“Beyond our wildest dreams”: Fermi scope bags 16 gamma-ray-only pulsars

July 6, 2009 | 12:34 pm

This all-sky map shows the positions and names of 16 new pulsars (yellow) and eight millisecond pulsars (magenta) studied using Fermi's LAT. (Image: NASA)

After only one year of operation, the Fermi Gamma-ray Space Telescope has already outperformed researchers’ best expectations. In two papers published in the July 2 edition of Science Express, the researchers reported a new class of pulsar and evidence that helps explain how gamma-ray emission occurs.

The team examined 300 sites of gamma-ray radiation for the study, using data collected by FGST’s Large Area Telescope between August 2008 and January 2009 along with data collected by a predecessor, the Energetic Gamma Ray Experiment Telescope. Within these sites they were able to identify 16 new gamma-ray only pulsars.

The LAT, one of the two main instruments on FGST, was assembled at SLAC National Accelerator Laboratory. The SLAC Instrument Science Operations Center processes all data from the LAT, converting the raw information into a form that scientists can analyze.

“With Fermi we are really opening the window on the possibility of discovering a new class of pulsars, using gamma-ray emissions,” said LAT Analysis Coordinator Nicola Omodei, who is visiting SLAC this year from the physics lab INFN in Pisa. “We basically saw something that no one has seen before.”

Pulsars are fast-spinning neutron stars: super-dense stellar remnants left in the wake of supernovae. As the neutron stars spin, they emit radio waves from their poles that sweep across the universe like giant searchlights. Radio telescopes on Earth or in orbit can detect the pulsars if the beam crosses the Earth’s path.

So far, more than 1800 pulsars have been identified by scanning the galaxy for radio waves. In some cases, though, the radio waves can’t be seen. Fermi first demonstrated this in October 2008, when researchers announced the identification of the first gamma-ray only pulsar.

“Before launch, some predicted that Fermi might uncover a handful of new pulsars during its mission,” University of California, Santa Cruz Astronomer Marcus Ziegler said. “To discover 16 in its first five months of operation is really beyond our wildest dreams.”

The results also provide insight on how gamma-ray emissions arise. Where radio waves emissions are thought to arise from the poles of a neutron star, the results suggest that gamma rays might originate as far as 300 miles from the star’s surface.

Researchers were also able to use the data to closer examine unique objects called millisecond pulsars.

Normally, pulsars lose energy as they spin. As they lose energy, they slow down and stop emitting radiation. But if these aging neutron stars are in close proximity to other stars, something interesting happens: the neutron star’s enormous density causes it to accrete material from its neighbors, increasing its mass and restarting its spin.

For reasons that aren’t yet clear, these rejuvenated pulsars can spin even faster than their younger brethren. They spin so fast, in fact, that they can complete anywhere from 100 to 1000 full rotations every second.

Before the Fermi gamma-ray telescope, the mechanism of how these millisecond pulsars emitted energy was unclear. But comparing the emission spectrum of the millisecond pulsars to those of gamma-ray only pulsars, researchers found that they were nearly identical.

“Before Fermi launched, it wasn’t clear that pulsars with millisecond periods could emit gamma-rays at all,” said Lucas Guillemot at the Center for Nuclear Studies in Gradigna, near Bordeaux, France. “Now we know that they do. It’s also clear that, despite their differences, both normal and millisecond pulsars share similar mechanisms for emitting gamma-rays.”

NASA has a press release here.

Nicholas Bock, SLAC Today

Symmetry Intern

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Happy birthday, Fermi Gamma-ray Space Telescope

June 11, 2009 | 12:56 pm

The Fermi Gamma-ray Space Telescope launched one year ago. (Photo: NASA)

Today marks one year since the Fermi Gamma-ray Space Telescope was launched into orbit. Since then, the telescope has discovered a whole new set of pulsars, gained a new view of cosmic jets, seen the most extreme gamma-ray blasts ever, created new sky maps in gamma-rays, shown that blazars are more complex than previously thought, observed a mysterious excess of high-energy electrons from space that could be from pulsars or possibly a sign of dark matter, and spotted gamma-ray bursts that lasted for half an hour rather than the expected few minutes.

Happy birthday, Fermi Gamma-ray Space Telescope!

David Harris

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Gamma-ray bursts may last longer than previously thought

May 5, 2009 | 10:09 am

Artist's conception of a gamma-ray burst. (Image: NASA.)

Gamma-ray bursts, the most powerful explosions in the universe since the big bang, are thought to last mere seconds or a few short minutes. But new data from the Fermi Gamma-ray Space Telescope show at least some of them have much more staying power.

In March, FGST’s Large Area Telescope, or LAT–an incredibly sensitive gamma-ray and particle detector assembled and operated by SLAC National Accelerator Laboratory–spotted high-energy gamma rays from two separate bursts lasting many minutes after they occurred. Such burst durations have been observed only once before. In 1994, NASA’s EGRET instrument picked up gamma rays 1.5 hours after a blast.

“With only one observation, you never know how often something happens,” said SLAC physicist Roger Blandford, who works on the FGST project. “Now these delayed gamma rays are beginning to look like a common phenomenon.”

Gamma-ray bursts are mysterious. Astronomers have proposed that they occur when massive stars run out of nuclear fuel and collapse into black holes, releasing intense jets of radiation. The collision of two neutron stars orbiting in a binary system is another possible source. FGST’s observations could help scientists tease out the actual cause or causes.

The new data set is “an important constraint on the nature of these explosions,” Blandford said. “The source of bursts such as these must remain active for a relatively long time. This means that certain explanations are not viable.”

Neutron-star collisions may fall into this category, according to Blandford. Scientists believe that when these collapsed stellar remnants smash into each other, it’s all over within a few minutes. Gamma-ray bursts could result, but they wouldn’t last very long.

One potential burst source that would fit FGST’s data: the formation of a black hole with an accretion disc of gas and dust spinning around it. As the black hole’s immense gravity pulls this material in and compresses it, high-energy electromagnetic radiation such as gamma rays could be emitted.

“Accretion discs could provide fuel for an hour or more,” Blandford said.

The new bursts occurred March 23 and March 28. FGST picked them up with the LAT and its other instrument, the Gamma-ray Burst Monitor. The LAT then tracked the blasts long after the initial fireworks ended–the first time the instrument was directed to stare rather than scan in response to an event.

“We got those things right in our sights and stared at them for hours,” said SLAC physicist Jim Chiang, who helped analyze the data. “We knew we had to confirm the EGRET event. We had to chase that down.”

The LAT’s measurements, announced over the weekend at the American Physical Society meeting in Denver, Colorado indicated that the first burst, named GRB090323, probably lasted at least half an hour. The second, GRB090328A, continued for 15 minutes or more.

Finding radiation emanating from a burst for so long is not new. But all previous instruments–with the exception of EGRET–picked up “afterglows” of lower-energy frequencies, such as X-rays and ultraviolet light.

All gamma-ray bursts recorded thus far have occurred very far away, outside our galaxy. The two long-lasting blasts are no different. GRB090323 had a measured redshift of 3.57, which corresponds to a distance of 11.9 billion light years. GRB090328A’s redshift was 0.736, translating to 6.5 billion light years away. That’s just as well: a nearby burst aimed directly at Earth could cause mass extinctions.

These findings are the latest in a growing list of accomplishments for FGST. Since its launch last June, the telescope has already documented the most powerful gamma-ray burst ever seen and picked up an intriguing excess of cosmic electrons which is a possible signal of dark-matter annihilations. FGST will continue to sweep the gamma-ray sky through at least 2013, searching for signs of dark matter and clues about the most extreme events in the universe.

by Michael Wall

Symmetry Intern

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Pulsars or dark matter might be the source of high-energy cosmic electrons

May 2, 2009 | 10:00 am

Something in our galactic neighborhood seems to be producing large numbers of high-energy electrons, according to new data gathered by the Fermi Gamma-ray Space Telescope. The electrons could be coming from nearby pulsars-or they could be a longed-for signal of dark matter, the elusive, invisible material thought to make up nearly a quarter of the universe.

FGST’s Large Area Telescope, a collaboration between NASA, the US Department of Energy, and multiple international partners, has been scanning the skies for gamma rays and particles since its launch last summer. The LAT, which was assembled at the SLAC National Accelerator Laboratory in Menlo Park, California, measured a strikingly high number of electrons with energies between 100 billion and one trillion electronvolts­. It is not known from the LAT data alone if these electrons are coming from the distant background, or are the signal of a nearby source of high-energy particles.

“If these particles were emitted far away, they’d have lost a lot of their energy by the time they reached us,” said LAT collaborator Luca Baldini of the Istituto Nazionale di Fisica Nucleare in Pisa, Italy.

When combined with other recent results, the LAT finding provides compelling evidence that something close by is churning out high-energy particles. The European satellite PAMELA, for example, last fall reported detecting surprisingly large quantities of high-energy positrons, the antimatter counterparts of electrons.

“Between the PAMELA results and our results, it’s very hard to construct a conventional galactic cosmic-ray model” explaining these particle energies, said Elliott Bloom, a SLAC physicist who works on the LAT project. “You need relatively local sources of positrons and electrons.”

These local sources could be pulsars, rapidly rotating neutron stars that emit intense electromagnetic radiation, positrons, and electrons. Alternatively, they could be bits of dark matter annihilating when they crash into each other or decaying because they are unstable. Such annihilations and decays also release high-energy particles, theorists think.

Physicists infer the existence of dark matter-which doesn’t interact with any of the electromagnetic forces, making it invisible to our eyes and our instruments-from its gravitational effects on light and “normal matter” such as stars, planets and interstellar gas. Though studies suggest that dark matter is more than five times as abundant as normal matter, nobody has yet directly measured the strange material or characterized its nature. The LAT team isn’t claiming they have detected dark matter.

“Occam’s Razor says pulsars are the most prosaic, and therefore perhaps most likely, explanation,” Bloom said. “But dark matter is also a possibility. This is particle astrophysics at its most exciting, trying to track down what’s going on here.”

A few other projects have recently mapped the spread of electron energies in space. One, the ATIC collaboration, found an even larger number of high-energy electrons than LAT did. However, the balloon-based ATIC must deal with atmospheric interference, which the orbiting LAT doesn’t have to worry about. And LAT is a remarkably precise instrument.

“This measurement provides the definitive determination of the spectrum of electrons outside of Earth’s atmosphere,” said SLAC physicist Greg Madejski, another LAT team member. Without such an accurate spectrum, suppositions about pulsars, dark matter or any other source of high-energy particles are on shaky ground. The current measurement allows the Fermi LAT team to constrain astrophysical models but the young mission needs to collect further data to say definitively whether or not there is a signal due to dark matter.

The LAT measurements, presented in the opening plenary session at the American Physical Society meeting in Denver, Colorado on May 2 and published in the journal Physical Review Letters, are difficult to make. Luca Latronica of INFN, Pisa, Alex Moiseev of NASA’s Goddard Space Flight Center, and Stefano Profumo of the University of California, Santa Cruz, will present further details of the results and their interpretation on behalf of the Fermi LAT collaboration at the APS meeting on Monday, May 4.

For each electron that hits LAT’s detectors, 50 to 100 other charged particles, mainly protons, come through as well. “It’s like finding a needle in a haystack,” Baldini said. “It requires a lot of simulations, a lot of cross-checking and a lot of study about how electrons behave in the detector.”

The LAT team is currently trying to pin down where exactly the electrons are coming from. The possibilities are that some electrons are coming from local sources, such as pulsars, supernova remnants, or from dark matter particle annihilations. They’re hoping to correlate any significant departures from the background with positions of known pulsars. And the LAT team is extending measurements even further, to energies of a few trillion electronvolts, according to collaborator Igor Moskalenko of Stanford University.

“What we will see at higher energies can only come from local sources,” he said. “If there are cosmic ray sources nearby, we may be able to find them.”

By Mike Wall

Update: The scientific paper has now been published. You can get to it free by going to the viewpoint in APS’ online publication Physics and clicking through free. If you go direct to the paper, you will need to be a subscriber to access it.

Symmetry Intern

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Gamma signature, Astronomer’s Telegram cast light on dazzling blazar

April 30, 2009 | 11:22 am

Fermi Gamma-ray Space Telescope's all-sky gamma map, with Blazar 3C 454.3 labeled in the lower left corner. (Image: NASA/DOE/International LAT collaboration.)

When it comes to watching the skies, two sets of eyes are always better than one, especially if one pair can see, say, radio waves, while the other has X-ray or even gamma-ray vision. The Fermi Gamma-ray Space Telescope’s Large Area Telescope collaboration has recently released a paper giving the gamma-ray perspective on an astronomical object that flared last summer, an active galactic nucleus-or quasar-known as 3C 454.3. The paper, accepted by the Astrophysical Journal and posted yesterday on the ArXiv preprint archive, reveals that the structure of these distant, energetic monsters is more complex than scientists had previously guessed. The paper also hints at a more comprehensive picture to come, next time unfolding in full color, using data from radio, infrared, optical, X-ray, and gamma bands.

A quasar is thought to be fueled by an enormous swirl of gas, or accretion disk, that has gathered around a massive black hole at the center of a distant galaxy. As gas particles stream into the black hole’s maw, protons and electrons near the black hole are propelled outward at close to light speeds in a jet perpendicular to the disk. When this jet outshines the surrounding galaxy, it’s often called a blazar-perhaps one of nature’s most powerful particle accelerators. In a process that’s poorly understood, the jets quake and shudder, shaking the high-energy beams and causing them to radiate.

“We don’t see the particles unless they produce some signature of their presence,” said SLAC National Accelerator Laboratory astrophysicist Greg Madejski, who with Benoit Lott of France’s National Institue of Nuclear and Particle Physics at Bordeaux, coordinated the work on the paper. “It’s like peeling an onion-we look at the radiation and look at what produces the radiation, then that tells us about the content and structure of the jet.”

In the summer of 2008, Blazar 3 C 454.3 flared violently, dazzling the Large Area Telescope, or LAT, with a gamma-ray blaze. Using data collected over the next month, astrophysicists uncovered unexpected complications in the gamma-ray portion of the blazar’s radiation-corresponding to energies between 20 MeV and 300 GeV. So far, gamma-ray observations of blazars have indicated that the amount of radiation emitted decreases as its energy increases. This relationship seems to follow a power law, so that a gamma ray with twice the energy of another is about one fourth as common. The new LAT data show a break in the expected smooth downward decline, with a sudden drop in the amount of radiation at about 2 GeV.

Today's astronomical telegrams are the electronic descendents of physical telegrams like this 1922 circular of the International Astronomical Union heralding the observation of a new comet. (Image: the International Astronomical Union.)

The gamma-ray data allow astrophysicists to make important revisions to the prevailing picture of blazars. Meanwhile, multi-wavelength data promise even deeper insights. As their own telescope tracked the gamma rays, the LAT collaboration leapt to action, alerting astronomers all over the world to the flaring blazar. Their July 24 posting on the Web site The Astronomer’s Telegram triggered an unplanned yet successful multi-wavelength campaign, securing observations in the radio, infrared, optical, ultraviolet and X-ray bands.

“This is really the only way to make any progress,” Madejski said. “It turns out what we really want to find out is who flares first-is it the gamma rays and then the optical and infrared, or vice versa? This tells us what the ‘prime mover’ behind these flares might be.”

For more than a century, astronomical news was sent from one observatory to another in Morse code, via miles of wire. Today’s astronomers use online resources, such as the International Astronomical Union e-mails and The Astronomer’s Telegram, to find the hottest things to watch. According to SLAC astrophysicist Jim Chiang, the LAT collaboration has posted more than 20 telegrams since January. As an all-sky monitor sensitive to gamma rays, the LAT is especially well-poised to launch broadband investigations of events like blazar flares, which are fleeting and tend to glow most brightly in the high-energy part of the spectrum.

“These guys will flare on a timescale of a few days, or they may stay hot. It’s important to trigger these campaigns on a timely basis,” Chiang said. “This is a new thing that we’re able to do with the LAT.”

“Understanding of Fermi data requires collaboration with astronomers and astrophysicists observing in other spectral bands,” Madejski added. “If we invite them to look at unusual celestial phenomena, it will shed light on these enigmatic sources of extremely energetic radiation.”

By Lauren Schenkman

This story first appeared in SLAC Today on April 30, 2009.

Symmetry Intern

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Gamma plus radio equals new view of cosmic jets

April 27, 2009 | 6:13 am

The radio jets of several active galaxies mapped by the Very Long Baseline Array (VLBA) are inset into the Fermi Gamma-ray Space Telescope’s map of the gamma-ray sky. Credit: NASA/DOE/Fermi LAT Collaboration and NRAO/AUI/MOJAVE Team/M. Kadler

The sky map and list of bright sources based on Fermi Gamma-ray Space Telescope’s first three months of operation are already yielding fruit. An international team of researchers participating in the MOJAVE program has correlated radio-wave data with Fermi’s gamma-ray findings to move toward a better understanding of the physics behind the universe’s most energetic objects.

The team used the Very Long Baseline Array, or VLBA, a set of ten radio telescopes operated by the National Radio Astronomy Observatory. Spanning North America from Hawaii in the west to the US Virgin Islands in the east, the telescopes act as one continent-sized dish, collecting radio-wave data on some of the brightest gamma-ray sources the Fermi telescope sees. These are active galactic nuclei, swirls of gas particles around supermassive black holes that spurt two opposing jets of highly energetic particles. The radio data show that nuclei with radio jets pointed straight at Earth are more likely to be detected by Fermi in gamma-ray wavelengths. The researchers also found that the jets from the brightest gamma-ray sources flare in radio waves at the same time.

Jim Chiang, an astrophysicist with Fermi’s Large Area Telescope collaboration at SLAC National Accelerator Laboratory, says the results support a model of active galactic nuclei in which the jets are responsible for both radio waves and gamma rays.

“This confirms that there is a connection between ejection of the blobs seen in radio waves and the production of gamma-ray emission,” Chiang says. “Now we actually have quantitative measurements of how the models should predict these effects. We can test these models concretely in terms of numbers.”

Stanford astrophysicist Peter Michelson, the lead scientist on the LAT, says he is pleased to see Fermi telescope data being used in a multi-wavelength study. Broadband examination of active galactic nuclei, he says, is one of the goals of the Fermi telescope, and critical to unraveling their mysteries.

“The bright-sources list was really intended primarily to inform the rest of the scientific community as to what we’re seeing with Fermi,” he says. That way telescopes working in other wavelengths, from radio waves to X-rays, can study these objects, “and we can correlate the emissions across as much of the electromagnetic spectrum as we can.

“We’re pretty happy with it, and we’ll see a lot more of this in the future,” he adds. “This is just the start, really.”

MOJAVE, which stands for Monitoring Of Jets in Active galactic nuclei with VLBA Experiments, involves astronomers throughout the United States and Europe. The results are reported in two papers in the May 1 issue of the Astrophysical Journal; they can be found here and here. For more information, see the NASA press release.

by Lauren Schenkman

Symmetry Intern

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Around the world in 80 telescopes

April 10, 2009 | 6:52 am

Without leaving your living room, you can get an inside look at the South Pole Telescope, a window to the early universe, that sits within walking distance of geographic bottom of the world, as well as insider views of 79 other renowned research telescopes.

The Web site “Around the World in 80 Telescopes” provides 24-hours of video from the most advanced observatories across the globe and in our skies. The European Southern Observatory originally created the program as a live Web cast April 3-4. 

The 24-hours worth of images has been archived by telescope on the Web site 100 hours of Astronomy, a keystone project of the International Year of Astronomy celebration.Among the highlighted telescopes are The Fermi Gamma-ray Space Telescope, the The Very Large Array (VLA) telescope in New Mexico and the The 10-meter South Pole Telescope/IceCube Neutrino Telescope in Antarctica.While many of the telescopes seek to take images of the stars and planets in our solar system, astrophysics often focus their telescopes radio and light waves to piece together a picture of how the universe looked 13.7 billion years ago and how it has evolved since then.

 A key investigator in this area is a team from nine institutions lead by University of Chicago professors John Carlstrom. They operate the 10-meter South Pole Telescope in a hunting for dark energy, thought to make cause the accelerating expanse of the universe, and evidence about the universe’s origins and past evolution.

The team uses the telescope to search for cosmic microwave background radiation, the afterglow of light left over from the big band and for extremely weak gravity waves, distortions in space and time that Einstein’s theory of general relativity predicts cosmic inflation should produce.

Construction of a new instrument, a  polarimeter,  attached to the telescope is designed to detect the electromagnetic radiation frequency of these gravity waves, found at submillimeter wavelengths, between microwaves and the infrared on the electromagnetic spectrum.

Finding the waves would go a long way to proving the theory of cosmic inflation launching the big bang as well as discounting other theories of how the universe began.

Tona Kunz

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New video of the frenetically twinkling gamma-ray sky

April 6, 2009 | 6:42 am

This map of the northern galactic sky shows Large Area Telescope counts of gamma rays with energies greater than 300 million electron volts. (Image courtesy of NASA/DOE/LAT collaboration. Click on image to view a movie of this swath of sky between August 4 and October 30, 2008.)

The gamma-ray sky is intensely frenetic, twinkling with abandon. And now, thanks to a series of time-lapse movies released Friday by NASA, the US Department of Energy, and the Fermi Gamma-ray Space Telescope’s Large Area Telescope collaboration, you too can enjoy the frenzy.

The movies, which were made public during a live webcast organized as part of the 100 Hours of Astronomy project, compress the space-based telescope’s first 87 days of science data into as little as 29 seconds. On this time scale, the sky is awash in softly blinking gamma-ray sources, which are again and again overwhelmed by the bright firework-like flashes of bright blazars.

Some of the most violent energy sources in the universe, blazars are galaxies that emit jets of particles traveling near the speed of light. In a blazar, one of these jets is oriented directly toward Earth, creating a very strong signal in many wavelengths—including gamma rays.

In one of the just-released movies of the Northern Hemisphere (accessible by clicking on the image at right), a careful examination reveals the sun scooting across the sky on the lower right corner of the frame. Although the sun does not directly emit gamma rays, cosmic rays streaming through the universe continually strike the sun’s gas and light, producing gamma rays. The sun’s position with respect to the background stars on the sky changes by about 1 degree per day as the earth advances in its orbit, leading to the steady progression in this highly compressed movie.

“The moon is just as strong a gamma-ray source as the sun, but moves around the earth many times faster and so is smeared out to invisibility in these daily images,” says Large Area Telescope deputy analysis coordinator Seth Digel, an experimental physicist at the Kavli Institute for Particle Astrophysics and Cosmology.

In the same movie, the faint but constant red speckles around the bottom and left edge of the frame correspond to the plane of the Milky Way galaxy, which is hot in gamma rays. And the two steady bright dots, one near the left edge and one just at the top, are pulsars in the Milky Way. The crushed cores left behind when massive stars explode, pulsars spin rapidly and sweep a lighthouse-like beam across the sky. When this beam is oriented so that it shines on Earth, we observe it to blink on and off as the star spins.

“It’s funny, but we [the Large Area Telescope collaboration] consider pulsars steady sources,” Digel says. “Unlike blazars, they don’t change in brightness, they only pulse.” Because the slowest gamma-ray pulsars flash a few times per second, their on-and-off nature isn’t visible in the highly compressed time of the movie. But in the telescope’s complete data, the flashes are quite clear; in fact, the Large Area Telescope was the first telescope to discern that one of these sources, LAT PSR J1836+5925 (the one on the left edge of the movie), is in fact a pulsar. Previously, it was known as a steady, unidentified gamma-ray object.

See what else you can discover in the Large Area Telescope time-lapse movies, which are all available on the NASA website.

Kelen Tuttle

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Fermi Gamma-ray Space Telescope reveals sky map and top-ten source list

March 11, 2009 | 8:01 am

This view from NASA's Fermi Gamma-ray Space Telescope is the deepest and best-resolved portrait of the gamma-ray sky to date. The image shows how the sky appears at energies more than 150 million times greater than that of visible light. Among the signatures of bright pulsars and active galaxies is something familiar--a faint path traced by the sun. Credit: NASA/DOE/Fermi LAT Collaboration

The Fermi Gamma-ray Space Telescope, a joint mission of NASA, the US Department of Energy, and international partners, today released a three-month sky-map of the gamma-ray sky and a list of the ten most interesting gamma-ray sources they have observed.

NASA’s press release and top-ten list (below) reveal a new perspective on the gamma-ray sky, highlighting interesting objects both within the galaxy and outside.

Here is what NASA has to say:

A new map combining nearly three months of data from NASA’s Fermi Gamma-ray Space Telescope is giving astronomers an unprecedented look at the high-energy cosmos. To Fermi’s eyes, the universe is ablaze with gamma rays from sources ranging from within the solar system to galaxies billions of light-years away.

“Fermi has given us a deeper and better-resolved view of the gamma-ray sky than any previous space mission,” said Peter Michelson, the lead scientist for the spacecraft’s Large Area Telescope (LAT) at Stanford University, Calif. “We’re watching flares from supermassive black holes in distant galaxies and seeing pulsars, high-mass binary systems, and even a globular cluster in our own.”

Read the rest of this entry »

David Harris

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Most extreme gamma-ray blast also probes quantum gravity

February 19, 2009 | 2:01 pm

GRB 080916C's X-ray afterglow appears orange and yellow in this view that merges images from Swift's UltraViolet/Optical and X-ray telescopes. Credit: NASA/Swift/Stefan Immler

A paper published today in Science Express details the most extreme gamma-ray blast ever observed, seen by the Fermi Gamma-ray Space Telescope. While I am sure this will make plenty of news around the world, the aspect I find most interesting isn’t the mere superlative native of the blast, but the science that comes from it.

In particular, the details of the blast place the tightest constraints yet on the constancy of the speed of light at different energies. This is important for the development of theories of quantum gravity, as many flavors of those theories predict that the speed of light does actually change with the frequency/color/wavelength of light. (So far, there has been no experimental evidence to suggest the speed of light changes, but it is possible for there to be small changes in the speed of light, consistent with everything that has ever been observed.)

The gamma rays from the burst came in spread out over 16.5 seconds. This time delay could be due to gamma-rays being emitted from different regions of space, some slightly closer to us than others. However, assuming that the gamma rays were all emitted from the same place at the same time, this spread of arrival times gives a maximum speed difference between the different gamma-ray energies. Doing the most conservative calculation possible shows that any change in the speed of light must be no more than a certain limit, and that limit is the tightest yet measured. (The assumption that the gamma rays come from the same place makes the constraint the most conservative, rather than invalidating this kind of conclusion.)

This movie compresses about 8 minutes of Fermi LAT observations of GRB 080916C into 6 seconds. Colored dots represent gamma rays of different energies. Visible light has energy between about 2 and 3 electron volts (eV). The blue dots represent lower-energy gamma rays (less than 100 million eV); green, moderate energies (100 million to 1 billion eV); and red, the highest energies (more than 1 billion eV). Credit: NASA/DOE/Fermi LAT Collaboration

Interestingly, this technique could be used to search for changes in the speed of light, called Lorentz violation in the argot. If the spread is due to spatial separation of the bursting elements, then the time delay between the start and end of the burst should not depend on the distance of the burst from us. However, if the speed of light varies, then we would expect the time delay to increase for bursts further away. The Fermi Gamma-ray Space Telescope will be in an excellent position to measure many of these bursts and collect precisely this kind of data.

Because the geometric layout of bursts isn’t well understood, nobody will be rushing to suggest that this technique can identify Lorentz violation. However, by doing these studies, the contraints on how much Lorentz violation could possible exist will get tighter and tighter, perhaps tight enough to rule out some theories of quantum gravity.

The press release issued by SLAC National Accelerator Laboratory is as follows:

Most extreme gamma-ray blast ever, seen by Fermi Gamma-ray Space Telescope

With the greatest total energy, the fastest motions, and the highest-energy initial emissions ever before seen, a gamma-ray burst recently observed by the Fermi Gamma-ray Space Telescope is one for the record books. The spectacular blast, which also raises new questions about gamma-ray bursts, was discovered by the FGST’s Large Area Telescope, a collaboration among NASA, the US Department of Energy (DOE) Office of Science and international partners.

“Burst emissions at these energies are still poorly understood, and Fermi is giving us the tools to figure them out,” says Large Area Telescope Principal Investigator Peter Michelson, a Stanford University physics professor affiliated with the Department of Energy’s SLAC National Accelerator Laboratory.

The explosion, designated GRB 080916C, occurred at 7:13 p.m. EDT Sept. 15 (after midnight GMT, Sept. 16) in the constellation Carina. FGST’s other instrument, the Gamma-ray Burst Monitor (GBM), simultaneously recorded the event. Together, the two instruments provide a view of the blast’s gamma-ray emission from energies ranging from 3000 to more than 5 billion times that of visible light.

A team led by Jochen Greiner at the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, established that the blast occurred 12.2 billion light-years away using the Gamma-Ray Burst Optical/Near-Infrared Detector (GROND) on the 2.2-meter telescope at the European Southern Observatory in La Silla, Chile.

“Already, this was an exciting burst,” says Julie McEnery, an FGST deputy project scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “But with the GROND team’s distance, it went from exciting to extraordinary.”

With the distance in hand, FGST team members showed that the blast exceeded the power of nearly 9,000 ordinary supernovae and that the gas bullets emitting the initial gamma rays must have moved at no less than 99.9999 percent the speed of light. This burst’s tremendous power and speed make it the most extreme recorded to date.

The burst is not only spectacular but also enigmatic: a curious time delay separates its highest-energy emissions from its lowest. Such a time lag has been seen clearly in only one earlier burst, and researchers have several explanations for why it may exist.

The environment around a gamma-ray burst is extremely complicated. Although the specifics vary from burst to burst, the surrounding area generally includes the remnants of a stellar explosion, a magnetic field, a black hole and various particles accelerated by the black hole’s gravitational pull, as well as huge amounts of radiation. It is possible that the delays could be explained by the structure of this environment, with the low- and high-energy gamma rays “coming from different parts of the jet or [being] created through a different mechanism,” Michelson says.

Another, far more speculative theory posits that perhaps time lags result not from anything in the environment around the black hole, but from the gamma rays’ long journey from the black hole to our telescopes. If the theorized idea of quantum gravity is correct, then at its smallest scale space is not a smooth medium but a tumultuous, boiling froth of “quantum foam.” Lower-energy (and thus lighter) gamma rays would travel faster through this foam than higher-energy (and thus heavier) gamma rays. Over the course of 12.2 billion light years, this very small effect could add up to a significant delay.

The FGST results provide the strongest test to date of the speed of light’s consistency at these extreme energies. As FGST observes more gamma-ray bursts, researchers can look for time lags that vary with respect to the bursts. If the quantum gravity effect is present, time lags should vary in relation to the distance. If the environment around the burst origin is the cause, the lag should stay relatively constant no matter how far away the burst occurred.

“This one burst raises all sorts of questions,” Michelson says. “In a few years, we’ll have a fairly good sample of bursts, and may have some answers.”

The team’s results appear in the February 19 edition of Science Express.

Gamma-ray bursts are the universe’s most luminous explosions. Astronomers believe most occur when exotic massive stars run out of nuclear fuel. As a star’s core collapses into a black hole, jets of material-powered by processes not yet fully understood-blast outward at nearly the speed of light. The jets bore all the way through the collapsing star and continue into space, where they interact with gas previously shed by the star. This generates bright afterglows that fade with time.

David Harris

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