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  • richardmitnick 11:40 am on May 9, 2016 Permalink | Reply
    Tags: , , Gamma Rays,   

    From New Scientist: “Water telescope’s first sky map shows flickering black holes” 


    New Scientist

    18 April 2016 [just appeared on social media]

    Lisa Grossman

    HAWC High Altitude Cherenkov Experiment, HAWC Collaboration
    HAWC High Altitude Cherenkov Gamma-Ray Collaboration, HAWC Observatory, Sierra Negra volcano near Puebla, Mexico

    Twinkle, twinkle, little black hole. The High Altitude Water Cherenkov observatory has released its first map of the sky, including the first measurements of how often black holes flicker on and off. It has also caught pulsars, supernova remnants, and other bizarre cosmic beasts.

    “This is our deepest look at two-thirds of the sky, as well as the highest energy photons we’ve ever seen from any source,” says Brenda Dingus of Los Alamos National Laboratory, who presented the map at the American Physical Society meeting in Salt Lake City, Utah on 18 April. “We’re at the high energy frontier.”

    HAWC has been operating from the top of a mountain in central Mexico for about a year, and has caught some of the highest-energy photons ever observed. It is sensitive to gamma rays between 0.1 and 100 teraelectronvolts (TeV) in energy – more than 7 times higher energy than the particles produced in the Large Hadron Collider. The most energetic photon they’ve picked up so far is 60 TeV.

    HAWC’s map of the gamma ray sky. HAWC Collaboration

    But this is no normal telescope. “HAWC doesn’t look or work like any other observatory,” Dingus says. The detector is made up of 300 water tanks filled with 200,000 litres of purified water each (see main image, above). When high-energy particles go through the water, they emit a blue light called Cherenkov radiation. Physicists can use that light to reconstruct where the particles came from.

    HAWC doesn’t observe the extremely high-energy photons directly. They are blocked by our atmosphere – luckily for us, as they can damage living tissue. Instead the detector catches the spray of secondary particles that gamma rays produce when they strike the atmosphere, called air showers.

    “20,000 air shower particles per second hit our detector,” Dingus says. “In fact they’re hitting us right now.”

    In the first year of data, HAWC picked up 40 distinct sources of gamma rays, 10 of which had not been seen in gamma rays before. The team is now working to figure out if they were associated with any other known objects that have been seen in other wavelengths like visible or infrared light.

    One, for instance, was associated with a known supernova remnant from an energetic pulsar, says Michelle Hui of NASA’s Marshall Spaceflight Center in Huntsville, Alabama. When massive stars die as supernovas, they slough off material in a cloud called a supernova remnant. The shock wave from the explosion then sweeps through the cloud and accelerates particles in it to extremely high energies, where they radiate gamma rays.

    Another source is a known pulsar 26,000 light years away. A nearby third is still being identified, and might be related to the supernova remnant.

    Three new sources of gamma rays spotted by HAWC. HAWC Collaboration

    Flickering black holes

    HAWC can also pick up gamma rays from galaxies outside the Milky Way, the sources of which are much more mysterious. We think they are caused by the black holes at galactic centres, but the details of how the photons gain so much energy are murky.

    Because it is watching 24 hours a day, the detector can pick up changes in gamma ray brightness more reliably than ever before. Just 10 days ago, HAWC spotted a flare in a galaxy called Markarian 501.

    “On April 5 we didn’t see it, on April 6 it got very bright, and by April 8 it had nearly disappeared again,” said Robert Lauer of the University of New Mexico. The team put out an announcement on the Astronomer’s Telegram network to alert other observatories to follow up in different wavelengths, although it has not had any responses yet.

    Previous telescopes that could catch such energetic photons could only look at one part of the sky at once, so they couldn’t measure the frequency of these flares. Over the next five years, HAWC will be able to make the first measurements of how often they happen. This level of flaring seems to happen about 5 to 10 times per year, Lauer says, but it seems to vary from galaxy to galaxy.

    HAWC will also be able to see such a flare from the black hole at the centre of our own galaxy.

    “We know these things happen, so we expect them to happen here,” Hui says. “We just don’t know how often.”

    See the full article here .

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  • richardmitnick 7:27 pm on April 18, 2016 Permalink | Reply
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    From phys.org: “HAWC Gamma-ray Observatory reveals new look at the very-high-energy sky” 


    April 18, 2016

    View of two-thirds of the entire sky with very-high-energy gamma rays observed by HAWC. Many sources are clearly visible in our own Milky Way galaxy, as well as two other galaxies: Markarian 421 and Markarian 501. Some well-known constellations are shown as a reference. The center of the Milky Way is located toward Sagittarius. Credit: HAWC Collaboration

    The United States and Mexico constructed the High Altitude Water Cherenkov (HAWC) Gamma-ray Observatory to observe some of the most energetic phenomena in the known universe—the aftermath when massive stars die, glowing clouds of electrons around rapidly spinning neutron stars, and supermassive black holes devouring matter and spitting out powerful jets of particles. These violent explosions produce high-energy gamma rays and cosmic rays, which can travel large distances—making it possible to see objects and events far outside our own galaxy.

    HAWC High Altitude Cherenkov Experiment
    HAWC High Altitude Cherenkov Experiment

    Today, scientists operating HAWC released a new survey of the sky made from the highest energy gamma rays ever observed. The new sky map, which uses data collected since the observatory began running at full capacity last March, offers a deeper understanding of high-energy processes taking place in our galaxy and beyond.

    “HAWC gives us a new way to see the high-energy sky,” said Jordan Goodman, professor of physics at the University of Maryland, and U.S. lead investigator and spokesperson for the HAWC collaboration. “This new data from HAWC shows the galaxy in unprecedented detail, revealing new high-energy sources and previously unseen details about existing sources.”

    HAWC researchers presented the new observation data and sky map April 18, 2016, at the American Physical Society meeting. They also participated in a press conference at the meeting.

    The new sky map shows many new gamma ray sources within our own Milky Way galaxy. Because HAWC observes 24 hours per day and year-round with a wide field-of-view and large area, the observatory boasts a higher energy reach especially for extended objects. In addition, HAWC can uniquely monitor for gamma ray flares by sources in our galaxy and other active galaxies, such as Markarian 421 and Markarian 501.

    HAWC observations show that a previously known gamma ray source in the Milky Way galaxy, TeV J1930+188, which is probably due to a pulsar wind nebula, is far more complicated than originally thought. Where researchers previously identified a single gamma ray source, HAWC identified several hot spots. Credit: HAWC Collaboration

    One of HAWC’s new observations provides a better understanding of the high-energy nature of the Cygnus region—a northern constellation lying on the plane of the Milky Way. A multitude of neutron stars and supernova remnants call this star nursery home. HAWC scientists observed previously unknown objects in the Cygnus region and identified objects discovered earlier with sharper resolution.

    In a region of the Milky Way where researchers previously identified a single gamma ray source named TeV J1930+188, HAWC identified several hot spots, indicating that the region is more complicated than previously thought.

    “Studying these objects at the highest energies can reveal the mechanism by which they produce gamma rays and possibly help us unravel the hundred-year-old mystery of the origin of high-energy cosmic rays that bombard Earth from space,” said Goodman.

    HAWC—located 13,500 feet above sea level on the slopes of Mexico’s Volcán Sierra Negra—contains 300 detector tanks, each holding 50,000 gallons of ultrapure water with four light sensors anchored to the floor. When gamma rays or cosmic rays reach Earth’s atmosphere they set off a cascade of charged particles, and when these particles reach the water in HAWC’s detectors, they produce a cone-shaped flash of light known as Cherenkov radiation. The effect is much like a sonic boom produced by a supersonic jet, because the particles are traveling slightly faster than the speed of light in water when they enter the detectors.

    The light sensors record each flash of Cherenkov radiation inside the detector tanks. By comparing nanosecond differences in arrival times at each light sensor, scientists can reconstruct the angle of travel for each particle cascade. The intensity of the light indicates the primary particle’s energy, and the pattern of detector hits can distinguish between gamma rays and cosmic rays. With 300 detectors spread over an area equivalent to more than three football fields, HAWC “sees” these events in relatively high resolution.

    “Unlike traditional telescopes, with HAWC we have now an instrument that surveys two-thirds of the sky at the highest energies, day and night,” said Andrés Sandoval, Mexico spokesperson for HAWC.

    HAWC exhibits 15-times greater sensitivity than its predecessor—an observatory known as Milagro that operated near Los Alamos, New Mexico, and ceased taking data in 2008. In eight years of operation, Milagro found new sources of high-energy gamma rays, detected diffuse gamma rays from the Milky Way galaxy and discovered that the cosmic rays hitting earth had an unexpected non-uniformity.

    “HAWC will collect more data in the next few years, allowing us to reach even higher energies,” said Goodman. “Combining HAWC observations with data from other instruments will allow us to extend the reach of our understanding of the most violent processes in the universe.”

    See the full article here .

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  • richardmitnick 6:29 pm on April 18, 2016 Permalink | Reply
    Tags: , , Gamma Rays, ,   

    From NASA Fermi: “NASA’s Fermi Telescope Poised to Pin Down Gravitational Wave Sources” 

    NASA Fermi Banner

    NASA/Fermi Telescope
    NASA Fermi

    April 18, 2016
    Francis Reddy
    NASA’s Goddard Space Flight Center, Greenbelt, Maryland

    On Sept. 14, waves of energy traveling for more than a billion years gently rattled space-time in the vicinity of Earth. The disturbance, produced by a pair of merging black holes, was captured by the Laser Interferometer Gravitational-Wave Observatory (LIGO) facilities in Hanford, Washington, and Livingston, Louisiana.

    Black holes merging Swinburne Astronomy Productions
    Black holes merging Swinburne Astronomy Productions

    Cornell SXS team. Two merging black holes simulation
    Cornell SXS team. Two merging black holes simulation

    MIT/Caltech Advanced aLIGO Hanford Washington USA installation
    MIT/Caltech Advanced aLIGO Hanford Washington USA installation

    This event marked the first-ever detection of gravitational waves and opens a new scientific window on how the universe works.

    Less than half a second later, the Gamma-ray Burst Monitor (GBM) on NASA’s Fermi Gamma-ray Space Telescope picked up a brief, weak burst of high-energy light consistent with the same part of the sky. Analysis of this burst suggests just a 0.2-percent chance of simply being random coincidence. Gamma-rays arising from a black hole merger would be a landmark finding because black holes are expected to merge “cleanly,” without producing any sort of light.

    Access mp4 video here . This visualization shows gravitational waves emitted by two black holes (black spheres) of nearly equal mass as they spiral together and merge. Yellow structures near the black holes illustrate the strong curvature of space-time in the region. Orange ripples represent distortions of space-time caused by the rapidly orbiting masses. These distortions spread out and weaken, ultimately becoming gravitational waves (purple). The merger timescale depends on the masses of the black holes. For a system containing black holes with about 30 times the sun’s mass, similar to the one detected by LIGO in 2015, the orbital period at the start of the movie is just 65 milliseconds, with the black holes moving at about 15 percent the speed of light. Space-time distortions radiate away orbital energy and cause the binary to contract quickly. As the two black holes near each other, they merge into a single black hole that settles into its “ringdown” phase, where the final gravitational waves are emitted. For the 2015 LIGO detection, these events played out in little more than a quarter of a second. This simulation was performed on the Pleiades supercomputer at NASA’s Ames Research Center. Credits: NASA/J. Bernard Kelly (Goddard), Chris Henze (Ames) and Tim Sandstrom (CSC Government Solutions LLC)

    “This is a tantalizing discovery with a low chance of being a false alarm, but before we can start rewriting the textbooks we’ll need to see more bursts associated with gravitational waves from black hole mergers,” said Valerie Connaughton, a GBM team member at the National Space, Science and Technology Center in Huntsville, Alabama, and lead author of a paper on the burst now under review by The Astrophysical Journal.

    Detecting light from a gravitational wave source will enable a much deeper understanding of the event. Fermi’s GBM sees the entire sky not blocked by Earth and is sensitive to X-rays and gamma rays with energies between 8,000 and 40 million electron volts (eV). For comparison, the energy of visible light ranges between about 2 and 3 eV.

    NASA Fermi Gamma-ray Space Telescope  Gamma-ray Burst Monitor (GBM)
    NASA Fermi Gamma-ray Space Telescope Gamma-ray Burst Monitor (GBM)

    With its wide energy range and large field of view, the GBM is the premier instrument for detecting light from short gamma-ray bursts (GRBs), which last less than two seconds. They are widely thought to occur when orbiting compact objects, like neutron stars and black holes, spiral inward and crash together. These same systems also are suspected to be prime producers of gravitational waves.

    “With just one joint event, gamma rays and gravitational waves together will tell us exactly what causes a short GRB,” said Lindy Blackburn, a postdoctoral fellow at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, and a member of the LIGO Scientific Collaboration. “There is an incredible synergy between the two observations, with gamma rays revealing details about the source’s energetics and local environment and gravitational waves providing a unique probe of the dynamics leading up to the event.” He will be discussing the burst and how Fermi and LIGO are working together in an invited talk at the American Physical Society meeting in Salt Lake City on Tuesday.

    Currently, gravitational wave observatories possess relatively blurry vision. This will improve in time as more facilities begin operation, but for the September event, dubbed GW150914 after the date, LIGO scientists could only trace the source to an arc of sky spanning an area of about 600 square degrees, comparable to the angular area on Earth occupied by the United States.

    “That’s a pretty big haystack to search when your needle is a short GRB, which can be fast and faint, but that’s what our instrument is designed to do,” said Eric Burns, a GBM team member at the University of Alabama in Huntsville. “A GBM detection allows us to whittle down the LIGO area and substantially shrinks the haystack.”

    Access mp4 video here. Fermi’s GBM saw a fading X-ray flash at nearly the same moment LIGO detected gravitational waves from a black hole merger in 2015. This movie shows how scientists can narrow down the location of the LIGO source on the assumption that the burst is connected to it. In this case, the LIGO search area is reduced by two-thirds. Greater improvements are possible in future detections. Credits: NASA’s Goddard Space Flight Center

    Less than half a second after LIGO detected gravitational waves, the GBM picked up a faint pulse of high-energy X-rays lasting only about a second. The burst effectively occurred beneath Fermi and at a high angle to the GBM detectors, a situation that limited their ability to establish a precise position. Fortunately, Earth blocked a large swath of the burst’s likely location as seen by Fermi at the time, allowing scientists to further narrow down the burst’s position.

    The GBM team calculates less than a 0.2-percent chance random fluctuations would have occurred in such close proximity to the merger. Assuming the events are connected, the GBM localization and Fermi’s view of Earth combine to reduce the LIGO search area by about two-thirds, to 200 square degrees. With a burst better placed for the GBM’s detectors, or one bright enough to be seen by Fermi’s Large Area Telescope, even greater improvements are possible.

    The LIGO event was produced by the merger of two relatively large black holes, each about 30 times the mass of the sun. Binary systems with black holes this big were not expected to be common, and many questions remain about the nature and origin of the system.

    Black hole mergers were not expected to emit significant X-ray or gamma-ray signals because orbiting gas is needed to generate light. Theorists expected any gas around binary black holes would have been swept up long before their final plunge. For this reason, some astronomers view the GBM burst as most likely a coincidence and unrelated to GW150914. Others have developed alternative scenarios where merging black holes could create observable gamma-ray emission. It will take further detections to clarify what really happens when black holes collide.

    Albert Einstein predicted the existence of gravitational waves in his general theory of relativity a century ago, and scientists have been attempting to detect them for 50 years. Einstein pictured these waves as ripples in the fabric of space-time produced by massive, accelerating bodies, such as black holes orbiting each other. Scientists are interested in observing and characterizing these waves to learn more about the sources producing them and about gravity itself.

    For more information about NASA’s Fermi Gamma-ray Space Telescope, please visit:


    See the full article here .

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    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.

  • richardmitnick 4:37 pm on April 18, 2016 Permalink | Reply
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    From AAS NOVA: “Found: A Galaxy’s Missing Gamma Rays” 


    American Astronomical Society

    Recent observations have detected high-energy gamma-ray emission for the first time from the massive, star-forming galaxy Arp 220 (shown here in optical wavelengths, as imaged by Hubble). [NASA/ESA/C. Wilson (McMaster University)]

    NASA/ESA Hubble Telescope
    NASA/ESA Hubble Telescope

    Recent reanalysis of data from the Fermi Gamma-ray Space Telescope has resulted in the first detection of high-energy gamma rays emitted from a nearby galaxy.

    NASA/Fermi Telescope
    NASA/Fermi Telescope

    This discovery reveals more about how supernovae interact with their environments.

    Colliding Supernova Remnant

    Supernova remnant Crab nebula. NASA/ESA Hubble
    Supernova remnant Crab nebula. NASA/ESA Hubble

    After a stellar explosion, the supernova’s ejecta expand, eventually encountering the ambient interstellar medium. According to models, this generates a strong shock, and a fraction of the kinetic energy of the ejecta is transferred into cosmic rays — high-energy radiation composed primarily of protons and atomic nuclei. Much is still unknown about this process, however. One open question is: what fraction of the supernova’s explosion power goes into accelerating these cosmic rays?

    In theory, one way to answer this is by looking for gamma rays.

    Gamma rays from the Fermi Gamma-ray Space Telescope, could be produced by proposed dark matter interactions
    Gamma rays from the Fermi Gamma-ray Space Telescope, could be produced by proposed dark matter interactions

    In a starburst galaxy, the collision of the supernova-accelerated cosmic rays with the dense interstellar medium is predicted to produce high-energy gamma rays. That radiation should then escape the galaxy and be visible to us.

    Pass 8 to the Rescue

    Observational tests of this model, however, have been stumped by Arp 220. This nearby ultraluminous infrared galaxy is the product of a galaxy merger ~700 million years ago that fueled a frenzy of starbirth. Due to its dusty interior and extreme levels of star formation, Arp 220 has long been predicted to emit the gamma rays produced by supernova-accelerated cosmic rays. But though we’ve looked, gamma-ray emission has never been detected from this galaxy … until now.

    In a recent study*, a team of scientists led by Fang-Kun Peng (Nanjing University) reprocessed 7.5 years of Fermi observations using the new Pass 8 analysis software. The resulting increase in resolution revealed the first detection of GeV emission from Arp 220!

    Gamma-ray luminosity vs. total infrared luminosity for LAT-detected star-forming galaxies and Seyferts. Arp 220’s luminosities are consistent with the scaling relation. [Peng et al. 2016]

    Acceleration Efficiency

    Peng and collaborators argue that this emission is due solely to cosmic-ray interactions with interstellar gas. This picture is supported by the lack of variability in the emission, and the fact that Arp 220’s gamma-ray luminosity is consistent with the scaling relation between gamma-ray and infrared luminosity for star-forming galaxies. The authors also argue that, due to Arp 220’s high gas density, all cosmic rays will interact with the gas before escaping.

    Under these two assumptions, Peng and collaborators use the gamma-ray luminosity and the known supernova rate in Arp 220 to estimate how efficiently cosmic rays are accelerated by supernova remnants in the galaxy. They determine that 4.2 ± 2.6% of the supernova remnant’s kinetic energy is used to accelerate cosmic rays above 1 GeV.

    This is the first time such a rate has been measured directly from gamma-ray emission, but it’s consistent with estimates of 3-10% efficiency in the Milky Way. Future analysis of other ultraluminous infrared galaxies like Arp 220 with Fermi (and Pass 8!) will hopefully reveal more about these recent-merger, starburst environments.

    Fang-Kun Peng et al 2016* ApJ 821 L20. doi:10.3847/2041-8205/821/2/L20

    Science paper:

    See the full article here .

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  • richardmitnick 8:39 am on April 14, 2016 Permalink | Reply
    Tags: , Dark Energy/Dark Matter, Gamma Rays,   

    From NOVA: “Dark Matter’s Invisible Hand” 



    13 Apr 2016
    Charles Q. Choi

    Dark matter is currently one of the greatest mysteries in the universe.

    Dark matter cosmic web and the large-scale structure it forms The Millenium Simulation, V. Springel et al
    Dark matter cosmic web and the large-scale structure it forms The Millenium Simulation, V. Springel et al

    It’s thought to be an invisible substance that makes up roughly five-sixths of all matter in the cosmos, a dark fog suffusing the universe that rarely interacts with ordinary matter. But when it does, according to an unexpected finding by theoretical physicist Lisa Randall, the consequences could be momentous.

    Astronomers first detected dark matter through its gravitational pull, which apparently keeps the Milky Way and other galaxies from ripping themselves apart given the speeds at which they spin.

    Scientists have mostly ruled out all known ordinary materials as candidates for dark matter. The current consensus is that dark matter lies outside the Standard Model of particle physics, currently the best description of how all known subatomic particles behave.

    The Standard Model of elementary particles , with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth.
    The Standard Model of elementary particles , with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth.

    Specifically, physicists have suggested that dark matter is composed of new kinds of particles that have very weak interactions—not just with ordinary matter but also with themselves.

    Gamma rays from the Fermi Gamma-ray Space Telescope, could be produced by proposed dark matter interactions
    Gamma rays from the Fermi Gamma-ray Space Telescope, could be produced by proposed dark matter interactions.

    NASA/Fermi Gamma Ray Telescope
    NASA/Fermi Gamma Ray Telescope

    However, Randall and other scientists have suggested that dark matter might interact more strongly with itself than we suspect, experiencing as-yet undetected “dark forces” that would influence dark matter particles alone. Just as electromagnetism can make particles of ordinary matter attract or repel each other and emit and absorb light, so too might “dark electromagnetism” cause similar interactions between dark matter particles and cause them to emit “dark light” that’s invisible to ordinary matter.

    Differentiated Dark Matter

    The evidence for this theory can be seen in potential discrepancies between predictions and observations of the way matter is distributed in the universe on relatively modest scales, such as that of dwarf galaxies, says Randall, a professor at Harvard University.

    Dwarf Galaxies with Messier 101  Allison Merritt  Dragonfly Telephoto Array
    Dwarf Galaxies with Messier 101 Allison Merritt Dragonfly Telephoto Array

    For example, repulsive interactions between dark matter particles might keep these particles apart and reduce their overall density, explaining why current estimates of the density of the innermost portion of galaxies are higher than what is actually seen.

    Most dark matter models suggest that dark matter particles are all of one type—they either all interact with each other or they all do not. However, Randall and her colleagues propose a more complex version that they call “partially-interacting dark matter,” where dark matter has both a non-interacting component and a self-interacting one. A similar example in real particles can be seen with protons, electrons, and neutrons—positively charged protons and negatively charged electrons attract one another, while neutrally charged neutrons are not attracted to either protons or electrons.

    “There’s no reason to think that dark matter is composed of all the same type of particle,” Randall says. “We certainly see a diversity of particles in the one sector of matter we do know about, ordinary matter. Why shouldn’t we think the same of dark matter?”

    In this model, Randall and her colleagues suggest that only a small portion of dark matter—maybe about 5%—experiences interactions reminiscent of those seen in ordinary matter. However, this fraction of dark matter could influence not only the evolution of the Milky Way, but of life on Earth as well, an idea Randall explores in her latest book, Dark Matter and the Dinosaurs: The Astounding Interconnectedness of the Universe.

    Standard dark matter models predict that dwarf galaxies orbiting larger galaxies should be scattered in spherical patterns around their parents. However, astronomical data suggest that many dwarf galaxies orbiting the Milky Way and Andromeda lie roughly in the same plane as each other. Randall and her colleagues suggest that if dark matter particles can interact with each other, they can shed energy, potentially creating a structure that could not only solve this dwarf galaxy mystery, but also have triggered the cosmic disruption that doomed the dinosaurs.

    Dark Disks

    In the partially interacting dark matter scenario, the non-interacting component would still form spherical clouds around galaxies, consistent with what astronomers know of their general structure. However, self-interacting dark matter particles would lose energy and cool as they jostled with each other. Cooling would slow these particles down, and gravity would make them cluster together. If these clouds were relatively immobile, they would simply shrink into smaller balls.

    However, since they likely rotate—just like the rest of the matter in their galaxies—this rotation would make these clouds of self-interacting dark matter collapse into flat disks, in much the same way as spherical clouds of ordinary matter collapsed to form the spiral disks of the Milky Way and many other galaxies. Conservation of angular momentum causes these would-be spheres to flatten out. While cooling would still cause them to collapse vertically, they would not collapse along the same plane as their rotation.

    If dark matter in large galaxies was concentrated in disks, it’s likely that at least some of the orbiting dwarf galaxies would be concentrated in flat planes because of the gravitational pull of dark matter on the dwarf galaxies, Randall and her colleagues say. The researchers suggest these “dark disks” should be embedded in the visible disk of larger galaxies.

    But here’s where dark matter begins to exert its influence. The relationship between the dark disk and the stars in the galaxy is not entirely stable. The Sun, for example, completes a circuit around the Milky Way’s core roughly every 240 million years. During its orbit, it bobs up and down in a wavy motion through the galactic plane about every 32 million years. Coincidentally, some researchers previously suggested that meteor impacts on Earth rise and fall in cycles about 30 million to 35 million years long, leading to regular mass extinctions.

    Earlier researchers proposed a cosmic trigger for this deadly cycle, such as a potential companion star for the Sun dubbed “Nemesis” that would ensnare meteoroids and send them hurling toward Earth. Instead, Randall and her colleagues suggest that the Sun’s regular passage through the Milky Way’s dark disk might have warped the orbits of comets in the outer solar system, flinging them inward. Such disruption may have then led to disastrous cosmic impacts on Earth, including the collision about 67 million years ago that likely caused the Cretaceous-Tertiary extinction event, the most recent and most familiar mass extinction which killed off all dinosaurs (except those which would evolve into birds).

    The main suspect behind this disaster is an impact from an asteroid or comet that left behind a gargantuan crater more than 110 miles wide near the town of Chicxulub in Mexico. The collision, likely caused by a meteor about 6 miles across, would have released as much energy as 100 trillion tons of TNT, more than a billion times more than the atomic bombs that destroyed Hiroshima and Nagasaki, killing off at least 75% of life on Earth.

    In research* detailed in 2014 in the journal Physical Review Letters, Randall and her colleague Matthew Reece analyzed craters that are more than 12.4 miles in diameter and were created in the past 250 million years. When they compared the ages of these craters against the 35-million-year cycle they proposed, they discovered that it was three times more likely that the craters matched the dark matter cycle instead of simply occurring randomly.

    “I want to be clear that I did not set out to explain the extinction of the dinosaurs,” Randall says. “This work was about exploring the story of how our universe came about, to explore one possible connection between many different levels of the universe, from the universe down to the Milky Way, the solar system, Earth, and life on Earth.”

    Geologist Michael Rampino at New York University, who did not participate in this study, finds a potential link between a dark disk and mass extinctions “an interesting idea,” he says. “If true, it ties together events that happened on Earth to large-scale cycles in the rest of the solar system and even the galaxy in general.”

    Dark Life?

    However, not everyone agrees that Randall and her colleagues present a convincing case. “They tie mass extinctions to the cratering record, but there are all kinds of estimates one can make with the cratering record depending on what craters one think makes the cut—do you accept all craters above a certain size or craters out to a certain age?” says astrobiophysicist Adrian Melott at the University of Kansas, who did not take part in this research. “You can get all different kind of answers, from cycles 20 million to 37 million years long.”

    Moreover, other research suggests that the cycle of mass extinctions on Earth is actually roughly 27 million years long, Melott says. “That’s way too short a duration for motion oscillating back and forth through the disk.”

    Randall notes that data from the European Space Agency’s satellite Hipparcos, launched in 1989 to precisely measure the positions and velocities of stars, allowed for the theoretical existence of a dark disk. She adds that ESA’s Gaia mission, launched in 2013 to create a precise 3D map of matter throughout the Milky Way, could reveal or refute the dark disk’s existence.

    ESA/Gaia satellite
    ESA/Gaia satellite

    One intriguing possibility raised by interacting dark matter models is the existence of dark atoms that might have given rise to dark life, neither of which would be easily detected, Randall says. Although she admits that the concept of dark life might be far-fetched, “life is complicated, and we have yet to understand life and what’s necessary for it.”

    *Science paper:
    Dark Matter as a Trigger for Periodic Comet Impacts

    Science team:
    Lisa Randall and Matthew Reece

    Department of Physics, Harvard University, Cambridge, Massachusetts 02138, USA

    See the full article here .

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    NOVA is the highest rated science series on television and the most watched documentary series on public television. It is also one of television’s most acclaimed series, having won every major television award, most of them many times over.

  • richardmitnick 12:23 pm on February 18, 2016 Permalink | Reply
    Tags: , , Gamma Rays, , ,   

    From Penn State: “New clues in the hunt for the sources of cosmic neutrinos” 

    Penn State Bloc

    Pennsylvania State University

    cosmic-ray accelerator hidden
    This illustration is an example of a hidden cosmic-ray accelerator. Cosmic rays are accelerated up to extremely high energies in dense environments close to black holes. High-energy gamma rays (marked by the “Y” gamma symbol) are blocked from escaping, while neutrinos (marked by the “V”nu symbol) easily escape and can reach the Earth. Credit: Bill Saxton at NRAO/AUI/NSF, modified by Kohta Murase at Penn State University

    The sources of the high-energy cosmic neutrinos that are detected by the IceCube Neutrino Observatory buried in the Antarctic ice may be hidden from observations of high-energy gamma rays, new research reveals.

    ICECUBE neutrino detector
    IceCube neutrino detector interior
    U Wisconsin/NSF/ICECUBE

    These high-energy cosmic neutrinos, which are likely to come from beyond our Milky Way Galaxy, may originate in incredibly dense and powerful objects in space that prevent the escape of the high-energy gamma rays that accompany the production of neutrinos. A paper describing the research will be published in the early online edition of the journal Physical Review Letters on February 18, 2016.

    “Neutrinos are one of the fundamental particles that make up our universe,” said Kohta Murase, assistant professor of physics and of astronomy and astrophysics at Penn State and the corresponding author of the studies. “High-energy neutrinos are produced along with gamma rays by extremely high-energy radiation known as cosmic rays in objects like star-forming galaxies, galaxy clusters, supermassive black holes, or gamma-ray bursts [GRB’s]. It is important to reveal the origin of these high-energy cosmic neutrinos in order to better understand the underlying physical mechanisms that produce neutrinos and other extremely high-energy astroparticles and to enable the use of neutrinos as new probes of particle physics in the universe.”

    Neutrinos are neutral particles, so they are not affected by electromagnetic forces as they travel through space. Neutrinos detected here on Earth therefore trace a direct path back to their distant astrophysical sources. Additionally, these neutrinos rarely interact with other kinds of matter — many pass directly through the Earth without interacting with other particles — making them incredibly difficult to detect, but ensuring that they escape the incredibly dense environments in which they are produced.

    The high-energy cosmic neutrinos detected by IceCube are believed to originate from cosmic-ray interactions with matter (proton-proton interactions); from cosmic-ray interactions with radiation (proton-photon interactions); or from the decay or destruction of heavy, invisible dark matter. Because these processes generate both high-energy neutrinos and high-energy gamma rays, the scientists compared the IceCube neutrino data to high-energy gamma rays detected by the Fermi Gamma-ray Space Telescope.

    NASA Fermi Telescope
    Fermi Gamma-ray Space Telescope

    “If all of the high-energy gamma rays are allowed to escape from the sources of neutrinos, we had expected to find corresponding data from IceCube and Fermi,” said Murase. In previous papers, including one that was featured as an Editorial Suggestion in Physical Review Letters in 2015, Murase and his colleagues showed the power of such a “multi-messenger” comparison. Now, the researchers suggest that the new neutrino data collected by IceCube has lead to intriguing contradictions with the gamma-ray data collected by Fermi.

    “Using sophisticated calculations and a detailed comparison of the IceCube data with the gamma-ray data from Fermi has led to new and interesting implications for the sources of high-energy cosmic neutrinos,” said Murase. “Surprisingly, with the latest IceCube data, we don’t see matching high-energy gamma-ray data detected by Fermi, which suggests a ‘hidden accelerator’ origin of high-energy cosmic neutrinos that Fermi has not detected.”

    In order to explain the multi-messenger data without any of the intriguing contradictions, the scientists propose that the high-energy gamma rays must be blocked from escaping the sources that created them. The researchers then asked what kinds of astrophysical events could produce high-energy neutrinos but also could suppress the high-energy gamma rays detectable by Fermi. “Interestingly, we found that the suppression of high-energy gamma rays should naturally occur when neutrinos are produced via proton-photon interactions,” said Murase. The low-energy photons that interact with protons to produce neutrinos in these events simultaneously prevent high-energy gamma rays from escaping via a process called ‘two-photon annihilation.’ The new finding implies that the amount of high-energy gamma rays associated with the neutrinos that reach the Earth can easily be below the level detectable by Fermi.

    According to the researchers, the results imply that high-energy cosmic neutrinos can be used as special probes of dense astrophysical environments that cannot be seen in high-energy gamma rays. Candidate sources include supermassive black holes and certain types of gamma-ray bursts. The results also motivate further theoretical and observational studies, such as the use of lower-energy gamma rays or X rays to help scientists understand the origin of high-energy neutrinos and cosmic rays.

    “The next decade will be a golden era for multi-messenger particle astrophysics with high-energy neutrinos detected in IceCube as well as gravitational waves detected with advanced-LIGO,” said Murase.

    Caltech Ligo
    MIT/Caltech Advanced aLIGO

    “Our work demonstrates that multi-messenger approaches are indeed very powerful tools for probing fundamental questions in particle astrophysics. I believe that the future is bright and that we will be able to find sources of neutrinos and cosmic rays, probably with other surprising new discoveries.”

    In addition to Murase, the research team includes Dafne Guetta from the Osservatorio Astronomic di Roma in Italy and ORT Braude College in Israel, and Markus Ahlers from the University of Wisconsin.

    The research was funded by Penn State University, the U.S. Nation Science Foundation (grant numbers OPP-0236449 and PHY-0236449) and by the U.S. Israel Binational Science Foundation.

    See the full article here.

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    We teach students that the real measure of success is what you do to improve the lives of others, and they learn to be hard-working leaders with a global perspective. We conduct research to improve lives. We add millions to the economy through projects in our state and beyond. We help communities by sharing our faculty expertise and research.

    Penn State lives close by no matter where you are. Our campuses are located from one side of Pennsylvania to the other. Through Penn State World Campus, students can take courses and work toward degrees online from anywhere on the globe that has Internet service.

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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” 


    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” 


    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.


    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 .


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

    See the full article here .

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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


    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

    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

    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” 


    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

    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

    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:


    See the full article here .

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