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Posts Tagged ‘GRB’

The Swift Satellite

Data from the Swift satellite's ultraviolet/optical and x-ray telescopes show one of the most distant gamma-ray bursts ever detected. It's so distant that the star that died to create it exploded when the universe was about 630 million years old. Credit: NASA/Swift/Stefan Immler

In the June edition of The Astronomer’s Universe, I talk about gamma-ray bursts with Dr. Dale Frail of the Very Large Array (VLA) in New Mexico.  Gamma-ray bursts (commonly referred to as GRBs) are strong blasts of gamma rays that astronomers have detected from nearly every direction in the universe. The first ones were detected by a military satellite back in the early days of the space age, and nobody knew quite what to make of them.  Today, we have satellites in constant watch for GRBs (as they are called), including NASA’s Swift mission.

Swift is an international mission with scientists from U.S., Italy, and the United Kingdom participating.  It has three main instrument packages that let it study GRBs in great detail. In the six years since its launch, Swift has seen more than 500 GRBs and it continues to monitor the skies constantly for more of these outbursts.

What instruments does Swift use to watch for these titanic outbursts that appear so briefly in our skies?  The first is the so-called Burst Alert Telescope (BAT). It looks f0r bursts in the energy range of 15 to 150 keV (shorthand for kilo-electron volt, a measure of energy). The BAT calculates the position of the burst that it sees and if the burst is strong enough, the whole spacecraft slews around to take a closer look.

Swift also has an x-ray telescope and UV/optical telescope, both of which study the afterglows of the gamma-ray burst in their respective wavelength ranges.  A GRB is going to have a certain amount of its radiation in the optical, ultraviolet, and x-ray ranges, as well. Studying all four (gamma, x-ray, optical, and ultraviolet) gives the most complete “picture” of the GRB.

The positions of GRBs that Swift finds are relayed to ground-based observatories so that astronomers can continue to study the afterglows of GRBs long after the inital burst has faded away. The data that astronomers get from Swift and all the other instruments used to monitor the GRBs (such as the radio telescopes of the VLA), are going a long way toward giving astronomers a much fuller picture of just what causes these monster explosions in distant space.

Swift Discovers 500th Burst

This all-sky map shows the locations of the 500 gamma-ray bursts Swift has detected. Credit: NASA/Swift.

In 2004, NASA launched the Swift satellite to study explosive, high-energy events in the universe. Five years later, the spacecraft has recorded its 500th burst. This satellite was launched to study these strong, distant explosions, some of which are thought to occur when two neutron stars in a binary system merge, sending out a pulse of gravitational waves and what’s called a short gamma-ray burst.

Swift’s job is to focus on a burst and report its position so that other observatories can immediately start to study the phenomenon. As soon as it alerts observers, the satellite then focuses on the same explosion with an array of x-ray and ultraviolet/optical-sensitive telescopes. The resulting data are giving astronomers a much better sense of where these outbursts are occurring and a clearer look at the characteristics of each explosion (that is, the light-curves they exhibit).

Swift also studies other explosive events in the universe, such as exploding stars (supernovae, for example), outbursts from the regions near black holes, and high-energy surges from neutron stars. It is also doing a long-term survey of the entire sky in x-ray wavelengths.

Gamma-ray bursters are extremely fascinating to astronomers. This is because gamma-rays themselves are signals from the highest-energy types of events in the cosmos. A gamma-ray burster is a sign that something extremely energetic has occurred. What could it be? The answer to that question kept astronomers busy for years, coming up with models to explain just what kind of event could trigger such an energetic explosion.  Some key breakthroughs came in 1997 when an Italian-Dutch satellite called Beppo-SAX provided very precise locations for some gamma-ray bursters. From that data, astronomers could zero in on the sources and spend time figuring out what the sources of the burst were.  It was a cosmic detective story that Swift is continuing to follow up on. For more information on the 500th burst, check out the NASA press release.

In addition, I’m working on a special June 2010 segment of The Astronomer’s Universe about gamma-ray bursters, so stay tuned for more on these bright, formerly mysterious explosions that seem to come at us from every direction of the cosmos!

In an announcement and paper published this week in the journal Nature, available online here, astronomers have calculated that the gamma ray burst known as GRB 090423 is now the most distant gamma ray burst detected. With a redshift of 8.2, it is estimated that the gamma ray burst actually occurred about 620 million years after the big bang formation of the universe. The previous highest recorded redshift of a gamma ray burst was 6.7, which would make that occurence taking place about 820 million years ago.

In a paper released today, available online now here, astronomers (lots of them) present their findings in the search for gravitational waves from 137 gamma-ray bursts (GRB) detected by the Swift satellite. The LIGO instrument is actually not fully completed, but with this set of findings astronomers report that “No plausible gravitational-wave signals were identified.”

In a paper submitted to the Astrophysical Journal, available online now here, an international team of astronomers have reported on their attempts to detect muon neutrinos from gamma-ray bursts (GRB). They use data from the IceCube neutrino telescope located at the south pole. It should be noted that neutrinos do not interact much with ordinary matter. In fact, the first neutrino detector ever built could not distinguish between the three known types of neutrinos, the electron neutrino, muon neutrino and tau neutrino. It could only detect electron neutrinos.

In a paper to be published in the Astrophysical Journal, available as a pre-print online now here an international team of astronomers including scientists associated with the NASA Swift Mission, examine the nature of the so-called dark gamma ray bursts (GRB). Most GRBs discovered by the Swift spacecraft have an associated optical afterglow. There is an elaborate network of telescopes set to lock in on any of the GRBs detected by the Swift spacecraft to catch these afterglows. However, some GRBs have been detected but they have not had an associated optical afterglow noted, but have had x-ray radiation detected. These authors conclude that “the previously suggested correlation between the optical darkness and the X-ray/gamma-ray brightness is merely an observational selection effect.” They propose a new classification method for categorizing these dark GRBs.

One of the major questions that remain for the gamma-ray observers, especially those associated with the NASA SWIFT spacecraft, is the nature of the so-called dark gamma-ray bursts (GRB). Usually, after the SWIFT spacecraft detects a GRB, optical telescopes that are part of a network of telescopes supporting the SWIFT mission, take a look in the direction of the GRB and find what is called an optical afterglow. In a number of cases, no optical afterglow is found, and these are the so-called “dark” GRBs. In this paper submitted to the Astrophysical Journal, an international team of astronomers present their searches for the host galaxies of these dark GRBs. The authors conclude that “the host galaxies of dark GRBs seem to have normal optical colors.” So what may be causing this “dark” phenomenon? The problem appears to be dust, and the authors present evidence that “the source of obscuring dust is local to the vicinity of the GRB.” However, in the end, the authors conclude that “the location and nature of this high-redshift dust remains unknown.” Read more about the search for the nature of dark GRBs online now here.

In a fascinating paper to appear in the journal Astrophysics and Space Science, a group of Brazilian and Cuban scientists consider the effects of gamma ray bursts on Earth’s atmosphere and the life below the atmosphere. Associated with upper atmospheric gamma ray interactions is a phenomenon known as ultraviolet flash. The UV flash can cause mutations in genetic material of life on Earth. This is considered to be “the only important short-term effect on life” caused by such gamma ray bursts. However, the consequences that this team concludes may result from this brief effect, may be very influential in the evolution of certain life forms on Earth. Read more about the possible consequences of gamma ray bursts on life on Earth online now here.

One of the hot topics in astronomy today is gamma ray bursts or GRBs. Scientists are still debating the details of the nature of these GRBs, and today a team from NYU released their computer models of GRBs. You can see these computer animations of GRBs online at http://cosmo.nyu.edu/~wqzhang/movies/ or read the details of how the authors developed their models and animations online at http://xxx.lanl.gov/PS_cache/arxiv/pdf/0902/0902.2396v1.pdf

Astronomers are only recently incorporating the extremely intense magnetic fields in the vicinity of active galactic nuclei (AGN) and gamma-ray bursts (GRB) into the models they use to help understand what is going on the vicinity of these celestial manifestations. Astronomers from NYU and Stanford released today a paper with their results of a 3D computer simulation of the influence of magnetic fields in the vicinity of AGNs and GRBs. Their results will be published in Astrophysical Journal Letters later this year, but their results are available online at http://xxx.lanl.gov/PS_cache/arxiv/pdf/0811/0811.3638v2.pdf If you wish to see their visualizations, they’ve put those online as well. Take a look at some spectacular animations online at http://cosmo.nyu.edu/~wqzhang/movies/