Gamma-ray burst

Drawing shows the life of a massive star as nuclear fusion converts lighter elements into heavier ones. When fusion no longer generates enough pressure to counteract gravity, the star rapidly collapses to form a black hole. Theoretically, energy is released during the collapse along the axis of rotation to form a gamma-ray burst. Credit: Nicolle Rager Fuller/NSF

Gamma-ray bursts (GRBs) are the most luminous electromagnetic events in the universe since the Big Bang. They are flashes of gamma rays emanating at random from distant galaxies. The duration of a gamma-ray burst is typically a few seconds, but can range from a few milliseconds to several minutes. The characteristics of the light curve vary significantly and are independent of the total duration of the burst. The initial burst is usually followed by a longer-lived "afterglow" emitting at longer wavelengths (X-ray, ultraviolet, optical, infrared, and radio).

GRBs were first detected in 1967 by the Vela satellites, a series of satellites designed to detect nuclear explosions in space. Since then, hundreds of theoretical models have been created in an attempt to explain these bursts, such as collisions between comets and neutron stars. Little information was available to support any of these models until the discovery of X-ray and optical afterglows and the determination of the redshift of GRB 970508. Where the scientific community had once been divided over how far away GRBs occur from Earth, there is now consensus that they occur in distant galaxies. It has been hypothesized that a gamma-ray burst in the Milky Way could cause mass extinctions on Earth.

Most observed GRBs appear to be collimated emissions caused by the collapse of the core of a rapidly rotating, high-mass star into a black hole. A subclass of GRBs (the "short" bursts) appear to originate from a different process, the leading theory being the merger of neutron stars orbiting in a binary system. All observed GRBs have originated from outside the Milky Way galaxy, though a related class of phenomena, soft gamma repeater flares, are associated with galactic magnetars. The sources of most GRBs have been billions of light years away.

There are several gamma-ray burst research missions currently in progress. Swift, launched in November 2004, features an extremely sensitive gamma ray detector and the ability to point on-board telescopes towards the direction of a new burst in less than one minute after the burst is detected. INTEGRAL, launched in March 2006, was the first observatory capable of simultaneously observing objects at gamma ray, X-ray, and visible wavelengths.

History

Main article: History of gamma-ray burst research

Gamma-ray bursts were discovered in the late 1960s by the U.S. Vela nuclear test detection satellites. The Velas were built to detect gamma radiation pulses emitted by nuclear weapon tests in space. The United States suspected that the USSR might attempt to conduct secret nuclear tests after signing the Nuclear Test Ban Treaty in 1963. On July 2 1967, at 14:19 UTC, the Vela 4 and Vela 3 satellites detected a flash of gamma radiation that were unlike any known nuclear weapons signatures. Unclear what had happened but not considering the matter particularly urgent, the team at the Los Alamos Scientific Laboratory, led by Ray Klebesadel, filed the data away for investigation. As additional Vela satellites were launched with better instruments, the Los Alamos team continued to find inexplicable gamma-ray bursts in their data. By analyzing the different arrival times of the bursts on different satellites, the team was able to determine rough estimates for the sky positions of sixteen bursts. In 1973, Ray Klebesadel, Roy Olson, and Ian Strong of the University of California Los Alamos Scientific Laboratory published Observations of Gamma-Ray Bursts of Cosmic Origin, identifying a cosmic source for the previously unexplained observations of gamma-rays. Between 1973 and 2001, more than 5300 papers were published on GRBs.

Many speculative theories were advanced to explain the existence of gamma-ray bursts, most of which posited nearby galactic sources. Little progress was made until the 1991 launch of the Compton Gamma Ray Observatory and its Burst and Transient Source Explorer (BATSE) instrument, an extremely sensitive gamma-ray detector. This instrument provided crucial data indicating that the distribution of GRBs is isotropic — not biased towards any particular direction in space, such as toward the galactic plane or the galactic center. Due to the flat structure of the Milky Way galaxy, gamma-ray bursts originating from within it would not be distributed isotropically across the sky, but would be concentrated in the plane of the galaxy. If these bursts were to occur in other galaxies they would have to be extremely energetic to be detectable at such great distances. Most astronomers concluded that it was more likely that the bursts were less energetic and occurred within the Milky Way, but the bursts' distribution provided very strong evidence to the contrary.

For decades after the discovery of GRBs, astronomers searched for a counterpart: any astronomical object in positional coincidence with a recently observed burst. Astronomers considered many distinct classes of objects, including white dwarfs, pulsars, supernovae, globular clusters, quasars, Seyfert galaxies, and BL Lac objects. Researchers specifically looked for objects with unusual properties which might relate to gamma-ray bursts: high proper motion, polarization, variations in brightness, extreme colors, emission lines, or an unusual shape. From the discovery of GRBs through the 1980s, GRB 790305b was the only event to have been identified with a candidate source object: nebula N49 in the Large Magellanic Cloud. All other attempts failed due to poor resolution of the available detectors. The best hope seemed to lie in finding a fainter, fading, longer wavelength emission after the burst itself, the "afterglow" of a GRB.

In the early 1980s, an Italian research group headed by Livio Scarsi at the Sapienza University of Rome began working on Satellite per Astronomia X, an X-ray astronomy research satellite. The project developed into a collaboration between the Italian Space Agency and the Netherlands Agency for Aerospace Programmes. Though the satellite was originally intended to study X-rays, Enrico Costa of the Istituto di Astrofisica Spaziale suggested that the satellite's four protective shields could also serve as gamma-ray burst detectors. After 10 years of delays and a final cost of approximately $350 million, the satellite, renamed BeppoSAX in honor of Giuseppe Occhialini, was launched on April 30, 1996.

Success for the BeppoSAX team came in February 1997, less than one year after it had been launched. BeppoSAX detected a gamma-ray burst (GRB 970228), and when the X-ray camera was pointed towards the direction from which the burst had originated, it detected a fading X-ray emission. Ground-based telescopes later identified a fading optical counterpart as well. The location of this event having been identified, once the GRB faded, deep imaging was able to identify a faint, very distant host galaxy in the GRB's location. Within only a few weeks the long controversy about the distance scale ended: GRBs were extragalactic events originating inside faint galaxies at enormous distances. By finally establishing the distance scale, characterizing the environments in which GRBs occur, and providing a new window on GRBs both observationally and theoretically, this discovery revolutionized the study of GRBs.

Two major breakthroughs occurred with the next event registered by BeppoSAX, GRB 970508. This event was localized within 4 hours of its discovery, allowing research teams to begin making observations much sooner than any previous burst. By comparing photographs of the error box taken on May 8 and May 9 (the day of the event and the day after), one object was found to have increased in brightness. Between May 10 and May, Charles Steidel recorded the spectrum of the variable object from the W. M. Keck Observatory. Mark Metzger analyzed the spectrum and determined a redshift of z=0.835, placing the burst at a distance of roughly 6 billion light years from Earth. This was the first accurate determination of the distance to a GRB, and it further proved that GRBs occur in extremely distant galaxies.

The next burst to have its redshift calculated was GRB 971214 with a redshift of 3.42, a distance of roughly 12 billion lightyears from earth. Using the redshift and the accurate brightness measurements made by both BATSE and BeppoSAX, Shrinivas Kulkarni, who had recorded the redshift at the W. M. Keck Observatory, calculated the amount of energy released by the burst in half a minute to be 3×10⁵³ ergs, several hundred times more energy than is released by the sun in 10 billion years. The burst was proclaimed to be the most energetic explosion to have ever occurred since the Big Bang, earning it the nickname Big Bang 2. This explosion presented a dilemma for GRB theoreticians: either this burst produced more energy than could possibly be explained by any of the existing models, or the burst did not emit energy in all directions, but instead in very narrow beams which happened to have been pointing directly at earth. While the beaming explanation would reduce the total energy output to a very small fraction of Kulkarni's calculation, it also implies that for every burst observed on earth, several hundred occur which are not observed because their beams are not pointed towards earth.

Current missions

Swift Spacecraft

NASA's Swift satellite was launched in November 2004. It combines a sensitive gamma-ray detector with the ability to point on-board X-ray and optical telescopes towards the direction of a new burst in less than one minute after the burst is detected. Swift's discoveries include the first observations of short burst afterglows and vast amounts of data on the behavior of GRB afterglows at early stages during their evolution, even before the GRB's gamma-ray emission has stopped. The mission has also discovered large X-ray flares appearing within minutes to days after the end of the GRB.

INTEGRAL, the European Space Agency's International Gamma-Ray Astrophysics Laboratory Announcements, was launched on March 16 2006. It is the first observatory capable of simultaneously observing objects at gamma ray, X-ray, and visible wavelengths. On June 11, 2008 NASA's Gamma-ray Large Area Space Telescope (GLAST), later renamed the Fermi Gamma-ray Space Telescope, was launched. The mission objectives include "crack[ing] the mysteries of the stupendously powerful explosions known as gamma-ray bursts." Other gamma-ray burst observation missions include EXIST and AGILE. As GRBs are detected, their positions are automatically distributed via the Gamma-ray Burst Coordinates Network so that researchers may promptly focus their instruments on the source of the burst to observe the afterglows.

Types of bursts

Gamma-ray burst light curves

Most astronomical eruptions have a very simple and consistent time structure. During novas and supernovas, power and brightness rise rapidly and decline slowly. Gamma-ray bursts are unusual in the complexity and diversity of their time structures. No two gamma-ray bursts are identical. Each has a distinctive pattern of emissions over time as shown by their observable light curves. Researchers generally consider two broad classes of GRBs: Short GRBs have an average duration of 0.3 seconds and range from a few milliseconds to 2 seconds, and Long GRBs have an average duration of 30 seconds and range from 2 seconds to several minutes. Some theories suggest that short and long bursts are caused by two distinct physical processes.

Gamma-ray bursts can also be divided into two other categories: Those that have a single maximum in the light curve, and those that have multiple maxima. While the existence of a maximum may be accepted or rejected depending on the level of confidence chosen by the researchers, the majority of bursts have multiple maxima. The multiplicity or singularity of peaks are not directly related to the duration of the burst. GRB 811201, for example, lasted 3.5 seconds but only had one peak in its light curve, whereas much shorter events have been observed to be double peaked. In addition, the amount of radiation between these peaks, or "subpulses," varies from burst to burst. In some events, there is a steady elevated level of radiation between the subpulses. In others, the emission recedes to the background level, meaning that the burster is emitting no radiation at all.

Several events have been recorded whose light curves have a periodic structure. As such, another classification scheme exists: bursts which are very brief, bursts with two peaks or a roughly periodic time structure, and bursts that are long and have irregular time structures. The time history of GRB 790305b, recorded by Venera 12, displayed 22 cycles of a period of 8 seconds, as well as quasi-periodic pulsations at roughly 23 ms. GRB 771029 also strongly exhibited periodicity with 6 cycles of a period of 4.2 seconds. In other events, periodicity may not be as obvious, and often the decision to classify an event as being periodic depends on the methodology of the research team.

Gamma-ray burst spectra cover a fairly wide energy range, both from event to event and within the duration of a single burst. At the extremes, burst spectra have been measured with energies as low as 2 keV, whereas some were higher than 10 MeV. The energy emitted by gamma-ray bursts is divided into three segments: the low energy continuum, which ranges from 2 keV to 30 keV, the intermediate energy continuum, from 30 keV to 1 MeV, and the high energy continuum, which covers all energy levels greater than 1 MeV. The first two GRBs to be observed in the low energy range were GRB 720427, which was detected by the Apollo 16 gamma-ray spectrometer, and GRB 720514, which was observed by the UCSD Solar X-Ray Spectrometer and by Vela 5b.

Distance scale and energetics

Galactic vs. extragalactic models

Prior to the launch of BATSE, the distance scale to GRBs from Earth was unknown. Data from the Vela satellites provided a lower bound of approximately 1,000,000 miles (1,600,000 km), and the observations from interplanetary networks later increased this lower bound to 10 astronomical units (1.5×10⁹ km), which excluded only the inner solar system. Theories for the location of these events ranged from the outer regions of our own solar system to the edges of the known universe. The discovery that bursts were isotropic—coming from completely random directions—reduced these possibilities greatly, though many scientists were still adamant that the events were occurring within the Milky Way. One explanation for the isotropic distribution was that GRBs were somehow related to the cloud of comets in the outer solar system. The first papers to advocate the theory of cosmological distances were those published by Soviet astrophysicist Vladimir Usov in 1975; his arguments were largely ignored by the scientific community.

Soft gamma repeaters (SGRs), highly magnetized galactic neutron stars, are known to periodically erupt in bright flares at gamma-ray and other wavelengths. Supporters of the galactic model hypothesized that there might be an unobserved population of similar objects at greater distances, producing GRBs. However, the sheer brightness of a typical gamma-ray burst observed on Earth would need enormous energy to be released if such an event occurred in a distant galaxy. Supporters of the extragalactic model claimed that the galactic neutron-star hypothesis involved too many ad-hoc assumptions in order to reproduce the degree of isotropy reported by BATSE, and that an extragalactic model would more closely fit the available data.

The argument over the distance scale culminated in 1995 in a formal debate organized by Robert Nemiroff. The debate featured Bohdan Paczyński and Don Lamb, and was structured based on The Great Debate. Lamb represented the local model theorists and presented the idea that GRBs came from Milky Way's supposed corona, a spherical cloud of neutron stars. This, if true, would be consistent with the previously observed isotropic distribution of bursts. Paczyński pointed out that only two isotropic distributions are known to exist: that of bright stars in the direct vicinity of the sun, and the most distant galaxies of the universe. Paczyński argued that it was highly improbable for GRBs to exist only in the direct vicinity of the sun, and therefore GRBs must occur in distant galaxies. Both researchers agreed that the solution would not be found without newer satellites with more accurate detectors, as well as more rapid relays between the satellites and researchers.

The discovery of afterglow emission associated with faraway galaxies confirmed the extragalactic hypothesis. Not only are GRBs extragalactic events, but they are also observable to the limits of the visible universe; a typical GRB has a redshift of at least $z=1.0$ (corresponding to a distance of 8 billion light-years), while the most distant known (GRB 080913) had a redshift of $z=6.7$ (corresponding to a distance of 12.8 billion light years). GRB 080913's lookback time reveals that the burst occurred less than 825 million years after the universe began. The previous record holder was a burst with a redshift of $z=6.29$, which placed it 70 million light-years closer than GRB 080913. As observers are able to acquire spectra of only a fraction of bursts—usually the brightest ones—some GRBs may actually originate from even higher redshifts.

Jets of collimated emissions

The prevailing theory to explain GRB emissions is that they are created by a rapidly rotating central engine, such as a dying star that collapses to form a black hole. The newly formed black hole absorbs infalling matter and releases enormous amounts of energy as relativistic jets along the axis of rotation to form collimated emissions, material and radiation traveling along parallel trajectories. These jets are focused into narrow beams as they drill through the layers of stellar material to reach the surface of the dying star. Observations have confirmed the presence of dying stars at the source of long gamma-ray bursts. Evidence suggests the beams have an opening angle of only a few degrees and travel at more than 99.995% the speed of light. Many GRBs have been observed to undergo a "jet break" in their light curve. In a jet break, the optical afterglow of a GRB undergoes an abrupt change in its rate of decay as the jet decelerates and expands.

Features suggestive of significant asymmetry have been observed in at least one nearby type Ic supernova—which may have the same progenitor stars as GRBs—and have been observed to accompany GRBs in some cases (see "Progenitors", below). The jet opening angle (degree of beaming), however, varies greatly, from 2 degrees to more than 20 degrees. There is some evidence which suggests the jet angles and apparent energy released are correlated in such a way that the true energy release of long GRBs is approximately constant—about 10⁴⁴ J, or the energy equivalent to 1/2000 of a solar mass. This is comparable to the energy released in a bright type Ib/c supernova (sometimes termed a "hypernova"). Bright hypernovae appear to accompany some GRBs, suggesting that hypernovae may be a source.

The fact that GRBs are jetted suggests that there are far more events occurring in the Universe than those actually seen, even when factoring in the limited sensitivity of available detectors. Most jetted GRBs will "miss" the Earth and never be seen; only a small fraction happen to be pointed such that they can be detected. Still, even with these considerations, the rate of GRBs is very small—about once per galaxy per 100,000 years.

Short GRBs, while also extragalactic, appear to come from a lower-redshift population and are less luminous than long GRBs. They appear to be generally less beamed or possibly not beamed at all, intrinsically less energetic than their longer counterparts, and probably more frequent in the universe despite rarely being observed.

Progenitors

Main article: Gamma-ray burst progenitors

The immense distances of most gamma-ray burst sources from Earth have made investigation of the progenitors, the systems that produce these explosions, extremely difficult. The most widely-accepted model for the origin of long duration GRBs is called the collapsar model, in which the core of an extremely massive, low-metallicity, rapidly-rotating star collapses into a black hole. The collapsar model originally explained the formation of black holes and was later applied to GRBs.

While this model is currently popular, various other models have been strongly supported throughout the history of GRB research. In 1974, less than a decade after GRBs had first been discovered, Marvin Ruderman of Columbia University presented a review listing dozens of proposed models. By the end of the 1970s the number of models included on this list had grown to more than 100. These models varied by the type of energy converted into GRBs, including gravitational, thermonuclear, rotational, and magnetic. By the late 1990s consensus had been reached among the scientific community that GRB emissions were non-thermal. The list of models varied by the types of objects involved (black holes, neutron stars, white dwarf stars, comets, etc.). In 1973, Martin Harwit and Edwin Salpeter of Cornell University first presented the idea that GRBs are produced by comets falling onto neutron stars. Because comets have a wide range of sizes and shapes and can collide with neutron stars at a wide range of angles, this model was flexible enough to account for the vast range of characteristics displayed by GRBs.

Stellar wind from highly magnetized, newly-formed neutron stars (proto-magnetars), accretion-induced collapse of older neutron stars, and the mergers of binary neutron stars have all been proposed as alternative models. The different models are not mutually exclusive, and it is possible that different types of bursts have different progenitors. For example, there is good evidence that some short gamma-ray bursts (GRBs with a duration of less than about two seconds) occur in galaxies without massive stars, strongly suggesting that this subset of events is associated with a different progenitor population than longer bursts—such as merging neutron stars. However, in 2007 the detection of 39 short gamma-ray bursts could not be associated with gravitational waves which are hypothesized to be observable in such compact mergers. This is not surprising as the current sensitivity of even the best gravitational waves detectors is not sufficient to detect such signals even from the nearest short GRBs detected so far.

Emission mechanisms

Main article: Gamma-ray burst emission mechanisms

The means by which gamma-ray bursts convert energy into radiation remains poorly understood, and as of 2007 there was still no generally accepted model for how this process occurs. A successful model of GRBs must explain both the energy source and the physical process for generating an emission of gamma-rays which matches the durations, light spectra, and other characteristics observed. The nature of the longer-wavelength afterglow emission ranging from X-ray through radio that follows gamma-ray bursts has been modeled much more successfully as synchrotron emission from a relativistic shock wave propagating through interstellar space, but this model has had difficulty explaining the observed features of some observed GRB afterglows (particularly at early times and in the X-ray band), and may be incomplete or in some ways inaccurate.

Inverse Compton scattering may cause gamma-ray emissions observed after GRBs. If a GRB progenitor, such as a Wolf-Rayet star, were to explode within a stellar cluster, the resulting shock wave could generate gamma-rays by scattering photons from neighboring stars. About 30% of known galactic Wolf-Rayet stars are located in dense clusters of O stars with intense ultraviolet radiation fields. Therefore, a substantial fraction of GRBs are expected to occur in such clusters. As the relativistic matter ejected from an explosion slows and interacts with ultraviolet-wavelength photons, some photons gain energy, generating gamma-rays.

Mass extinction events

In 1995, physicist Stephen Thorsett at Princeton University suggested that a nearby gamma-ray burst could significantly affect the Earth's atmosphere and potentially cause severe damage to the biosphere. Current models suggest that gamma-ray bursts occur within the Milky Way galaxy every 100,000–1,000,000 years. If such a GRB were pointing at Earth, the gamma-ray radiation would far exceed even the most intense solar flares. The absorption of radiation in the atmosphere would cause photodissociation of nitrogen, generating nitric oxide which would act as a catalyst to destroy ozone.

In 2005, scientists at NASA and the University of Kansas released a more detailed study suggesting that the Ordovician-Silurian extinction events, which occurred approximately 450 million years ago, could have been triggered by a gamma-ray burst. They did not have direct evidence to suggest that such a burst resulted in the ancient extinction, rather they created a model of resulting atmospheric changes and the likely consequences of a nearby GRB. Gamma-ray radiation from a relatively nearby star explosion, hitting the Earth for only ten seconds, could deplete up to half of the atmosphere's protective ozone layer, the recovery for which would take at least five years. With the ozone layer damaged, ultraviolet radiation from the Sun would kill much of the life on land and near the surface of oceans and lakes. While this wouldn't directly affect all forms of life, the food chain would be affected dramatically. This, in turn, could lead to mass extinctions. While gamma-ray bursts in the Milky Way galaxy are indeed rare, NASA scientists estimate that at least one nearby event has probably hit the Earth in the past billion years. Life has existed on Earth for at least 3.5 billion years, therefore it is possible that such an event could have caused a mass extinction.

In 2006, researchers at Ohio State University conducted a comparative study on galaxies in which GRBs have occurred. They found that metal-deficient galaxies are the most likely to contain sources of highly energetic, long GRBs. Due to the fact that the Milky Way has been too metal-rich to host a long GRB since the Earth formed, in their opinion it is most unlikely that a nearby GRB has caused mass extinction events on Earth.

The Wolf-Rayet star WR 104, located 8000 light years from Earth, has been found to have a rotational axis aligned within 16° of the solar system, suggesting that if it produced a GRB, one of the jets might be pointed towards Earth. The chance of WR 104 producing a gamma-ray burst are small, and the effects on Earth from such a potential event are not fully understood.

Notable gamma-ray bursts

The optical afterglow of gamma-ray burst GRB-990123 was imaged on January 23, 1999. The burst is seen as a bright dot denoted by a square on the left, with an enlarged cutout on the right. The object above it with the finger-like filaments is the originating galaxy. This galaxy seems to be distorted by a collision with another galaxy.

On July 2, 1967, the first GRB, 670702, was detected by the Vela 4 satellite. Many gamma-ray bursts have been detected since then, including several of significant historical or scientific importance.

On February 27, 1997 the BeppoSAX satellite detected GRB 970228 and its afterglow. This was the first GRB with a successfully detected afterglow. The location of the afterglow was coincident with a very faint galaxy, providing strong evidence that GRBs are extragalactic.

On May 9, 1997, the BeppoSAX satellite detected GRB 970508. GRB 970508 was the first with a measured redshift, $z=0.835$, confirming that GRBs are extragalactic events. The extent to which radiation is redshifted allows astronomers to calculate an estimate of the distance to the event from Earth.

Astronomers obtained a visible-light image of GRB 990123 as it occurred on January 23, 1999, using the ROTSE-I telescope, sited in Los Alamos, New Mexico. The robotic telescope was fully automated, responding to signals from NASA's BATSE instrument aboard the Compton Gamma Ray Observatory within seconds, without human intervention. This was the first GRB for which optical emission was detected before the gamma-ray emission had ceased. GRB 990123 had the brightest measured optical afterglow until GRB 080319B. GRB 990123 momentarily reached magnitude 8.9, and would have been visible with an ordinary pair of binoculars despite being nearly 10 billion light years from Earth.

On May 9, 2005, NASA's Swift achieved the first accurate localization of a short GRB, GRB 050509b. It became the first GRB associated with a host galaxy, the E1 elliptical galaxy 2MASX J12361286+2858580, in the galaxy cluster NSC J123610+285901. It may also be the first observation of a GRB with a black hole-neutron star (BH-NS) or NS-NS merger progenitor.

On March 19, 2008, NASA's Swift detected GRB 080319B, later referred to as the "naked-eye GRB". It was the most luminous event observed in optical and infrared wavelengths, and the most distant event observed that would be theoretically visible to the naked eye (7.5 Gly). Additionally, its rotational axis was closely aligned with Earth, allowing more detailed observation of the jet. In September 2008, a team of astronomers announced the discovery of a previously unknown "inner jet".

On September 13, 2008, NASA's Swift detected GRB 080913. Subsequent terrestrial observations by VLT and GROND showed that it was 12.8 Gly distant, making it the most distant GRB observed to date. This stellar explosion occurred around 825 million years after the Big Bang.

On September 16, 2008, the Fermi Gamma-ray Space Telescope detected GRB 080916C in the constellation Carina. This burst has been confirmed to have had "the greatest total energy, the fastest motions, and the highest-energy initial emissions" ever detected. The explosion had the power of about 9,000 ordinary supernovae, and the gas bullets emitting the initial gamma rays must have moved at 99.9999 percent the speed of light. The tremendous power and speed make this blast the most extreme recorded to date.

See also

• Category:Gamma-ray telescopes
• Gamma-ray astronomy
• Stellar evolution
• Terrestrial gamma-ray flashes

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External links

GRB Catalogs and Circulars

• Gamma-ray Burst Real-time Sky Map based on Swift data
• GRBlog: A Gamma-Ray Burst Database at University of Texas
• Gamma Ray Burst Coordinates Network
• GRBOX, a feature-rich online (grbox.net) catalog of gamma-ray bursts and their properties for public use
• Greiner's GRB Catalog

GRB General Information

• Gamma-ray burst information and image from Kyoto University
• Gamma-ray burst FAQ from Caltech
• Gamma-ray burst information from NASA/Swift

GRB Mission Sites

• Official NASA Swift Homepage: The Swift Gamma-Ray Burst Mission
• UK Swift Science Data Centre
• Swift Mission Operations Center at Penn State
• HETE-2
• INTEGRAL
• BATSE: Burst and Transient Source Explorer
• Fermi Gamma-ray Large Area Space Telescope launch date: June 11, 2008
• Agile
• EXIST: Energetic X-ray Survey Telescope

GRB Follow-up Programs

• PROMPT: Panchromatic Robotic Optical Monitoring and Polarimetry Telescopes (Wiki entry)
• RAPTOR: Rapid Telescopes for Optical Response
• ROTSE: Robotic Optical Transient Search Experiment
• PAIRITEL: Peters Automated Infrared Imaging Telescope
• MASTER: Mobile Astronomical System of the Telescope-Robots
• KAIT: The Katzman Automatic Imaging Telescope
• BOOTES
• REM: Rapid Eye Mount

News Articles and Media

• PBS NOVA: Death Star (gamma-ray bursts)
• GRB 971214: Most energetic event in the universe
• GRB 971214: Space Science Update Webcast (RealMedia)
• Animation of Gamma-Ray Burst (Quicktime)
• GRB 980326: Evidence for a massive star connection
• Gamma-ray bursters segment of Science Friday, 3 June 2005 (RealAudio)
• Most distant cosmic blast sighted (BBC reports a registered GRB from about 13 billion light years away)
• Cosmological Gamma-Ray Bursts and Hypernovae Conclusively Linked (ESO)
• GRB 080319B Naked Eye Burst (2008-03-19) reported by NASA, from 7.5 billion light years and Animation of the event
• New Gamma-Ray Burst is Most Distant Naked Eye Object.