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  • richardmitnick 9:45 pm on July 31, 2014 Permalink | Reply
    Tags: , , , , NASA Fermi,   

    From SLAC Lab: “Despite Extensive Analysis, Fermi Bubbles Defy Explanation” 


    SLAC Lab

    July 31, 2014

    Scientists from Stanford and the Department of Energy’s SLAC National Accelerator Laboratory have analyzed more than four years of data from NASA’s Fermi Gamma-ray Space Telescope, along with data from other experiments, to create the most detailed portrait yet of two towering bubbles that stretch tens of thousands of light-years above and below our galaxy.

    https://www6.slac.stanford.edu/sites/www6.slac.stanford.edu/files/styles/lightbox_large_image/public/images/Fermi_bubbles-ST.jpg
    This artist’s representation shows the Fermi bubbles towering above and below the galaxy. (NASA’s Goddard Space Flight Center)

    NASA Fermi Telescope
    NASA/Fermi

    The bubbles, which shine most brightly in energetic gamma rays, were discovered almost four years ago by a team of Harvard astrophysicists led by Douglas Finkbeiner who combed through data from Fermi’s main instrument, the Large Area Telescope.

    NASA Fermi LAT Large Area Telescope
    NASA/Fermi LAT

    The new portrait, described in a paper that has been accepted for publication in The Astrophysical Journal, reveals several puzzling features, said Dmitry Malyshev, a postdoctoral researcher at the Kavli Institute for Particle Astrophysics and Cosmology who co-led on the analysis.

    For example, the outlines of the bubbles are quite sharp, and the bubbles themselves glow in nearly uniform gamma rays over their colossal surfaces, like two 30,000-light-year-tall incandescent bulbs screwed into the center of the galaxy.

    Their size is another puzzle. The farthest reaches of the Fermi bubbles boast some of the highest energy gamma rays, but there’s no discernable cause for them that far from the galaxy.

    Finally, although the parts of the bubbles closest to the galactic plane shine in microwaves as well as gamma rays, about two-thirds of the way out the microwaves fade and only gamma rays are detectable. Not only is this different from other galactic bubbles, but it makes the researchers’ work that much more challenging, said Malyshev’s co-lead, KIPAC postdoctoral researcher Anna Franckowiak.

    two
    KIPAC researchers Dmitry Malyshev (left) and Anna Franckowiak with the magazine issues that contain the articles about the Fermi bubbles they co-authored for the general public. Malyshev’s is in the July 2014 issue of Scientific American, while Franckowiak’s article is in the July 2014 issue of Physics Today. (SLAC National Accelerator Laboratory)

    “Since the Fermi bubbles have no known counterparts in other wavelengths in areas high above the galactic plane, all we have to go on for clues are the gamma rays themselves,” she said.

    What Blew The Bubbles?

    Soon after the initial discovery theorists jumped in, offering several explanations for the bubbles’ origins. For example, they could have been created by huge jets of accelerated matter blasting out from the supermassive black hole at the center of our galaxy. Or they could have been formed by a population of giant stars, born from the plentiful gas surrounding the black hole, all exploding as supernovae at roughly the same time.

    “There are several models that explain them, but none of the models is perfect,” Malyshev said. “The bubbles are rather mysterious.”

    Creating the portrait wasn’t easy.

    “It’s very tricky to model,” said Franckowiak. “We had to remove all the foreground gamma-ray emissions from the data before we could clearly see the bubbles.”

    From the vantage point of most Earth-bound telescopes, all but the highest-energy gamma rays are completely screened out by our atmosphere. It wasn’t until the era of orbiting gamma-ray observatories like Fermi that scientists discovered how common extra-terrestrial gamma rays really are. Pulsars, supermassive black holes in other galaxies and supernovae are all gamma rays point sources, like distant stars are point sources of visible light, and all those gamma rays had to be scrubbed from the Fermi data. Hardest to remove were the galactic diffuse emissions, a gamma ray fog that fills the galaxy from cosmic rays interacting with interstellar particles.

    “Subtracting all those contributions didn’t subtract the bubbles,” Franckowiak said. “The bubbles do exist and their properties are robust.” In other words, the bubbles don’t disappear when other gamma-ray sources are pulled out of the Fermi data – in fact, they stand out quite clearly.

    Franckowiak says more data is necessary before they can narrow down the origin of the bubbles any further.

    “What would be very interesting would be to get a better view of them closer to the galactic center,” she said, “but the galactic gamma ray emissions are so bright we’d need to get a lot better at being able to subtract them.”

    Fermi is continuing to gather the data Franckowiak wants, but for now, both researchers said, there are a lot of open questions.

    See the full article here.

    SLAC Campus
    SLAC is a multi-program laboratory exploring frontier questions in photon science, astrophysics, particle physics and accelerator research. Located in Menlo Park, California, SLAC is operated by Stanford University for the DOE’s Office of Science.
    i1


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  • richardmitnick 4:10 pm on July 31, 2014 Permalink | Reply
    Tags: , , , , , NASA Fermi   

    From NASA/Fermi: “NASA’s Fermi Space Telescope Reveals New Source of Gamma Rays” 

    NASA Fermi

    July 31, 2014
    No Writer Credit

    Observations by NASA’s Fermi Gamma-ray Space Telescope of several stellar eruptions, called novae, firmly establish these relatively common outbursts almost always produce gamma rays, the most energetic form of light.

    novae
    These images show Fermi data centered on each of the four gamma-ray novae observed by the LAT. Colors indicate the number of detected gamma rays with energies greater than 100 million electron volts (blue indicates lowest, yellow highest). Image Credit: NASA/DOE/Fermi LAT Collaboration

    “There’s a saying that one is a fluke, two is a coincidence, and three is a class, and we’re now at four novae and counting with Fermi,” said Teddy Cheung, an astrophysicist at the Naval Research Laboratory in Washington, and the lead author of a paper reporting the findings in the Aug. 1 edition of the journal Science.

    A nova is a sudden, short-lived brightening of an otherwise inconspicuous star caused by a thermonuclear explosion on the surface of a white dwarf, a compact star not much larger than Earth. Each nova explosion releases up to 100,000 times the annual energy output of our sun. Prior to Fermi, no one suspected these outbursts were capable of producing high-energy gamma rays, emission with energy levels millions of times greater than visible light and usually associated with far more powerful cosmic blasts.

    Fermi’s Large Area Telescope (LAT) scored its first nova detection, dubbed V407 Cygni, in March 2010. The outburst came from a rare type of star system in which a white dwarf interacts with a red giant, a star more than a hundred times the size of our sun. Other members of the same unusual class of stellar system have been observed “going nova” every few decades.

    NASA Fermi LAT Large Area Telescope
    NASA/Fermi LAT

    image
    The white dwarf star in V407 Cygni, shown here in an artist’s concept, went nova in 2010. Scientists think the outburst primarily emitted gamma rays (magenta) as the blast wave plowed through the gas-rich environment near the system’s red giant star. Image Credit: NASA’s Goddard Space Flight Center/S. Wiessinger

    In 2012 and 2013, the LAT detected three so-called classical novae which occur in more common binaries where a white dwarf and a sun-like star orbit each other every few hours.

    “We initially thought of V407 Cygni as a special case because the red giant’s atmosphere is essentially leaking into space, producing a gaseous environment that interacts with the explosion’s blast wave,” said co-author Steven Shore, a professor of astrophysics at the University of Pisa in Italy. “But this can’t explain more recent Fermi detections because none of those systems possess red giants.”

    Fermi detected the classical novae V339 Delphini in August 2013 and V1324 Scorpii in June 2012, following their discovery in visible light. In addition, on June 22, 2012, the LAT discovered a transient gamma-ray source about 20 degrees from the sun. More than a month later, when the sun had moved farther away, astronomers looking in visible light discovered a fading nova from V959 Monocerotis at the same position.

    Astronomers estimate that between 20 and 50 novae occur each year in our galaxy. Most go undetected, their visible light obscured by intervening dust and their gamma rays dimmed by distance. All of the gamma-ray novae found so far lie between 9,000 and 15,000 light-years away, relatively nearby given the size of our galaxy.

    Novae occur because a stream of gas flowing from the companion star piles up into a layer on the white dwarf’s surface. Over time — tens of thousands of years, in the case of classical novae, and several decades for a system like V407 Cygni — this deepening layer reaches a flash point. Its hydrogen begins to undergo nuclear fusion, triggering a runaway reaction that detonates the accumulated gas. The white dwarf itself remains intact.

    shock
    Novae typically originate in binary systems containing sun-like stars, as shown in this artist’s rendering. A nova in a system like this likely produces gamma rays (magenta) through collisions among multiple shock waves in the rapidly expanding shell of debris. Image Credit: NASA’s Goddard Space Flight Center/S. Wiessinger

    One explanation for the gamma-ray emission is that the blast creates multiple shock waves that expand into space at slightly different speeds. Faster shocks could interact with slower ones, accelerating particles to near the speed of light. These particles ultimately could produce gamma rays.

    “This colliding-shock process must also have been at work in V407 Cygni, but there is no clear evidence for it,” said co-author Pierre Jean, a professor of astrophysics at the University of Toulouse in France. This is likely because gamma rays emitted through this process were overwhelmed by those produced as the shock wave interacted with the red giant and its surroundings, the scientists conclude.

    See the full article here.

    The Fermi Gamma-ray Space Telescope , formerly referred to as the Gamma-ray Large Area Space Telescope (GLAST), is a space observatory being used to perform gamma-ray astronomy observations from low Earth orbit. Its main instrument is the Large Area Telescope (LAT), with which astronomers mostly intend to perform an all-sky survey studying astrophysical and cosmological phenomena such as active galactic nuclei, pulsars, other high-energy sources and dark matter. Another instrument aboard Fermi, the Gamma-ray Burst Monitor (GBM; formerly GLAST Burst Monitor), is being used to study gamma-ray bursts. The mission is a joint venture of NASA, the United States Department of Energy, and government agencies in France, Germany, Italy, Japan, and Sweden.


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  • richardmitnick 6:59 pm on July 22, 2014 Permalink | Reply
    Tags: , , , NASA Fermi   

    From NASA/Fermi: “NASA’s Fermi Finds A ‘Transformer’ Pulsar” 

    NASA Fermi

    July 22, 2014
    Francis Reddy, NASA’s Goddard Space Flight Center, Greenbelt, Maryland

    In late June 2013, an exceptional binary containing a rapidly spinning neutron star underwent a dramatic change in behavior never before observed. The pulsar’s radio beacon vanished, while at the same time the system brightened fivefold in gamma rays, the most powerful form of light, according to measurements by NASA’s Fermi Gamma-ray Space Telescope.

    split
    These artist’s renderings show one model of pulsar J1023 before (top) and after (bottom) its radio beacon (green) vanished. Normally, the pulsar’s wind staves off the companion’s gas stream. When the stream surges, an accretion disk forms and gamma-ray particle jets (magenta) obscure the radio beam. Image Credit: NASA’s Goddard Space Flight Center

    “It’s almost as if someone flipped a switch, morphing the system from a lower-energy state to a higher-energy one,” said Benjamin Stappers, an astrophysicist at the University of Manchester, England, who led an international effort to understand this striking transformation. “The change appears to reflect an erratic interaction between the pulsar and its companion, one that allows us an opportunity to explore a rare transitional phase in the life of this binary.”

    A binary consists of two stars orbiting around their common center of mass. This system, known as AY Sextantis, is located about 4,400 light-years away in the constellation Sextans. It pairs a 1.7-millisecond pulsar named PSR J1023+0038 – J1023 for short — with a star containing about one-fifth the mass of the sun. The stars complete an orbit in only 4.8 hours, which places them so close together that the pulsar will gradually evaporate its companion.

    When a massive star collapses and explodes as a supernova, its crushed core may survive as a compact remnant called a neutron star or pulsar, an object squeezing more mass than the sun’s into a sphere no larger than Washington, D.C. Young isolated neutron stars rotate tens of times each second and generate beams of radio, visible light, X-rays and gamma rays that astronomers observe as pulses whenever the beams sweep past Earth. Pulsars also generate powerful outflows, or “winds,” of high-energy particles moving near the speed of light. The power for all this comes from the pulsar’s rapidly spinning magnetic field, and over time, as the pulsars wind down, these emissions fade.

    More than 30 years ago, astronomers discovered another type of pulsar revolving in 10 milliseconds or less, reaching rotational speeds up to 43,000 rpm. While young pulsars usually appear in isolation, more than half of millisecond pulsars occur in binary systems, which suggested an explanation for their rapid spin.

    “Astronomers have long suspected millisecond pulsars were spun up through the transfer and accumulation of matter from their companion stars, so we often refer to them as recycled pulsars,” explained Anne Archibald, a postdoctoral researcher at the Netherlands Institute for Radio Astronomy (ASTRON) in Dwingeloo who discovered J1023 in 2007.

    During the initial mass-transfer stage, the system would qualify as a low-mass X-ray binary, with a slower-spinning neutron star emitting X-ray pulses as hot gas raced toward its surface. A billion years later, when the flow of matter comes to a halt, the system would be classified as a spun-up millisecond pulsar with radio emissions powered by a rapidly rotating magnetic field.

    To better understand J1023’s spin and orbital evolution, the system was regularly monitored in radio using the Lovell Telescope in the United Kingdom and the Westerbork Synthesis Radio Telescope in the Netherlands. These observations revealed that the pulsar’s radio signal had turned off and prompted the search for an associated change in its gamma-ray properties.

    Lovell
    Lovell at Jodrell Bank

    westerbrook
    Westerbork

    A few months before this, astronomers found a much more distant system that flipped between radio and X-ray states in a matter of weeks. Located in M28, a globular star cluster about 19,000 light-years away, a pulsar known as PSR J1824-2452I underwent an X-ray outburst in March and April 2013. As the X-ray emission dimmed in early May, the pulsar’s radio beam emerged.

    While J1023 reached much higher energies and is considerably closer, both binaries are otherwise quite similar. What’s happening, astronomers say, are the last sputtering throes of the spin-up process for these pulsars.

    In J1023, the stars are close enough that a stream of gas flows from the sun-like star toward the pulsar. The pulsar’s rapid rotation and intense magnetic field are responsible for both the radio beam and its powerful pulsar wind. When the radio beam is detectable, the pulsar wind holds back the companion’s gas stream, preventing it from approaching too closely. But now and then the stream surges, pushing its way closer to the pulsar and establishing an accretion disk.

    Gas in the disk becomes compressed and heated, reaching temperatures hot enough to emit X-rays. Next, material along the inner edge of the disk quickly loses orbital energy and descends toward the pulsar. When it falls to an altitude of about 50 miles (80 km), processes involved in creating the radio beam are either shut down or, more likely, obscured.

    The inner edge of the disk probably fluctuates considerably at this altitude. Some of it may become accelerated outward at nearly the speed of light, forming dual particle jets firing in opposite directions — a phenomenon more typically associated with accreting black holes. Shock waves within and along the periphery of these jets are a likely source of the bright gamma-ray emission detected by Fermi.

    The findings were published in the July 20 edition of The Astrophysical Journal. The team reports that J1023 is the first example of a transient, compact, low-mass gamma-ray binary ever seen. The researchers anticipate that the system will serve as a unique laboratory for understanding how millisecond pulsars form and for studying the details of how accretion takes place on neutron stars.

    “So far, Fermi has increased the number of known gamma-ray pulsars by about 20 times and doubled the number of millisecond pulsars within in our galaxy,” said Julie McEnery, the project scientist for the mission at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “Fermi continues to be an amazing engine for pulsar discoveries.”

    See the full article here.

    The Fermi Gamma-ray Space Telescope , formerly referred to as the Gamma-ray Large Area Space Telescope (GLAST), is a space observatory being used to perform gamma-ray astronomy observations from low Earth orbit. Its main instrument is the Large Area Telescope (LAT), with which astronomers mostly intend to perform an all-sky survey studying astrophysical and cosmological phenomena such as active galactic nuclei, pulsars, other high-energy sources and dark matter. Another instrument aboard Fermi, the Gamma-ray Burst Monitor (GBM; formerly GLAST Burst Monitor), is being used to study gamma-ray bursts. The mission is a joint venture of NASA, the United States Department of Energy, and government agencies in France, Germany, Italy, Japan, and Sweden.


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  • richardmitnick 4:04 am on June 4, 2014 Permalink | Reply
    Tags: , , , , NASA Fermi   

    From NASA/FERMI: “Black Hole ‘Batteries’ Keep Blazars Going and Going” 


    Fermi

    June 3, 2014
    No Writer Credit

    Astronomers studying two classes of black-hole-powered galaxies monitored by NASA’s Fermi Gamma-ray Space Telescope have found evidence that they represent different sides of the same cosmic coin. By unraveling how these objects, called blazars, are distributed throughout the universe, the scientists suggest that apparently distinctive properties defining each class more likely reflect a change in the way the galaxies extract energy from their central black holes.

    blazar
    Typical look of a blazar

    “We can think of one blazar class as a gas-guzzling car and the other as an energy-efficient electric vehicle,” said lead researcher Marco Ajello, an astrophysicist at Clemson University in South Carolina. “Our results suggest that we’re actually seeing hybrids, which tap into the energy of their black holes in different ways as they age.”

    Active galaxies possess extraordinarily luminous cores powered by black holes containing millions or even billions of times the mass of the sun. As gas falls toward these supermassive black holes, it settles into an accretion disk and heats up. Near the brink of the black hole, through processes not yet well understood, some of the gas blasts out of the disk in jets moving in opposite directions at nearly the speed of light.

    smbh
    An artist’s conception of a supermassive black hole and accretion disk.

    Blazars are the highest-energy type of active galaxy and emit light across the spectrum, from radio to gamma rays. They make up more than half of the discrete gamma-ray sources cataloged by Fermi’s Large Area Telescope, which has detected more than 1,000 to date. Astronomers think blazars appear so intense because they happen to tip our way, bringing one jet nearly into our line of sight. Looking almost directly down the barrel of a particle jet moving near the speed of light, emissions from the jet and the region producing it dominate our view.

    To be considered a blazar, an active galaxy must show either rapid changes in visible light on timescales as short as a few days, strong optical polarization, or glow brightly at radio wavelengths with a “flat spectrum” — that is, one exhibiting relatively little change in brightness among neighboring frequencies.

    Astronomers have identified two models in the blazar line. One, known as flat-spectrum radio quasars (FSRQs), show strong emission from an active accretion disk, much higher luminosities, smaller black hole masses and lower particle acceleration in the jets. The other, called BL Lacs, are totally dominated by the jet emission, with the jet particles reaching much higher energy and the accretion disk emission either weak or absent.

    disc
    Ring Around a Suspected Black Hole in Galaxy NGC 4261.
    Image taken by Hubble space telescope of what may be gas accreting onto a black hole in elliptical galaxy NGC 4261

    Speaking at the American Astronomical Society meeting in Boston on Tuesday, Ajello said he and his team wanted to probe how the distribution of these objects changed over the course of cosmic history, but solid distance information for large numbers of gamma-ray-producing BL Lac objects was hard to come by.

    “One of our most important tools for determining distance is the movement of spectral lines toward redder wavelengths as we look deeper into the cosmos,” explained team member Dario Gasparrini, an astronomer at the Italian Space Agency’s Science Data Center in Rome. “The weak disk emission from BL Lacs makes it extremely difficult to measure their redshift and therefore to establish a distance.”

    So the team undertook an extensive program of optical observations to measure the redshifts of BL Lac objects detected by Fermi.

    “This project has taken several years and simply wouldn’t have been possible without the extensive use of many ground-based observatories by our colleagues,” said team member Roger Romani, an astrophysicist at the Kavli Institute for Particle Astrophysics and Cosmology, a facility run jointly by Stanford University and the SLAC National Accelerator Laboratory in Menlo Park, California.

    The redshift survey included 25 nights on the Hobby-Eberly Telescope at McDonald Observatory in Texas, led by Romani; eight nights on the 200-inch telescope at Palomar Observatory and nine nights on the 10-meter Keck Telescope in Hawaii, led by Anthony Readhead at Caltech in Pasadena, California; and nine nights on telescopes at the European Southern Observatory in Chile, led by Garret Cotter at the University of Oxford in England. In addition, important observations were provided by the Chile-based GROND camera, led by Jochen Greiner at the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, and the Ultraviolet/Optical Telescope on NASA’s Swift satellite, led by Neil Gehrels at Goddard Space Flight Center in Greenbelt, Maryland.

    Hobby-Eberly Telescope
    Hobby-Eberly Telescope

    Caltech Palomar Observatory
    Palomar

    Keck Observatory
    Keck

    With distances for about 200 BL Lacs in hand — the largest and most comprehensive sample available to date — the astronomers could compare their distribution across cosmic time with a similar sample of FSRQs. What emerged suggests that, starting around 5.6 billion years ago, FSRQs began to decline while BL Lacs underwent a steady increase in numbers. The rise is particularly noticeable among BL Lacs with the most extreme energies, which are known as high-synchrotron-peaked blazars based on a particular type of emission.

    “What we think we’re seeing here is a changeover from one style of extracting energy from the central black hole to another,” adds Romani.

    Large galaxies grew out of collisions and mergers with many smaller galaxies, and this process occurs with greater frequency as we look back in time. These collisions provided plentiful gas to the growing galaxy and kept the gas stirred up so it could more easily reach the central black hole, where it piled up into a vast, hot, and bright accretion disk like those seen in “gas-guzzling” FSRQs. Some of the gas near the hole powers a jet while the rest falls in and gradually increases the black hole’s spin.

    As the universe expands and the density of galaxies decreases, so do galaxy collisions and the fresh supply of gas they provide to the black hole. The accretion disk becomes depleted over time, but what’s left is orbiting a faster-spinning and more massive black hole. These properties allow BL Lac objects to maintain a powerful jet even though relatively meager amounts of material are spiraling toward the black hole.

    In effect, the energy of accretion from the galaxy’s days as an FSRQ becomes stored in the increasing rotation and mass of its black hole, which acts much like a battery. When the gas-rich accretion disk all but disappears, the blazar taps into the black hole’s stored energy that, despite a lower accretion rate, allows it to continue operating its particle jet and producing high-energy emissions as a BL Lac object.

    One observational consequence of the hybrid blazar notion is that the luminosity of BL Lacs should decrease over time as the black hole loses energy and spins down.

    The astronomers say they are eager to test this idea with larger blazar samples provided in part by Fermi’s continuing all-sky survey. Understanding the details of this transition also will require better knowledge of the jet, the black hole mass and the galaxy environment for both blazar classes.

    See the full article here.


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  • richardmitnick 1:18 pm on April 3, 2014 Permalink | Reply
    Tags: , , NASA Fermi, , ,   

    From Symmetry: “Possible hints of dark matter in Fermi data” 

    April 03, 2014
    No Writer Credit

    A new study of gamma-ray light from the center of our galaxy makes the strongest case to date that some of this emission may arise from dark matter, an unknown substance making up most of the material universe.

    Using publicly available data from NASA’s Fermi Gamma-ray Space Telescope, independent scientists at the Fermi National Accelerator Laboratory, the Harvard-Smithsonian Center for Astrophysics, the Massachusetts Institute of Technology and the University of Chicago have developed new maps showing that the galactic center produces more high-energy gamma rays than can be explained by known sources and that this excess emission is consistent with some forms of dark matter.

    NASA Fermi Telescope
    Fermi

    “The new maps allow us to analyze the excess and test whether more conventional explanations, such as the presence of undiscovered pulsars or cosmic-ray collisions on gas clouds, can account for it,” says Dan Hooper, an astrophysicist at Fermilab and a lead author of the study. “The signal we find cannot be explained by currently proposed alternatives and is in close agreement with the predictions of very simple dark matter models.”

    gc

    The galactic center (shown above) teems with gamma-ray sources, from interacting binary systems and isolated pulsars to supernova remnants and particles colliding with interstellar gas. It’s also where astronomers expect to find the galaxy’s highest density of dark matter, which only affects normal matter and radiation through its gravity. Large amounts of dark matter attract normal matter, forming a foundation upon which visible structures, like galaxies, are built.

    No one knows the true nature of dark matter, but WIMPs, or Weakly Interacting Massive Particles, represent a leading class of candidates. Theorists have envisioned a wide range of WIMP types, some of which may either mutually annihilate or produce an intermediate, quickly decaying particle when they collide. Both of these pathways end with the production of gamma rays—the most energetic form of light—at energies within the detection range of Fermi’s Large Area Telescope, or LAT.

    NASA Fermi Large Area Telescope
    LAT

    When astronomers carefully subtract all known gamma-ray sources from LAT observations of the galactic center, a patch of leftover emission remains (shown above, on right). This excess appears most prominent at energies between 1 and 3 billion electron volts—roughly a billion times greater than that of visible light—and extends outward at least 5000 light-years from the galactic center.

    Hooper and his colleagues conclude that annihilations of dark matter particles with a mass between 31 and 40 GeV provide a remarkable fit for the excess based on its gamma-ray spectrum, its symmetry around the galactic center, and its overall brightness. Writing in a paper submitted to the journal Physical Review D, the researchers say that these features are difficult to reconcile with other explanations proposed so far, although they note that plausible alternatives not requiring dark matter may yet materialize.

    “Dark matter in this mass range can be probed by direct detection and by the Large Hadron Collider, so if this is dark matter, we’re already learning about its interactions from the lack of detection so far,” says co-author Tracy Slatyer, a theoretical physicist at MIT in Cambridge, Massachusetts. “This is a very exciting signal, and while the case is not yet closed, in the future we might well look back and say this was where we saw dark matter annihilation for the first time.”

    The researchers caution that it will take multiple sightings—in other astronomical objects, the LHC or in some of the direct-detection experiments now being conducted around the world—to validate their dark matter interpretation.

    “Our case is very much a process-of-elimination argument. We made a list, scratched off things that didn’t work, and ended up with dark matter,” says co-author Douglas Finkbeiner, a professor of astronomy and physics at the CfA, also in Cambridge.

    “This study is an example of innovative techniques applied to Fermi data by the science community,” says Peter Michelson, a professor of physics at Stanford University in California and the LAT principal investigator. “The Fermi LAT Collaboration continues to examine the extraordinarily complex central region of the galaxy, but until this study is complete we can neither confirm nor refute this interesting analysis.”

    While the great amount of dark matter expected at the galactic center should produce a strong signal, competition from many other gamma-ray sources complicates any case for a detection. But turning the problem on its head provides another way to attack it. Instead of looking at the largest nearby collection of dark matter, look where the signal has fewer challenges.

    Dwarf galaxies orbiting the Milky Way lack other types of gamma-ray emitters and contain large amounts of dark matter for their size – in fact, they’re the most dark-matter-dominated sources known. But there’s a tradeoff. Because they lie much farther away and contain much less total dark matter than the center of the Milky Way, dwarf galaxies produce a much weaker signal and require many years of observations to establish a secure detection.

    For the past four years, the LAT team has been searching dwarf galaxies for hints of dark matter. The published results of these studies have set stringent limits on the mass ranges and interaction rates for many proposed WIMPs, even eliminating some models. In the study’s most recent results, published in Physical Review D on February 11, the Fermi team took note of a small but provocative gamma-ray excess.

    “There’s about a one-in-12 chance that what we’re seeing in the dwarf galaxies is not even a signal at all, just a fluctuation in the gamma-ray background,” explained Elliott Bloom, a member of the LAT Collaboration at the Kavli Institute for Particle Astrophysics and Cosmology, jointly located at the SLAC National Accelerator Laboratory and Stanford University. If it’s real, the signal should grow stronger as Fermi acquires additional years of observations and as wide-field astronomical surveys discover new dwarfs. “If we ultimately see a significant signal,” he added, “it could be a very strong confirmation of the dark matter signal claimed in the galactic center.”

    See the full article here.

    Symmetry is a joint Fermilab/SLAC publication.



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  • richardmitnick 1:03 pm on March 21, 2014 Permalink | Reply
    Tags: , , , , , , NASA Fermi   

    From Fermilab: “If it looks like dark matter and acts like dark matter …” 


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

    Friday, March 21, 2014
    Dan Hooper

    Are we seeing dark matter in the gamma-ray sky? It sure looks that way.

    Since early in the mission of the Fermi Gamma-ray Space Telescope, a number of scientists have noticed an interesting and fairly bright signal coming from the direction of the Galactic Center. Lisa Goodenough, then of New York University, and I wrote the first couple of papers on this observation in 2009 and 2010. What especially captured our attention was that the spectrum and spatial shape of this signal seemed to match what had been predicted to come from annihilating dark matter particles — an intriguing hint indeed.

    NASA Fermi Telescope
    NASA Fermi Gamma Ray Telescope

    The motivation for using gamma-ray telescopes to look for dark matter is simple. In many (if not most) theories of dark matter, when pairs of dark matter particles interact, they can annihilate each other, producing other kinds of energetic particles in their place. Given the large densities of dark matter that are present around the Galactic Center, dark matter particles are expected to annihilate there at a high rate, producing large fluxes of energetic gamma rays.

    In our new analysis, we reduced background contamination by making use of only the best-reconstructed events. We also performed a large number of tests and cross checks, many of which had not been carried out before. We examined multiple variations in our background model and looked for anything that might masquerade as a signal. What we found was remarkable: The signal from the Galactic Center was not only robust and statistically significant, but in every respect we could measure, it looked like annihilating dark matter.

    The resemblance was astonishing. First, the shape of the observed gamma ray spectrum is in excellent agreement with what we would expect from dark matter particles with a mass of about 35 GeV. Second, the spatial distribution of the photons looks very much like what we calculate based on numerical simulations, approximately spherically symmetric and falling off rapidly with distance from the Galactic Center. And third, the overall brightness of the gamma-ray signal implies a dark matter annihilation cross section (times relative velocity) of about 2×10-26 cm3/s, which is almost exactly the value predicted for a generic dark matter species that was produced in the big bang.

    Although one can never be completely certain in science, and future observations and analysis related to this signal will be very important, this gamma-ray signal does look remarkably like annihilating dark matter. If so, it would represent the first detection of dark matter particles. It is an exciting time to be hunting for dark matter.

    Learn more about the finding in our new paper, which describes in more detail our updated analysis of the Fermi telescope’s gamma-ray data.

    See the full article here.

    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.


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  • richardmitnick 10:18 pm on February 21, 2014 Permalink | Reply
    Tags: , , , , NASA Fermi,   

    From Symmetry: “‘Black widow’ pulsars consume their mates” 

    February 21, 2014
    With a deadly embrace, ‘spidery’ pulsars devour their partners. One such pulsar is the first rapidly spinning black widow to be discovered using only gamma rays.

    No Writer Credit

    Black widow spiders and their Australian cousins, known as redbacks, are notorious for killing and devouring their partners. Astronomers have noted similar behavior among two rare breeds of binary system that contain rapidly spinning neutron stars, also known as pulsars, and have named them accordingly.

    “The essential features of black widow and redback binaries are that they place a normal but very low-mass star in close proximity to a [rapidly spinning] pulsar, which has disastrous consequences for the star,” says Roger Romani, a member of the Kavli Institute for Particle Astrophysics and Cosmology, an institute run jointly by Stanford and SLAC National Accelerator Laboratory.

    So far, astronomers have found at least 18 black widows and nine redbacks within the Milky Way, and additional members of each class have been discovered within the dense globular star clusters that orbit our galaxy. The main difference between the two is that black widow systems contain stars that are both physically smaller and of much lower mass than those found in redbacks.

    bw
    Courtesy of NASA’s Goddard Space Flight Center

    ‘Spider’ pulsars

    When a massive star explodes as a supernova, the crushed core it leaves behind—a neutron star—squeezes more mass than the sun into a ball no larger than Washington, DC.

    Young, an isolated neutron stars rotate a few thousand times per minute and emit beams of radio, visible light, X-rays and gamma rays. They also generate powerful outflows, or “winds,” of high-energy particles. The power for all this derives from the neutron star’s rapidly spinning magnetic field. Over time, as solitary pulsars wind down, their emissions fade.

    Thirty-two years ago, astronomers discovered a new, much faster class of pulsars. These neutron stars spin at astonishing speeds, up to 43,000 revolutions per minute. Today, more than 300 of these so-called millisecond pulsars have been cataloged.

    While young pulsars usually appear in isolation, more than half of millisecond pulsars have a stellar partner, suggesting that interactions with a normal star can make neutron stars spin faster. But how did isolated millisecond pulsars get their kick?

    Enter black widows and their kin.

    “The high-energy emission and wind from the pulsar basically heats and blows off the normal star’s material and, over millions to billions of years, can eat away the entire star,” says Alice Harding, an astrophysicist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “These systems can completely consume their companion stars, and that’s how we think solitary millisecond pulsars form.”

    For astronomers, an exciting aspect of the black widow and redback systems is the opportunity to observe how the stellar companion intercepts energy from the pulsar. In effect, the star serves as a vanity mirror, showing the pulsar’s emissions in tremendous detail.

    J1311

    The Fermi Gamma-ray Space Telescope, which orbits the Earth, excels at locating millisecond pulsars, with more than four dozen found to date. Pulsars stand out to Fermi as prominent gamma-ray sources, but searching for their pulsations in Fermi data is extraordinarily difficult without knowing more about the system. Follow-up surveys with radio telescopes are usually the first to pick up actual pulses, providing confirmation that the object is indeed a pulsar. By narrowing down the timing and other parameters, radio studies also enable Fermi scientists to also tease out the gamma-ray pulses from Fermi data.

    NASA Fermi Telescope
    NASA/Fermi

    When Romani began investigating a source of pulses found by Fermi now known as PSR J1311-3430 (J1311, for short), he imaged the system in visible light. This revealed a faint star that changed color from an intense blue to a dull red—hot and cold, for stars—every hour and a half. Romani conjectured that the star was orbiting and being dramatically heated by a compact object, most likely a pulsar, and suggested that the system was a new black widow.

    His measurements indicate that the side of the star facing the pulsar is heated to more than 21,000 degrees Fahrenheit, more than twice as hot as the sun’s surface. The cool red side reveals the true color of the pipsqueak star, glowing at a temperature of 5000 Fahrenheit or lower. From these temperatures, the scientists estimate that the companion is between 12 and 17 times the mass of Jupiter.

    Holger Pletsch at the Albert Einstein Institute in Hannover, Germany, led an international team on an effort to comb through four years of Fermi LAT data in a search for gamma-ray pulses from J1311. The orbital information established by Romani’s work significantly narrowed the search, but the unknown pulsar parameters still left 100 million billion combinations to explore. Nevertheless, armed with a new, more efficient method, they detected a millisecond pulsar that rotates 390 times a second—more than 23,000 rpm.

    J1311 is the first millisecond pulsar ever detected using only gamma rays.

    J1311 and other black widow and redback binaries offer unique natural laboratories for studying pulsars up close through the disastrous effects on their partners, which are distorted by the neutron star’s tidal pull, inflamed by its gamma rays, pummeled with particles accelerated to near the speed of light, and ultimately evaporated in a breakup of cosmic proportions.

    See the full article here.

    NASA video from Goddard added.

    Symmetry is a joint Fermilab/SLAC publication.



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  • richardmitnick 6:34 am on February 6, 2014 Permalink | Reply
    Tags: , , , , NASA Fermi,   

    From Symmetry: “Astrophysicists use lens to study black-hole jet” 

    January 08, 2014
    No Writer Credit

    Using an entire galaxy as a lens to look at an object in the far distance, researchers are learning more about powerful jets emitted when matter falls into a black hole.

    Using an astronomer’s trick, scientists analyzing data from the Fermi Gamma-ray Space Telescope are learning about how powerful jets of light are produced by matter spiraling into a black hole.

    trick
    Courtesy of NASA/Goddard Space Flight Center Conceptual Image Lab

    NASA Fermi Telescope
    Fermi Gamma Ray Telescope

    As light from a distant object—such as matter falling into a black hole—travels through the universe, it sometimes encounters a massive object such as a large galaxy. Due to the nature of gravity and space, when this happens, the light rays bend—or “gravitationally lens”—around the galaxy. The result is a lensing that amplifies the original light, revealing more information than could be gathered from the same light rays traveling a straight path.

    The Fermi telescope measurements, the first to use this type of gravitational lensing with gamma-ray light, could help provide new insights into how powerful black-hole jets work and could even help researchers better measure universe’s rate of expansion.

    “We began thinking about the possibility of making this observation a couple of years after Fermi launched, and all of the pieces finally came together in late 2012,” said Teddy Cheung in a NASA press release issued this week. Cheung is the lead scientist for the finding, which was made by researchers at the Kavli Institute for Particle Astrophysics and Cosmology at SLAC in California, the Naval Research Laboratory in Washington DC, NASA Goddard Space Flight Center in Maryland, INAF Istituto di Radioastronomia in Italy and 25 other institutions around the globe.

    In September 2012, Fermi’s Large Area Telescope detected a series of bright gamma-ray flares from a source known as B0218+357, located 4.35 billion light-years from Earth in the direction of a constellation called Triangulum. These powerful flares, in a known gravitational lens system, provided the key to making the lens measurement.

    Astronomers classify B0218+357 as a blazar—a type of active galaxy noted for its intense emissions and unpredictable behavior. At the blazar’s heart is a supersized black hole with a mass millions to billions of times that of the sun. As matter spirals toward the black hole, some of it blasts outward as jets of particles traveling near the speed of light.

    Long before the light from one of these jets reaches us, it passes directly through a face-on spiral galaxy—one very much like our own—about 4 billion light-years away.

    The galaxy’s gravity bends the light into different paths, so astronomers see the background blazar as dual images. With just a third of an arcsecond (less than 0.0001 degree) between them, the B0218+357 images hold the record for the smallest separation of any lensed system known.

    While radio and optical telescopes can resolve and monitor the individual blazar images, Fermi’s LAT cannot. Instead, the Fermi team exploited a “delayed playback” effect.

    “One light path is slightly longer than the other, so when we detect flares in one image we can try to catch them days later when they replay in the other image,” said team member Jeff Scargle, an astrophysicist at NASA’s Ames Research Center.

    In September 2012, when the blazar’s flaring activity made it the brightest gamma-ray source outside of our own galaxy, Cheung realized it was a golden opportunity. He was granted a week of LAT target-of-opportunity observing time, from Sept. 24 to Oct. 1, to hunt for delayed flares.

    At the American Astronomical Society meeting in National Harbor, Md., Cheung said the team had identified three episodes of flares showing playback delays of 11.46 days.

    Intriguingly, the gamma-ray delay is about a day longer than radio observations report for this system. And while the flares and their playback show similar gamma-ray brightness, in radio wavelengths one blazar image is about four times brighter than the other.

    Astronomers don’t think the gamma rays arise from the same regions as the radio waves, so these emissions likely take slightly different paths, with correspondingly different delays and amplifications, as they travel through the lens.

    “Over the course of a day, one of these flares can brighten the blazar by 10 times in gamma rays but only 10 percent in visible light and radio, which tells us that the region emitting gamma rays is very small compared to those emitting at lower energies,” said team member Stefan Larsson, an astrophysicist at Stockholm University in Sweden.

    As a result, the gravity of small concentrations of matter in the lensing galaxy may deflect and amplify gamma rays more significantly than lower-energy light. Disentangling these so-called microlensing effects poses a challenge to taking further advantage of high-energy lens observations.

    The scientists say that comparing radio and gamma-ray observations of additional lens systems could help provide new insights into the workings of powerful black-hole jets and establish new constraints on important cosmological quantities like the Hubble constant, which describes the universe’s rate of expansion.

    The most exciting result, the team said, would be the LAT’s detection of a playback delay in a flaring gamma-ray source not yet identified as a gravitational lens in other wavelengths.

    A paper describing the research will appear in a future edition of The Astrophysical Journal Letters.

    See the full article here.

    Symmetry is a joint Fermilab/SLAC publication.



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  • richardmitnick 5:00 pm on January 13, 2014 Permalink | Reply
    Tags: , , , , NASA Fermi, ,   

    From Symmetry: “Scientists pinpoint ‘very peculiar’ pulsar” 

    Scientists studying five years of data from the Fermi Gamma-ray Space Telescope have found the first gamma-ray variable pulsar. But is it really what it seems?

    NASA Fermi Telescope
    NASA/Fermi

    January 13, 2014
    Lori Ann White

    Astrophysicists studying the gamma-ray sky have gone back over five years of survey data from the Fermi Gamma-ray Space Telescope and discovered something new: a pulsar that varies in the amount of gamma-ray radiation it emits.

    gamma
    Courtesy of NASA/DOE/Fermi LAT Collaboration

    Pulsars have a reputation as the cosmic versions of lighthouses: These neutron stars emit beams of electromagnetic energy. The beams sweep across the sky with the pulsar’s rotations, like beacons sweeping across space. Astrophysicists have known for some time, though, that the clockwork precision of pulsars is an illusion; not only do they rotate more and more slowly as they lose energy over millennia, the amount of energy they emit in radio waves and X-rays at any one time can change too.

    But the knowledge that pulsars also vary in gamma rays is a bit surprising, says Fermi scientist Luigi Tibaldo. “The X-rays and radio waves emitted by pulsars are generated through different processes than the gamma waves,” he says. When the Energetic Gamma Ray Experiment Telescope, or EGRET, the predecessor to Fermi’s main instrument, the Large Area Telescope, saw no signs of variability, astrophysicists considered the stability of pulsars in gamma rays to be an axiom—”No, more than an axiom. A motivated conclusion. We just didn’t think they varied,” Tibaldo says.

    The pulsar, known as PSR J2021+4026, is one of the first previously unknown pulsars found by Fermi after its launch in 2008. It resides in the Gamma Cygni region—the very heart of the Swan constellation. At first, it seemed like a perfectly ordinary pulsar. Yet as Tibaldo and his colleague Massimiliano Razzano reviewed the data, they saw hints of a small, steady increase in gamma rays from the beginning of Fermi’s mission in 2008 until mid-October 2011. Then, in less than a week, gamma-ray energies dropped by almost 20 percent, while the pulsar’s rotational speed got slower and slower.

    This was “very peculiar,” Tibaldo says. A pulsar slows down because it emits energy. “Think about it,” he continues. If the rotational speed continues to drop, “the pulsar should be emitting more energy, but we weren’t seeing it. Where was the energy going?”

    Their theory, which Tibaldo presented last month at the 27th Texas Symposium on Relativistic Astrophysics, is that a violent upheaval in the intense magnetic fields surrounding the pulsar caused the variability.

    “Here we are dealing with magnetic fields trillions of times more intense than Earth’s magnetic field,” says Razzano, who presented their results at the American Astronomical Society meeting in Washington, DC, earlier this month. “Fields of that strength are currently impossible to reproduce in the laboratory.”

    These magnetic fields essentially aim the pulsar’s energy beams, and, says Tibaldo, “the field lines broke and reconfigured in less than a week. This moved the beam slightly off our line of sight. It only looks like the gamma ray flux went down because we’re no longer directly in its path.”

    In other words, researchers can only see pulsars whose beams sweep across Earth. If the pulsar’s beams were to redirect away from Earth, the signal would drop—just what the Fermi telescope observed.

    PSR J2021+4026 remains the only known pulsar that’s variable in gamma rays. With a sample set of one, Tibaldo can’t say if his team’s theory is correct or not. “Having more cases is important to understanding what’s going on,” he says.

    As the Fermi mission continues, Tibaldo and Razzano—working with a team at laboratories and universities including SLAC National Accelerator Laboratory, the University of Pisa and Italy’s National Institute of Nuclear Physics—hope to gather data on additional gamma-ray-variable pulsars. With these, they may be able to determine if this new type of pulsars really stands apart from the rest, or if the only difference is in perspective.

    This work recently appeared in The Astrophysical Journal Letters.

    See the full article here.

    Symmetry is a joint Fermilab/SLAC publication.



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  • richardmitnick 8:19 pm on January 6, 2014 Permalink | Reply
    Tags: , , , , NASA Fermi   

    From NASA/Fermi: “NASA’s Fermi Makes First Gamma-ray Study of a Gravitational Lens” 


    Fermi

    Jan. 6, 2014
    Rachel Hoover
    Ames Research Center, Moffett Field, Calif.
    650-930-6149
    rachel.hoover@nasa.gov

    J.D. Harrington
    Headquarters, Washington
    202-358-5241
    j.d.harrington@nasa.gov

    Lynn Chandler
    Goddard Space Flight Center, Greenbelt, Md.
    301-286-2806
    lynn.chandler-1@nasa.gov

    An international team of astronomers, using NASA’s Fermi observatory, has made the first-ever gamma-ray measurements of a gravitational lens, a kind of natural telescope formed when a rare cosmic alignment allows the gravity of a massive object to bend and amplify light from a more distant source.

    gl
    In the heart of an active galaxy, matter falling toward a supermassive black hole creates jets of particles traveling near the speed of light. For active galaxies classified as blazars, one of these jets beams almost directly toward Earth.
    Image Credit: NASA/Goddard Space Flight Center Conceptual Image Lab

    This accomplishment opens new avenues for research, including a novel way to probe emission regions near supermassive black holes. It may even be possible to find other gravitational lenses with data from the Fermi Gamma-ray Space Telescope.

    “We began thinking about the possibility of making this observation a couple of years after Fermi launched, and all of the pieces finally came together in late 2012,” said Teddy Cheung, lead scientist for the finding and an astrophysicist at the Naval Research Laboratory in Washington.

    In September 2012, Fermi’s Large Area Telescope (LAT) detected a series of bright gamma-ray flares from a source known as B0218+357, located 4.35 billion light-years from Earth in the direction of a constellation called Triangulum. These powerful flares, in a known gravitational lens system, provided the key to making the lens measurement.

    Astronomers classify B0218+357 as a blazar — a type of active galaxy noted for its intense emissions and unpredictable behavior. At the blazar’s heart is a supersized black hole with a mass millions to billions of times that of the sun. As matter spirals toward the black hole, some of it blasts outward as jets of particles traveling near the speed of light in opposite directions.

    The extreme brightness and variability of blazars result from a chance orientation that brings one jet almost directly in line with Earth. Astronomers effectively look down the barrel of the jet, which greatly enhances its apparent emission.

    Long before light from B0218+357 reaches us, it passes directly through a face-on spiral galaxy — one very much like our own — about 4 billion light-years away.

    The galaxy’s gravity bends the light into different paths, so astronomers see the background blazar as dual images. With just a third of an arcsecond (less than 0.0001 degree) between them, the B0218+357 images hold the record for the smallest separation of any lensed system known.

    While radio and optical telescopes can resolve and monitor the individual blazar images, Fermi’s LAT cannot. Instead, the Fermi team exploited a “delayed playback” effect.

    “One light path is slightly longer than the other, so when we detect flares in one image we can try to catch them days later when they replay in the other image,” said team member Jeff Scargle, an astrophysicist at NASA’s Ames Research Center in Moffett Field, Calif.

    In September 2012, when the blazar’s flaring activity made it the brightest gamma-ray source outside of our own galaxy, Cheung realized it was a golden opportunity. He was granted a week of LAT target-of-opportunity observing time, from Sept. 24 to Oct. 1, to hunt for delayed flares.

    At the American Astronomical Society meeting in National Harbor, Md., Cheung said the team had identified three episodes of flares showing playback delays of 11.46 days, with the strongest evidence found in a sequence of flares captured during the week-long LAT observations.

    Intriguingly, the gamma-ray delay is about a day longer than radio observations report for this system. And while the flares and their playback show similar gamma-ray brightness, in radio wavelengths one blazar image is about four times brighter than the other.

    Astronomers don’t think the gamma rays arise from the same regions as the radio waves, so these emissions likely take slightly different paths, with correspondingly different delays and amplifications, as they travel through the lens.

    “Over the course of a day, one of these flares can brighten the blazar by 10 times in gamma rays but only 10 percent in visible light and radio, which tells us that the region emitting gamma rays is very small compared to those emitting at lower energies,” said team member Stefan Larsson, an astrophysicist at Stockholm University in Sweden.

    As a result, the gravity of small concentrations of matter in the lensing galaxy may deflect and amplify gamma rays more significantly than lower-energy light. Disentangling these so-called microlensing effects poses a challenge to taking further advantage of high-energy lens observations.

    The scientists say that comparing radio and gamma-ray observations of additional lens systems could help provide new insights into the workings of powerful black-hole jets and establish new constraints on important cosmological quantities like the Hubble constant, which describes the universe’s rate of expansion.

    The most exciting result, the team said, would be the LAT’s detection of a playback delay in a flaring gamma-ray source not yet identified as a gravitational lens in other wavelengths.

    A paper describing the research will appear in a future edition of The Astrophysical Journal Letters.

    See the full article here.


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