

    “When we looked at all the data from this event, from gamma rays to radio, we realized the measurements told us something we didn’t expect about how the black hole produced this energy,” said Jonathan Biteau at the Nuclear Physics Institute of Orsay, France. He led the study of results from the Very Energetic Radiation Imaging Telescope Array System (VERITAS), a gamma-ray telescope in Arizona.

    Veritas Telescope
    VERITAS

    Astronomers had assumed that light at different energies came from regions at different distances from the black hole. Gamma rays, the highest-energy form of light, were thought to be produced closest to the black hole.

    “Instead, the multiwavelength picture suggests that light at all wavelengths came from a single region located far away from the power source,” Biteau explained. The observations place the area roughly five light-years from the black hole, which is greater than the distance between our sun and the nearest star.

    The gamma rays came from a galaxy known as PKS 1441+25, a type of active galaxy called a blazar. Located toward the constellation Boötes, the galaxy is so far away its light takes 7.6 billion years to reach us. At its heart lies a monster black hole with a mass estimated at 70 million times the sun’s and a surrounding disk of hot gas and dust. If placed at the center of our solar system, the black hole’s event horizon — the point beyond which nothing can escape — would extend almost to the orbit of Mars.

    As material in the disk falls toward the black hole, some of it forms dual particle jets that blast out of the disk in opposite directions at nearly the speed of light. Blazars are so bright in gamma rays because one jet points almost directly toward us, giving astronomers a view straight into the black hole’s dynamic and poorly understood realm.

    Temp 1
    Black-hole-powered galaxies called blazars are the most common sources detected by NASA’s Fermi Gamma-ray Space Telescope. As matter falls toward the supermassive black hole at the galaxy’s center, some of it is accelerated outward at nearly the speed of light along jets pointed in opposite directions. When one of the jets happens to be aimed in the direction of Earth, as illustrated here, the galaxy appears especially bright and is classified as a blazar.Credits: M. Weiss/CfA

    In April, PKS 1441+25 underwent a major eruption. Luigi Pacciani at the Italian National Institute for Astrophysics in Rome was leading a project to catch blazar flares in their earliest stages in collaboration with the Major Atmospheric Gamma-ray Imaging Cerenkov experiment (MAGIC), located on La Palma in the Canary Islands.

    MAGIC Cherenkov gamma ray telescope
    MAGIC telescope

    Using public Fermi data, Pacciani discovered the outburst and immediately alerted the astronomical community. Fermi’s Large Area Telescope revealed gamma rays up to 33 billion electron volts (GeV), reaching into the highest-energy part of the instrument’s detection range. For comparison, visible light has energies between about 2 and 3 electron volts.

    NASA Fermi LAT
    NASA/Fermi LAT

    “Detecting these very energetic gamma rays with Fermi, as well as seeing flaring at optical and X-ray energies with NASA’s Swift satellite, made it clear that PKS 1441+25 had become a good target for MAGIC,” Pacciani said.

    NASA SWIFT Telescope
    NASA/Swift

    Following up on the Fermi alert, the MAGIC team turned to the blazar and detected gamma rays with energies ranging from 40 to 250 GeV. “Because this galaxy is so far away, we didn’t have a strong expectation of detecting gamma rays with energies this high,” said Josefa Becerra Gonzalez, a researcher at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, who analyzed Fermi LAT data as part of the MAGIC study. “There are fewer and fewer gamma rays at progressively higher energies, and fewer still from very distant sources.”

    The reason distance matters for gamma rays is that they convert into particles when they collide with lower-energy light. The visible and ultraviolet light from stars shining throughout the history of the universe forms a remnant glow called the extragalactic background light (EBL). For gamma rays, this is a cosmic gauntlet they must pass through to be detected at Earth. When a gamma ray encounters starlight, it transforms into an electron and a positron and is lost to astronomers. The farther away the blazar is, the less likely its highest-energy gamma rays will survive to be detected.

    Temp 2
    More distant blazars show a loss of higher-energy gamma rays thanks to the extragalactic background light (EBL), a “cosmic fog” of visible and ultraviolet starlight that permeates the universe. From studies of nearby blazars, scientists know how many gamma rays should be emitted at different energies. If a gamma ray on its way to Earth collides with lower-energy light in the EBL, it converts into a pair of particles and is lost to astronomers. As shown by the graphs at left in this illustration, the more distant the blazar, the fewer high-energy gamma rays we can detect. During the April 2015 outburst of PKS 1441+25, MAGIC and VERITAS saw rare gamma rays exceeding 100 GeV that managed to survive a journey of 7.6 billion light-years. Credits: NASA’s Goddard Space Flight Center

    Following the MAGIC discovery, VERITAS also detected gamma rays with energies approaching 200 GeV. Findings from both teams are detailed in papers published Dec. 15 in The Astrophysical Journal Letters.

    PKS 1441+25 is one of only two such distant sources for which gamma rays with energies above 100 GeV have been observed. Its dramatic flare provides a powerful glimpse into the intensity of the EBL from near-infrared to near-ultraviolet wavelengths and suggests that galaxy surveys have identified most of the sources responsible for it.

    NASA’s Fermi Gamma-ray Space Telescope is an astrophysics and particle physics partnership, developed in collaboration with the U.S. Department of Energy and with important contributions from academic institutions and partners in France, Germany, Italy, Japan, Sweden and the United States.

    For more information about NASA’s Fermi, visit:

    http://www.nasa.gov/fermi

    See the full article here .

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  • richardmitnick 5:30 pm on January 7, 2016 Permalink | Reply
    Tags: , , Gamma Rays,   

    From NASA Fermi: “NASA’s Fermi Space Telescope Sharpens its High-energy Vision” 


    Fermi

    Jan. 7, 2016
    Francis Reddy
    NASA’s Goddard Space Flight Center, Greenbelt, Maryland

    Major improvements to methods used to process observations from NASA’s Fermi Gamma-ray Space Telescope have yielded an expanded, higher-quality set of data that allows astronomers to produce the most detailed census of the sky yet made at extreme energies. A new sky map reveals hundreds of these sources, including 12 that produce gamma rays with energies exceeding a trillion times the energy of visible light. The survey also discovered four dozen new sources that remain undetected at any other wavelength.

    Temp 1
    This image, constructed from more than six years of observations by NASA’s Fermi Gamma-ray Space Telescope, is the first to show how the entire sky appears at energies between 50 billion (GeV) and 2 trillion electron volts (TeV). For comparison, the energy of visible light falls between about 2 and 3 electron volts. A diffuse glow fills the sky and is brightest in the middle of the map, along the central plane of our galaxy. The famous Fermi Bubbles, first detected in 2010, appear as red extensions north and south of the galactic center and are much more pronounced at these energies.

    Temp 2
    Gamma-Ray bubbles at the center of the Milky Way. Credit: NASA’s Goddard Space Flight Center

    Discrete gamma-ray sources include pulsar wind nebulae and supernova remnants within our galaxy, as well as distant galaxies called blazars powered by supermassive black holes. Labels show the highest-energy sources, all located within our galaxy and emitting gamma rays exceeding 1 TeV.
    Credits: NASA/DOE/Fermi LAT Collaboration

    “What made this advance possible was a complete reanalysis, which we call Pass 8, of all data acquired by Fermi’s Large Area Telescope (LAT),” said Marco Ajello, a Fermi team member at Clemson University in South Carolina. “The end result is effectively a complete instrument upgrade without our ever having to leave the ground.”

    By carefully reexamining every gamma-ray and particle detection by the LAT since Fermi’s 2008 launch, scientists improved their knowledge of the detector’s response to each event and to the background environment in which it was measured. This enabled the Fermi team to find many gamma rays that previously had been missed while simultaneously improving the LAT’s ability to determine the directions of incoming gamma rays. These improvements effectively sharpen the LAT’s view while also significantly widening its useful energy range.


    download mp4 video here .
    Watch Fermi scientists explain why they’re so excited about Pass 8, a complete reprocessing of all data collected by the mission’s Large Area Telescope. This analysis increased the LAT’s sensitivity, widened its energy range, and effectively sharpened its view through improved backtracking of incoming gamma rays. Credits: NASA’s Goddard Space Flight Center

    Using 61,000 Pass 8 gamma rays collected over 80 months, Ajello and his colleagues constructed a map of the entire sky at energies ranging from 50 billion (GeV) to 2 trillion electron volts (TeV). For comparison, the energy of visible light ranges from about 2 to 3 electron volts.


    download mp4 video here .
    Tour the best view of the high-energy gamma-ray sky yet seen. This video highlights the plane of our galaxy and identifies objects producing gamma rays with energies greater than 1 TeV. Credits: NASA’s Goddard Space Flight Center

    “Of the 360 sources we cataloged, about 75 percent are blazars, which are distant galaxies sporting jets powered by supermassive black holes,” said co-investigator Alberto Domínguez at the Complutense University in Madrid. “The highest-energy sources, all located in our galaxy, are mostly remnants of supernova explosions and pulsar wind nebulae, places where rapidly rotating neutron stars accelerate particles to near the speed of light.” One famous example, the Crab Nebula, tops the list of the highest-energy Fermi sources, producing a steady drizzle of gamma rays exceeding 1 TeV.

    Astronomers think these very high-energy gamma rays are produced when lower-energy light collides with accelerated particles. This results in a small energy loss for the particle and a big gain for the light, transforming it into a gamma ray.


    download mp4 video here.
    Gamma-ray emission from the highest-energy sources detected by Fermi is likely produced by what scientists call the inverse Compton process. When an electron moving near the speed of light strikes a low-energy photon, the collision slightly slows the electron and boosts the light’s energy into the gamma-ray regime. Credits: NASA’s Goddard Space Flight Center

    For the first time, Fermi data now extend to energies previously seen only by ground-based detectors. Because ground-based telescopes have much smaller fields of view than the LAT, which scans the whole sky every three hours, they have detected only about a quarter of the objects in the catalog. This study provides ground facilities with more than 280 new targets for follow-up observations.

    “An exciting aspect of this catalog is that we find many new sources that emit gamma rays over a comparatively large patch of the sky,” explained Jamie Cohen, a University of Maryland graduate student working with the Fermi team at NASA’s Goddard Space Flight Center in Greenbelt. “Finding more of these objects enables us to probe their structures as well as better understand mechanisms that accelerate the subatomic particles that ultimately produce gamma-ray emission.” The new catalog identifies 25 of these extended objects, including three new pulsar wind nebulae and two new supernova remnants.

    Ajello presented the findings Thursday at the 227th meeting of the American Astronomical Society in Kissimmee, Florida. A paper describing the catalog has been accepted for publication in The Astrophysical Journal Supplement.

    NASA’s Fermi Gamma-ray Space Telescope is an astrophysics and particle physics partnership, developed in collaboration with the U.S. Department of Energy and with important contributions from academic institutions and partners in France, Germany, Italy, Japan, Sweden and the United States.

    For more information about NASA’s Fermi, visit:

    http://www.nasa.gov/fermi

    See the full article here .

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  • richardmitnick 2:04 pm on December 15, 2015 Permalink | Reply
    Tags: , , , Gamma Rays, ,   

    From CfA: “VERITAS Detects Gamma Rays from Galaxy Halfway Across the Visible Universe” 

    Harvard Smithsonian Center for Astrophysics


    Center For Astrophysics

    December 15, 2015
    Christine Pulliam
    Media Relations Manager
    Harvard-Smithsonian Center for Astrophysics
    617-495-7463
    cpulliam@cfa.harvard.edu – See more at: https://www.cfa.harvard.edu/news/2015-29#sthash.X9b3ENBu.dpuf

    1

    In April 2015, after traveling for about half the age of the universe, a flood of powerful gamma rays from a distant galaxy slammed into Earth’s atmosphere. That torrent generated a cascade of light – a shower that fell onto the waiting mirrors of the Very Energetic Radiation Imaging Telescope Array System (VERITAS) in Arizona.

    CfA VERITAS
    VERITAS Gamma Ray telescope array

    The resulting data have given astronomers a unique look into that faraway galaxy and the black hole engine at its heart.

    Gamma rays are photons of light with very high energies. These gamma rays came from a galaxy known as PKS 1441+25, which is a rare type of galaxy known as a blazar. At its center it hosts a supermassive black hole surrounded by a disk of hot gas and dust.

    As material from the disk swirls toward the black hole, some of it gets channeled into twin jets that blast outward like water from a fire hose only much faster – close to the speed of light. One of those jets is aimed nearly in our direction, giving us a view straight into the galaxy’s core.

    “We’re looking down the barrel of this relativistic jet,” explains Wystan Benbow of the Harvard-Smithsonian Center for Astrophysics (CfA). “That’s why we’re able to see the gamma rays at all.”

    One of the unknowns in blazar physics is the exact location of gamma-ray emission. Using data from VERITAS, as well as the Fermi Gamma-Ray Space Telescope, the researchers found that the source of the gamma rays was within the relativistic jet but surprisingly far from the galaxy’s black hole.

    NASA Fermi Telescope
    NASA/Fermi

    The emitting region is at least a tenth of a light-year away, and most likely is 5 light-years away. (A light-year is the distance light travels in one year, or about 6 trillion miles.)

    Moreover, the region emitting gamma rays was larger than typically seen in an active galaxy, measuring about a third of a light-year across.

    “These jets tend to have clumps in them. It’s possible that two of those clumps may have collided and that’s what generated the burst of energy,” says co-author Matteo Cerruti of the CfA.

    Measuring high-energy gamma rays at all was a surprise. They tend to be either absorbed at the source or on their long journey to Earth. When the galaxy flared to life, it must have generated a huge flood of gamma rays.

    The finding also provides insight into a phenomenon known as extragalactic background light or EBL, a faint haze of light that suffuses the universe. The EBL comes from all the stars and galaxies that have ever existed, and in a sense can track the history of the universe.

    The EBL also acts like a fog to high-energy gamma rays, absorbing them as they travel through space. This new measurement sets an indirect limit on how abundant the EBL can be – too much, and it would have absorbed the gamma-ray flare. The results complement previous measurements based on direct observations.

    These results have been accepted for publication in The Astrophysical Journal Letters and are available online.

    See the full article here .

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    The Center for Astrophysics combines the resources and research facilities of the Harvard College Observatory and the Smithsonian Astrophysical Observatory under a single director to pursue studies of those basic physical processes that determine the nature and evolution of the universe. The Smithsonian Astrophysical Observatory (SAO) is a bureau of the Smithsonian Institution, founded in 1890. The Harvard College Observatory (HCO), founded in 1839, is a research institution of the Faculty of Arts and Sciences, Harvard University, and provides facilities and substantial other support for teaching activities of the Department of Astronomy.

     
  • richardmitnick 11:07 am on September 9, 2015 Permalink | Reply
    Tags: , Gamma Rays, HERMES III,   

    From Sandia: “Workhorse gamma ray generator HERMES III fires its 10,000th shot at Sandia Labs” 


    Sandia Lab

    September 9, 2015
    Neal Singer, nsinger@sandia.gov, (505) 845-7078

    1
    Chris Kirtley, top, and JJ Montoya adjust gamma ray generator HERMES III (High-Energy Radiation Megavolt Electron Source) for its next shot at Sandia National Laboratories. (Photo by Randy Montoya)

    The High-Energy Radiation Megavolt Electron Source, better known as HERMES III, has fired its 10,000th shot at Sandia National Laboratories.

    HERMES III, the world’s most powerful gamma ray generator, produces a highly energetic beam that tests how well electronics can survive a burst of radiation that approximates the output of a nuclear weapon. The machine can accommodate targets that range in size from a single transistor to a military tank.

    The machine generates an intense electron beam at energies approaching 20 mega-electron volts. The electron beam is then guided into a very dense target called a converter. That interaction produces copious amounts of gamma rays. The thinness of the converter permits most of the beam’s energy to pass through it rapidly; thus, the passage causes minimum damage. This enables HERMES III to fire multiple shots at a time without having to re-establish the vacuum in which the experiments take place.

    “HERMES III has gone hundreds of shots without any damage to its converter,” said Sandia manager Ray Thomas.

    To achieve its high voltage, HERMES III uses 20 inductively isolated modules arranged in series. In size and shape, the machine resembles a short subway train 17 feet wide, 50 feet long and 16.5 feet high. Each “car,” or unit, adds 1 million volts in series, reaching a total of 20 million volts. Its linear, voltage-adding geometry is distinct from the wagon-wheel-shaped architecture favored by other Sandia accelerators, arrangements more useful for adding current.

    Also helpful for rapid firing is that HERMES III test targets are placed at one end of the machine rather than at its center.

    “Our customers bring their own targets, place them at the front of the machine as we request and then remove them after the shot,” said technician Gary Tilley, who’s worked on HERMES III for 20 years. Other Sandia facilities, like its more famous Z machine, have to clean up the remnants of exploded targets placed at the center of their energy flows.

    2
    Technician Gary Tilley at Sandia Labs repairs a cavity at HERMES III. (Photo by Randy Montoya)

    Juan Diego Salazar is part of the team that watches to make sure each module receives the proper dose of power, at the right moment in time, to accelerate the beam.

    “Every firing is different,” he said. “The test targets always change.”

    Continual re-evaluation of the electrical power feeding the beam as it flows through its modules, and continual recalibration of the beam’s line of sight to the target, are necessary because an unobserved power or alignment failure somewhere within the system could mistakenly show a target more radiation-resistant than it actually is.

    Real-time adjustments would be too late: The achieved beam flashes for 20 billionths of a second, about the time it takes light to travel 20 feet.

    “Accurate results are important,” said Thomas. “That’s what we’re about.”

    See the full article here .

    Please help promote STEM in your local schools.

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    Sandia Campus
    Sandia National Laboratory

    Sandia National Laboratories is a multiprogram laboratory operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration. With main facilities in Albuquerque, N.M., and Livermore, Calif., Sandia has major R&D responsibilities in national security, energy and environmental technologies, and economic competitiveness.
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  • richardmitnick 3:18 pm on June 24, 2015 Permalink | Reply
    Tags: , , Gamma Rays, ,   

    From Symmetry: “Seeing in gamma rays” 

    Symmetry

    June 24, 2015
    Glenn Roberts Jr.

    1
    Courtesy of Fermi LAT collaboration

    The Fermi Gamma-ray Space Telescope creates maps of the gamma-ray sky.

    Maps from the Fermi Gamma-ray Space Telescope literally show the universe in a different light.

    NASA Fermi Telescope
    Fermi

    Fermi’s Large Area Telescope (LAT) has been watching the universe at a broad range of gamma-ray energies for more than seven years.

    Gamma rays are the highest-energy form of light in the cosmos. They come from jets of high-energy particles accelerated near supermassive black holes at the centers of galaxies, shock waves around exploded stars, and the intense magnetic fields of fast-spinning collapsed stars. On Earth, gamma rays are produced by nuclear reactors, lightning and the decay of radioactive elements.

    From low-Earth orbit, the Fermi Gamma-ray Space Telescope scans the entire sky for gamma rays every three hours. It captures new and recurring sources of gamma rays at different energies, and it can be diverted from its usual course to fix on explosive events known as gamma-ray bursts.

    Combining data collected over years, the LAT collaboration periodically creates gamma-ray maps of the universe. These colored maps plot the universe’s most extreme events and high-energy objects.

    The all-sky maps typically portray the universe as an ellipse that shows the entire sky at once, as viewed from Earth. On the maps, the brightest gamma-ray light is shown in yellow and progressively dimmer gamma-ray light is shown in red, blue, and black. These are false colors, though; gamma-rays are invisible.

    The maps are oriented with the center of the Milky Way at their center and the plane of our galaxy oriented horizontally across the middle. The plane of the Milky Way is bright in gamma rays. Above and below the bright band, much of the gamma-ray light comes from outside of our galaxy.

    “What you see in gamma rays is not so predictable,” says Elliott Bloom, a SLAC National Accelerator Laboratory professor and member of the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC) who is part of a scientific collaboration supporting Fermi’s principal instrument, the Large Area Telescope.

    Teams of researchers have identified mysterious, massive “bubbles” blooming 30,000 light-years outward from our galaxy’s center, for example, with most features appearing only at gamma-ray wavelengths.

    Scientists create several versions of the Fermi sky maps. Some of them focus only on a specific energy range, says Eric Charles, another member of the Fermi collaboration who is also a KIPAC scientist.

    “You learn a lot by correlating things in different energy ‘bins,’” he says. “If you look at another map and see completely different things, then there may be these different processes. What becomes useful is at different wavelengths you can make comparisons and correlate things.”

    But sometimes what you need is the big picture, says Seth Digel, a SLAC senior staff scientist and a member of KIPAC and the Fermi team. “There are some aspects you can only study with maps, such as looking at the extended gamma-ray emissions—not just the point sources, but regions of the sky that are glowing in gamma rays for different reasons.”

    See the full article here.

    Please help promote STEM in your local schools.

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    Symmetry is a joint Fermilab/SLAC publication.


     
  • richardmitnick 9:23 am on June 12, 2015 Permalink | Reply
    Tags: , , , Gamma Rays,   

    From FNAL- “Frontier Science Result: Fermi Gamma-Ray Space Telescope” 

    FNAL Home

    Fermilab is an enduring source of strength for the US contribution to scientific research world wide.

    June 12, 2015
    Dan Hooper

    NASA Fermi Telescope
    Fermi Gamma Ray Telescope

    2
    A team of astrophysicists is looking for dark matter in the form of subhalos. These clumps of dark matter within the Milky Way are predicted to produce a distinctive gamma-ray signal. Image courtesy of The Aquarius Project

    In addition to teaching us about pulsars, cosmic rays and supermassive black holes, the Fermi Gamma Ray Space Telescope is one of the world’s premier dark matter experiments. In many models, the interactions of dark matter particles can create energetic photons, known as gamma rays. Fermi provides us with our most sensitive view of the gamma-ray sky and is able to test many of our most promising theories of dark matter.

    Over the past several years, my collaborators and I have published a series of papers describing an excess of gamma rays from the region surrounding the center of the Milky Way. After many long discussions, arguments and debates, the majority of the gamma-ray astrophysics community seems to have reached a consensus that this excess is real and is in need of an explanation. One exciting possibility is that these gamma rays could be produced by dark matter particles. But even though this signal looks very much like what we expected from dark matter, we can’t entirely rule out other explanations, such as a series of recent outbursts of cosmic rays or some unknown population of faint gamma-ray sources.

    One way to potentially confirm a dark matter origin for this excess would be to observe the same spectrum of gamma rays from otherwise invisible clumps of dark matter — known as subhalos — elsewhere in the sky. In fact, if the gamma rays from the Galactic Center do come from dark matter particles, we estimate that Fermi should be able to detect a handful of these subhalos as bright gamma-ray sources. The challenge is that Fermi has detected hundreds of bright, unidentified sources, the vast majority of which are not related to dark matter. This large haystack of sources makes it hard to find the dark matter subhalos that are the needles we are looking for.

    But in one important respect, dark matter subhalos should look different from other kinds of gamma-ray sources: They should be slightly extended or “puffy.” My collaborators (Bridget Bertoni of the University of Washington and Tim Linden of the University of Chicago) and I have recently found evidence that some of Fermi’s unidentified sources are in fact extended, making them seem more likely to be dark matter subhalos. We continue to scrutinize the data, and although we’re not prepared to claim discovery yet, we are very excited that this new information might make it possible to independently test — and maybe even confirm — a dark matter origin for Fermi’s Galactic Center gamma-ray excess.

    See the full article here.

    Please help promote STEM in your local schools.

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    Fermilab Campus

    Fermi National Accelerator Laboratory (Fermilab), located just outside Batavia, Illinois, near Chicago, is a US Department of Energy national laboratory specializing in high-energy particle physics. Fermilab is America’s premier laboratory for particle physics and accelerator research, funded by the U.S. Department of Energy. Thousands of scientists from universities and laboratories around the world
    collaborate at Fermilab on experiments at the frontiers of discovery.

     
  • richardmitnick 10:13 am on May 27, 2015 Permalink | Reply
    Tags: , , , Gamma Rays, ,   

    From LLNL: “Lawrence Livermore scientists move one step closer to mimicking gamma-ray bursts” 


    Lawrence Livermore National Laboratory

    May. 26, 2015

    Anne M Stark
    stark8@llnl.gov (link sends e-mail)
    925-422-9799

    1
    The Centaurus A galaxy, at a distance of about 12 million light years from Earth, contains a gargantuan jet blasting away from a central supermassive black hole. In this image, red, green and blue show low, medium and high-energy X-rays. Photo courtesy NASA/CXC/U. Birmingham/M. Burke et al.

    Using ever more energetic lasers, Lawrence Livermore researchers have produced a record high number of electron-positron pairs, opening exciting opportunities to study extreme astrophysical processes, such as black holes and gamma-ray bursts.

    By performing experiments using three laser systems — Titan at Lawrence Livermore, Omega-EP at the Laboratory for Laser Energetics (link is external) and Orion at Atomic Weapons Establishment (link is external) (AWE) in the United Kingdom — LLNL physicist Hui Chen and her colleagues created nearly a trillion positrons (also known as antimatter particles). In previous experiments at the Titan laser in 2008, Chen’s team had created billions of positrons.

    Positrons, or “anti-electrons,” are anti-particles with the same mass as an electron but with opposite charge. The generation of energetic electron-positron pairs is common in extreme astrophysical environments associated with the rapid collapse of stars and formation of black holes. These pairs eventually radiate their energy, producing extremely bright bursts of gamma rays. Gamma-ray bursts (GRBs) are the brightest electromagnetic events known to occur in the universe and can last from ten milliseconds to several minutes. The mechanism of how these GRBs are produced is still a mystery.

    In the laboratory, jets of electron-positron pairs can be generated by shining intense laser light into a gold foil. The interaction produces high-energy radiation that will traverse the material and create electron-positron pairs as it interacts with the nucleus of the gold atoms. The ability to create a large number of positrons in a laboratory, by using energetic lasers, opens the door to several new avenues of antimatter research, including the understanding of the physics underlying extreme astrophysical phenomena such as black holes and gamma-ray bursts.

    “The goal of our experiments was to understand how the flux of electron-positron pairs produced scales with laser energy,” said Chen, who along with former Lawrence Fellow Frederico Fiuza (now at SLAC National Accelerator Laboratory), co-authored the article appearing in the May 18 edition of Physical Review Letters.

    “We have identified the dominant physics associated with the scaling of positron yield with laser and target parameters, and we can now look at its implication for using it to study the physics relevant to gamma-ray bursts,” Chen said. “The favorable scaling of electron-positron pairs with laser energy obtained in our experiments suggests that, at a laser intensity and pulse duration comparable to what is available, near-future 10-kilojoule-class lasers would provide 100 times higher antimatter yield.”

    The team used these scaling results obtained experimentally together with first-principles simulations to model the interaction of two electron positron pairs for future laser parameters. “Our simulations show that with upcoming laser systems, we can study how these energetic pairs of matter-antimatter convert their energy into radiation,” Fiuza said. “Confirming these predictions in an experiment would be extremely exciting.”

    Antimatter research could reveal why more matter than antimatter survived the Big Bang at the start of the universe. There is considerable speculation as to why the observable universe is apparently almost entirely matter, whether other places are almost entirely antimatter, and what might be possible if antimatter could be harnessed. Normal matter and antimatter are thought to have been in balance in the very early universe, but due to an “asymmetry” the antimatter decayed or was annihilated, and today very little antimatter is seen.

    In future work, the researchers plan to use the National Ignition Facility [NIF] to conduct laser antimatter experiments to study the physics of relativistic pair shocks in gamma-ray bursts by creating even higher fluxes of electron-positron pairs.

    LLNL NIF
    NIF

    The research was funded by LLNL’s Laboratory Directed Research and Development program and the LLNL Lawrence Fellowship.

    Chen and Fiuza were joined by Anthony Link, Andy Hazi, Matt Hill, David Hoarty, Steve James, Shaun Kerr, David Meyerhofer, Jason Myatt, Jaebum Park, Yasuhiko Sentoku and Jackson Williams from LLNL, AWE, University of Alberta, University of Rochester and University of Nevada, Reno.

    See the full article here.

    Please help promote STEM in your local schools.

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    LLNL Campus

    Operated by Lawrence Livermore National Security, LLC, for the Department of Energy’s National Nuclear Security
    Administration
    DOE Seal
    NNSA

     
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