They occur isotropically — that is, evenly over the entire sky, suggesting a cosmological distribution with no dipole and quadrupole components. The uniform distribution led most astronomers to conclude that the instruments were picking up some kind of event happening throughout the universe. The detection of radiation from bursts at other wavelengths would turn out to be essential. Visible light, for example, could reveal the galaxies in which the bursts took place, allowing their distances to be measured. Attempts were made to detect these burst counterparts, but they proved fruitless.
A watershed event occurred in , when the spacecraft BeppoSAX succeeded in obtaining high-resolution X-ray images  of the predicted fading afterglow of GRB — so named because it occurred on February 28, This detection, followed by a number of others at an approximate rate of 10 per year, led to positions accurate to about an arc minute, which allowed the detection and follow-up of the afterglows at optical and longer wavelengths .
Radio observations of its afterglow provided an essential clue. The large variations probably had nothing to do with the burst source itself; rather they involved the propagation of the afterglow light through space. Therefore, if GRB was scintillating at radio wavelengths and then stopped, its source must have grown from a mere point to a discernible disk. To reach this size the source must have been expanding at a considerable rate — close to the speed of light . The old concept of a sudden release of energy concentrated in a few brief seconds has been discarded.
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A striking development in the last several months has been the measurement and localization of fading X-ray signals from several short GRBs by Swift. Other missions, though not designed solely for GRB discovery, will also contribute. The Energetic X-ray Imaging Survey Telescope, planned for launch a decade from now, will have a sensitive gamma-ray instrument capable of detecting thousands of GRBs. The general trend is that the spectrum softens, and Epk decreases, with time.
More precise statements must however wait for larger area detectors.
Some obvious points should be emphasised. The time integrated spectrum on those detectors ranges from 25 keV to 10MeV . This gives unambiguous evidence for the importance of nonthermal processes in GRBs. Observed durations vary, however, by six orders of magnitude, from several milliseconds  to several thousand seconds .
The longest so far, GRB, displayed GeV activity one and a half hours after the main event . The bursts GRB a, GRBb, GRB a and GRB b occurred in the same region of the sky within two days ; if this gang of four is considered as a single event then the longest duration so far is two days!
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These observations may indicate that some sources display a continued activity at a variable level over a period of days. BATSE lightcurves of various individual bursts. The y axis is the photon count rate in the 0. They range from smooth, fast rise and quasi-exponential decay, through curves with several peaks, to variable curves with many peaks. Some of these bursts were also followed by low energy X-ray tails. The hardness ratio is a measure of the shape of the spectrum: larger values correspond to harder spectra. A bimodal duration distribution is measured, irrespective of whether the t50 or t90 durations are considered .
Figure 3 shows the hardness ratio as a function of the duration of the emission.
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This behaviour follows from a cosmological origin of GRB sources, with the decline in the number of faint bursts due to cosmic Theory of Gamma-Ray Burst Sources 83 expansion. A detection of soft X-ray emission features was also reported from GRB . The observed frequencies of the lines appear displaced from the laboratory frequency, as expected from the Doppler shift caused by the expansion of the universe, in agreement with the redshift measured in optical lines from the host galaxy.
A transient absorption feature was detected from GRB , and X-ray absorption in excess of the Galactic hydrogen column density has also been reported in GRB . The energy is calculated, assuming isotropic emission in a common comoving bandpass, for 22 BeppoSax GRBs with known redshift and spectra, using the compilation of .
As of early , plausible or certain host galaxies have been found for all but 1 or 2 of the bursts with optical, radio, or X-ray afterglows localised with arcsecond precision. The few missing cases are at least qualitatively consistent with being in the faint tail of the observed distribution of host galaxy magnitudes . We note also that the observations in the visible probe the UV in the rest frame, and are thus especially susceptible to extinction.
However, sub-mm detections of dusty GRB hosts are currently limited by the available technology to only a handful of ultraluminous sources . The majority of redshifts so far are from the spectroscopy of host galaxies, but an increasing number are based on the absorption-line systems seen in the spectra of the afterglows which are otherwise featureless power-law continua. Reassuring overlap exists in several cases; invariably, the highest-z absorption system corresponds to that of the host galaxy, and has the strongest lines. It is thus fully consistent with a progenitor population associated with the sites of massive star formation.
However, such measurements are completely insensitive to any fully obscured SFR components. The interpretation of the luminosities and observed star formation rates is vastly complicated by the unknown amount and geometry of extinction.
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The observed quantities in the visible trace only the unobscured stellar component, or the components seen through optically thin dust. Any stellar and star formation components hidden by optically thick dust cannot be estimated at all from these data, and require radio and sub-mm observations . The luminosities are plotted at the rest frequencies to facilitate a direct comparison. Panel from . Given the uncertainties of the geometry of optically thin and optically thick dust, optical colours of GRB hosts cannot be used to make any meaningful statements about their net star formation activity.
Supernova Partnership At cosmological distances, the total energy of GRBs is roughly of the same order of magnitude as that of supernovae; in GRBs, however, the energy is emitted much more rapidly. The reason is that in supernovae the energy except that in neutrinos is thermalised by a large amount of mass several solar 88 Enrico Ramirez-Ruiz masses. Independent of its connection with a GRB, SN bw is extraordinary for its very high radio luminosity near the peak of the SN lightcurve .
Because the sampling volume for low luminosity bursts such as is smaller than that of normal GRBs by a factor of , the rate per galaxy of the former events may well exceed those of the latter by a large factor. At the same time, the spectral energy distribution became very red. Theory of Gamma-Ray Burst Sources 89 A number of other GRBs since then have shown similar late deviations from a power-law decline the best example being GRB  , but they still lacked a clear spectroscopic detection of an underlying supernova.
Detection of such signature was recently reported by  for GRB Due to its extreme brightness and slow decay, spectroscopic observations were extensive. Beginning April 6 i. Over the next few days the SN features became more prominent as the afterglow faded and the SN brightened towards maximum. The afterglow spectrum of April 8, clearly supernova dominated, is shown in Figure 7. For comparison, spectra of SN bw at maximum and a week before maximum are also displayed .
The similarities are striking. There are three basic ways of learning about the evolution of luminous matter and gas in the universe. First, a direct detection of sources i. Second, the detection of galaxies selected in absorption along the lines of sight to luminous background sources, traditionally QSOs. Studies of GRB hosts and afterglows can contribute to all three of these methodological approaches, bringing in new, independent constraints for models of galaxy evolution and of the history of star formation in the universe [11, 89, 27].
This is an obvious culprit if indeed GRBs are associated with massive star formation. The census of optical afterglow detections for well-localised bursts can thus provide a completely new and independent estimate of the mean obscured star formation rate . Such a geometrical arrangement may be unlikely in most cases, and our argument probably still applies. A more careful treatment of the dust evaporation geometry is needed, but it is probably safe to say that GRBs can provide a valuable new constraint on the history of star formation in the universe. Absorption spectroscopy of GRB afterglows is now becoming a powerful new probe of the ISM in evolving galaxies, complementary to the traditional studies of quasar absorption line systems.
The growing body of data on GRB absorption systems shows exceptionally high column densities of gas, when compared to the typical quasar absorption systems; only the highest column density DLA systems themselves Theory of Gamma-Ray Burst Sources 91 ostensibly star-forming disks or dwarfs come close . This opens the interesting prospect of using GRB absorbers as a new probe of the chemical enrichment history in galaxies in a more direct fashion than is possible with the quasar absorbers, where there may be very complex dynamics of gas ejection, infall, and mixing at play.
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The bursts for which redshifts are known are bright enough to be detectable, in principle, out to much larger distances than those of the most luminous quasars or galaxies detected at present . Thus, promptly localized GRBs could serve as beacons which, shining through the pregalactic gas, provide information about much earlier epochs in the history of the universe.
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The presence of iron or other X-ray lines provides an additional tool for measuring GRB distances, which may be valuable for investigating the small but puzzling fraction of bursts which have been detected only in X-rays but not optically, perhaps due to a high dust content in the host galaxy. The presence in old stellar populations e. In addition, no bright supernova is observed to accompany short GRBs [18, 31, 47], in distinction from most nearby long GRBs . The newly launched Swift spacecraft is expected to yield localizations for about long and 10 short bursts per year.
This will permit much more detailed studies of the burst environment, the host galaxy, and the intergalactic medium between galaxies. This concludes our compendium of the facts. For ease of reference in the chapters that follow, they have been assembled here with a minimum of speculative interpretation. In these objects, accretion and the accompanying radiation is usually thought to be limited by the Eddington rate, a self—regulatory balance imposed by Newtonian gravity and radiation pressure.
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