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Gamma ray burst
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Gamma-ray bursts (GRBs) are the most luminous electromagnetic events occurring in the universe since the Big Bang. They are flashes of gamma rays emanating from seemingly random places in deep space at random times. The duration of a gamma-ray burst is typically a few seconds, but can range from a few milliseconds to several minutes, and the initial burst is usually followed by a longer-lived "afterglow" emitting at longer wavelengths (X-ray, ultraviolet, optical, infrared, and radio).
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Encyclopedia
Gamma-ray bursts (GRBs) are the most luminous electromagnetic events occurring in the universe since the Big Bang. They are flashes of gamma rays emanating from seemingly random places in deep space at random times. The duration of a gamma-ray burst is typically a few seconds, but can range from a few milliseconds to several minutes, and the initial burst is usually followed by a longer-lived "afterglow" emitting at longer wavelengths (X-ray, ultraviolet, optical, infrared, and radio). Gamma-ray bursts are detected by orbiting satellites about two to three times per week.
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.
A nearby gamma-ray burst could possibly cause mass extinctions on Earth. The short duration of a gamma-ray burst would limit the immediate damage to life. However, a nearby burst might alter atmospheric chemistry by reducing the ozone layer and generating acidic nitrogen oxides ultimately causing severe damage to the biosphere. Since GRBs in metal-rich galaxies like the Milky Way are rare, mass extinctions due to GRBs may only happen once per billion years.
History
Discovery
Gamma-ray bursts were discovered in the late 1960s by the US 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. While most satellites orbited at about 500 miles above Earth's surface, the Vela satellites orbited at an altitude of 65,000 miles. At this height, the satellites orbited above the Van Allen radiation belt, which reduced the noise in the sensors. The extra height also meant that the satellites could detect explosions behind the moon, a location where the United States government suspected the Soviet Union would try to conceal nuclear weapon tests. The Vela system generally had four satellites operational at any given time such that a gamma-ray signal could be detected at multiple locations. This made it possible to localize the source of the signal to a relatively compact region of space. While these characteristics were incorporated into the Vela system to improve the detection of nuclear weapons, these same characteristics were what made the satellites capable of detecting gamma-ray bursts.
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. Nuclear bombs produce a very brief, intense burst of gamma rays less than one millionth of a second. The radiation then steadily fades as the unstable nuclei decay. The signal detected by the Vela satellites had neither the intense initial flash nor the gradual fading, but instead there were two distinct peaks in the light curve. Solar flares and new supernovas were the two other possible explanations for the event, but neither had occurred on that day. Unclear on what had happened, but not considering the matter particularly urgent, the team at the Los Alamos Scientific Laboratory, lead by Ray Klebesadel, filed the data away for later investigation.
Vela 5 was launched on May 23 1969. Because the sensitivity and time resolution on these satellites were significantly more accurate then the instruments on Vela 4, the Los Alamos team expected these new satellites to detect more gamma-ray bursts. Despite an enormous amount of background signals picked up by the new detectors, the research team found twelve events which had not coincided with any solar flares or supernovas. Some of the new detections also showed the same double-peak pattern that had been observed by Vela 4.
Although their instrumentation offered no improvement over those on Vela 5, the Vela 6 satellites were launched on April 8 1970 with the intention of determining the direction from which the gamma rays were arriving. The orbits for the Vela 6 satellites were chosen to be as far away from Vela 5 as possible, generally on the order of 10000 kilometers apart. This separation meant that, despite gamma rays traveling at the speed of light, a signal would be detected at slightly different times by different satellites. By analyzing the arrival times, Klebesadel and his team successfully traced sixteen gamma-ray bursts. The random distribution of bursts across the sky made it clear that the bursts were not coming from the sun, moon, or other planets in our solar system.
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.ef>
Early research missions
Shortly after the discovery of gamma-ray bursts, a general consensus arose within the astronomical community that in order to determine what caused them, they would have to be identified with astronomical objects at other wavelengths, particularly visible light, as this approach had been successfully applied to the fields of radio X-ray astronomy. This method would require far more accurate positions of several gamma-ray bursts than the Vela system could provide. Greater accuracy required the detectors to be spaced farther apart. Instead of launching satellites only into Earth's orbit, it was deemed necessary to spread the detectors throughout the solar system.
By the end of 1978, the first Inter-Planetary Network (IPN) had been completed. In addition to the Vela satellites, the IPN was comprised of 5 new space probes: the Russian Prognoz 7, in orbit around the earth, the German Helios 2, in elliptical orbit around the Sun, and NASA's Pioneer Venus Orbiter, Venera 11, and Venera 12, each of which orbited Venus. The research team at the Russian Institute for Space Research in Moscow, lead by Kevin Hurley, was able to use the data collected by the IPN to accurately determine the position of gamma-ray bursts with an accuracy of a few minutes of arc. However, even when using the most powerful telescopes available, nothing of interest could be found within the determined regions.
To explain the existence of gamma-ray bursts, many speculative theories were advanced, most of which posited nearby galactic sources. Little progress was made, however, 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 GRBs are isotropic (not biased towards any particular direction in space, such as toward the galactic plane or the galactic center), and therefore ruling out nearly all galactic origins. Because the Milky Way galaxy has a very flat structure, if gamma-ray bursts were to originate from within the Milky Way, they would not be distributed isotropically across the sky, but instead concentrated in the plane of the Milky Way. Although the brightness of the flashes suggested that the bursts had to be originating within the Milky Way, the distribution indicated otherwise.
BATSE data also showed that GRBs fall into two distinct categories: short-duration, hard-spectrum bursts ("short bursts"), and long-duration, soft-spectrum bursts ("long bursts"). Short bursts are typically less than two seconds in duration and are dominated by higher-energy photons; long bursts are typically more than two seconds in duration and dominated by lower-energy photons. The separation is not absolute and the populations overlap observationally, but the distinction suggests two different classes of progenitors. However, some believe there is a third type of GRBs. The three kinds of GRBs are hypothesized to reflect three different origins: mergers of neutron star systems, mergers between white dwarfs and neutron stars, and the collapse of massive stars.
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 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, orbital brightness modulation, fast time scale flickering, extreme colors, emission lines, or an unusual shape. From the discovery of GRBs through the 1980s, GRB 790305b[GRBs are named after the date on which they are discovered: the first two digits being the year, followed by the two-digit month and two-digit day. If two or more GRBs occur on a given day, the name is appended with a letter 'A' for the first burst identified, 'B' for the second and so on.] 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.
As early as 1980, a research group headed by Livio Scarsi at the 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 serve the sole purpose of studying X-rays, Enrico Costa of the Istituto di Astrofisica Spaziale suggested that the satellite's four protective shields could easily 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.
In 1983, a team comprised of Stan Woosley, Don Lamb, Ed Fenimore, Kevin Hurley, and George Ricker began discussing plans for a new GRB research satellite, the High Energy Transient Explorer (HETE). Although many satellites were already providing data on GRBs, HETE would be the first satellite devoted entirely to GRB research. The goal was for HETE to be able to localize gamma-ray bursts with much greater accuracy than the BATSE detectors. The team submitted a proposal to NASA in 1986 under which the satellite would be equipped with four gamma ray detectors, an X-ray camera, and four electronic cameras for detecting visible and ultraviolet light. The project was to cost $14.5 million, and the launch was originally planned for the summer of 1994. The Pegasus XL launch, which occurred on November 4, 1994, was successful, but neither HETE nor SAC-B, an Argentinian research satellite also on board, uncoupled from the main rocket. Neither of the two satellites were able to direct their solar panels towards the sun, and within one day of the launch, all radio contact with the satellites was lost. The eventual successor to the mission, HETE 2, was successfully launched on 9 October 2000. It observed its first GRB on 13 February 2001.
Observations and analysis
BeppoSAX detected a gamma-ray burst on January 11 1997, and one of its Wide Field Cameras (WFC) also detected X-rays at the same moment. John Heise, project scientist for BeppoSAX's WFCs, quickly deconvolved the data from the WFCs and, in less than 24 hours, produced a sky position with an accuracy of about 10 arcminutes. Although this level of accuracy had already been surpassed by the interplanetary networks, they were unable to produce the data as quickly as Heise could. In the following days, Dale Frail, working with the Very Large Array, detected a single fading radio source within the error box, a BL Lac object. An article was written for Nature stating that this event proved that GRBs originated from active galaxies. However, Jean in 't Zand, a former gamma-ray spectroscopist at the Goddard Space Flight Center, rewrote the WFC deconvolution software to produce a position with an accuracy of 3 arcminutes, and the BL Lac object was no longer within the reduced error box. Despite BeppoSAX having observed both X-rays and a GRB and the position being known within that same day, the source of the burst was not identified.
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.[For more on galaxies hosting GRBs, see the GHostS database http://www.grbhosts.org] 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 also 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. This was the first accurate determination of the distance to a GRB, and it further proved that GRBs occur in extremely distant galaxies.
Prior to the localization of GRB 970228, opinions differed as to whether or not GRBs would emit detectable radio waves. Bohdan Paczynski and James Rhoads published an article in 1993 predicting radio afterglows, but Martin Rees and Peter Mészáros concluded that, due to the vast distances between GRBs and the earth, any radio waves produced would be too weak to be detected. Although GRB 970228 was accompanied by an optical afterglow, neither the Very Large Array nor the Westerbork Synthesis Radio Telescope were able to detect a radio afterglow. However, five days after GRB 970508, Dale Frail, working with the Very Large Array in New Mexico, observed radio waves from the afterglow at wavelengths of 3.5 cm, 6 cm, and 21 cm. The total luminosity varied widely from hour to hour, but not simultaneously in all wavelengths. Jeremy Goodman of Princeton University explained the erratic fluctuations as being the result of scintillation caused by vibrations in the Earth's atmosphere, which no longer occurs when the source has an apparent size larger than 3 micro-arcseconds. After several weeks, the luminosity fluctuations had dissipated. Using this piece of information and the distance to the event, it was determined that the source of radio waves had expanded almost at the speed of light. Never before had accurate information been obtained regarding the physical characteristics of a gamma-ray burst explosion.
Also, because GRB 970508 was observed at many different wavelengths, it was possible to form a very complete spectrum for the event. Ralph Wijers and Titus Galama attempted to calculate various physical properties of the burst, including the total amount of energy in the burst and the density of the surrounding medium. Using an extensive system of equations, they were able to compute these values as 3×1052 ergs and 30,000 particles per cubic meter, respectively. Although the observation data was not accurate enough for their results to be considered particularly reliable, Wijers and Galama did show that, in principle, it would be possible to determine the physical characters of GRBs based on their spectrums.
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
INTEGRAL, the European Space Agency's International Gamma-Ray Astrophysics Laboratory Announcements, was launched on March 16 2006. It is the first observatory capable simultaneously observing objects at gamma ray, X-ray, and visible wavelengths.
NASA's Swift satellite 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.
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 and AGILE. Discoveries of GRBs are made as they are detected 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
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. 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 each 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 prevailing trend is that 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 which are long and which 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 GRB720427, which was detected by the Apollo 16 gamma-ray spectrometer, and GRB720514, 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 , and the observations from interplanetary networks later increased this lower bound to , 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, though 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 in such an event if it 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, despite its various flaws, 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, featuring Bohdan Paczynski and Don Lamb, was structured based on The Great Debate. Lamb, representing the local model theorists, 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. Paczynski pointed 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. He 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 (corresponding to a distance of 8 billion light-years), while the most distant known (GRB 080913) had a redshift of (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 , 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—many 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. As these jets drill through the layers of stellar material to reach the surface of the dying star, they are focused into narrow beams. 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 that 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 also 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
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. Currently, the most widely-accepted model for the origin of most observed 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 popular today, 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 (gravitational, thermonuclear, rotational, magnetic) and 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 now 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—for example, 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.
Emission mechanisms
The means by which gamma-ray bursts convert energy into radiation remains poorly understood, and as of 2007 there is still no generally accepted model for how this process occurs. A successful model of GRBs must explain not only the energy source, but also 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 which suggested 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 the 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
On July 2, 1967, the first GRB, 670702, was detected by the Vela 4 satellite. Since then, many gamma-ray bursts have been detected, 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, , 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 exceeded magnitude 8.9, and would have been visible with an ordinary pair of binoculars in spite of its distance of 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 an "inner jet", previously unknown.
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.
GRB 080916C which occurred on September 16, 2008 in the constellation Carina and recorded by the Fermi telescope has been confirmed to have "the greatest total energy, the fastest motions, and the highest-energy initial emissions" ever seen. 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
Bibliography
External links
GRB Catalogs and Circulars
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- , a feature-rich online (grbox.net) catalog of gamma-ray bursts and their properties for public use
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GRB General Information
GRB Mission Sites
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- launch date: June 11, 2008
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GRB Follow-up Programs
News Articles and Media
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- segment of Science Friday, 3 June 2005 (RealAudio)
- (BBC reports a registered GRB from about 13 billion light years away)
- (ESO)
- reported by NASA, from 7.5 billion light years and
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