The Swift mission, scheduled for launch in 2004, is a multiwavelength observatory for gamma-ray burst (GRB) astronomy. It is a first-of-its-kind autonomous rapid-slewing satellite for transient astronomy and pioneers the way for future rapid-reaction and multiwavelength missions. It will be far more powerful than any previous GRB mission, observing more than 100 bursts yr � 1 and performing detailed X-ray and UV/optical afterglow observations spanning timescales from 1 minute to several days after the burst. The objectives are to (1) determine the origin of GRBs, (2) classify GRBs and search for new types, (3) study the interaction of the ultrarelativistic outflows of GRBs with their surrounding medium, and (4) use GRBs to study the early universe out to z >10. The mission is being developed by a NASA-led international collaboration. It will carry three instruments: a newgeneration wide-field gamma-ray (15‐150 keV) detector that will detect bursts, calculate 1 0 ‐4 0 positions, and trigger autonomous spacecraft slews; a narrow-field X-ray telescope that will give 5 00 positions and perform spectroscopy in the 0.2‐10 keV band; and a narrow-field UV/optical telescope that will operate in the 170‐ 600 nm band and provide 0B3 positions and optical finding charts. Redshift determinations will be made for most bursts. In addition to the primary GRB science, the mission will perform a hard X-ray survey to a sensitivity of � 1m crab (� 2;10 � 11 ergs cm � 2 s � 1 in the 15‐150 keV band), more than an order of magnitude better than HEAO 1 A-4. A flexible data and operations system will allow rapid follow-up observations of all types of

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The Swift Gamma-Ray Burst Mission

The discovery of afterglows associated with gamma-ray bursts at X-ray, optical and radio wavelengths and the measurement of the redshifts of some of these events has established that gamma-ray bursts lie at extreme distances, making them the most powerful photon-emitters known in the Universe. Here we report the discovery of transient optical emission in the error box of the gamma-ray burst GRB980425, the light curve of which was very different from that of previous optical afterglows associated with gamma-ray bursts. The optical transient is located in a spiral arm of the galaxy ESO 184-GS2, which has a redshift velocity of only 2,550 km/ s. Its optical spectrum and location indicate that it is a very luminous supernova, which has been identified as SN1998bw. If this supernova and GRB980425 are indeed associated, the energy radiated in gamma-rays is at least four orders of magnitude less than in other gamma-ray bursts, although its appearance was otherwise unremarkable: this indicates that very different mechanisms can give rise to gamma-ray bursts. But independent of this association, the supernova is itself unusual, exhibiting an unusual light curve at radio wavelengths that requires that the gas emitting the radio photons be expanding relativistically.

An Unusual Supernova in the Error Box of the Gamma-Ray Burst of 25 April 1998

Gamma-ray bursts (GRB's), short and intense pulses of low-energy $\ensuremath{\gamma}$ rays, have fascinated astronomers and astrophysicists since their unexpected discovery in the late sixties. During the last decade, several space missions---BATSE (Burst and Transient Source Experiment) on the Compton Gamma-Ray Observatory, BeppoSAX and now HETE II (High-Energy Transient Explorer)---together with ground-based optical, infrared, and radio observatories have revolutionized our understanding of GRB's, showing that they are cosmological, that they are accompanied by long-lasting afterglows, and that they are associated with core-collapse supernovae. At the same time a theoretical understanding has emerged in the form of the fireball internal-external shocks model. According to this model GRB's are produced when the kinetic energy of an ultrarelativistic flow is dissipated in internal collisions. The afterglow arises when the flow is slowed down by shocks with the surrounding circumburst matter. This model has had numerous successful predictions, like the predictions of the afterglow itself, of jet breaks in the afterglow light curve, and of the optical flash that accompanies the GRB's. This review focuses on the current theoretical understanding of the physical processes believed to take place in GRB's.

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The physics of gamma-ray bursts

The discovery of X-ray, optical and radio afterglows of gamma-ray bursts (GRBs) and the measurements of the distances to some of them have established that these events come from Gpc distances and are the most powerful photon emitters known in the Universe, with peak luminosities up to 10^52 erg/s. We here report the discovery of an optical transient, in the BeppoSAX Wide Field Camera error box of GRB980425, which occurred within about a day of the gamma-ray burst. Its optical light curve, spectrum and location in a spiral arm of the galaxy ESO 184-G82, at a redshift z = 0.0085, show that the transient is a very luminous type Ic supernova, SN1998bw. The peculiar nature of SN1998bw is emphasized by its extraordinary radio properties which require that the radio emitter expand at relativistical speed. Since SN1998bw is very different from all previously observed afterglows of GRBs, our discovery raises the possibility that very different mechanisms may give rise to GRBs, which differ little in their gamma-ray properties.

Discovery of the peculiar supernova 1998bw in the error box of GRB980425

https://ned.ipac.caltech.edu/level5/March04/Piran2/paper.pdf

Gamma-ray bursts and the fireball model

For almost a quarter of a century1, the origin of γ-ray bursts— brief, energetic bursts of high-energy photons—has remained unknown. The detection of a counterpart at another wavelength has long been thought to be a key to understanding the nature of these bursts (see, for example, ref. 2), but intensive searches have not revealed such a counterpart. The distribution and properties of the bursts3 are explained naturally if they lie at cosmological distances (a few Gpc)4, but there is a countervailing view that they are relatively local objects5, perhaps distributed in a very large halo around our Galaxy. Here we report the detection of a transient and fading optical source in the error box associated with the burst GRB970228, less than 21 hours after the burst6,7. The optical transient appears to be associated with a faint galaxy7,8, suggesting that the burst occurred in that galaxy and thus that γ-ray bursts in general lie at cosmological distance.

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Transient optical emission from the error box of the gamma-ray burst of 28 February 1997

We report on the results of a search for a radio counterpart to the strong gamma-ray burst GRB 940301. Observations with the Westerbork Synthesis Radio Telescope of the Compton Telescope error box region of GRB 940301 began on March 4, 1994, at 21 cm and April 2, 1994, at 92 cm. No flux density variations were detected at 92 cm above S= 10 mJy (5 (sigma)) within a period of 1 to 4 months after the burst. However, when we compared the field with Westerbork Northern Sky Survey data, taken two years prior to GRB 940301, we found two radio sources with significantly increased flux densities. These sources, only 17 min. apart, are located at the 2.3 and 2.6(sigma) Compton Telescope confidence contours. Their separation from the Inter Planetary Network annulus virtually excludes association with GRB 940301. Further observations in January 1996 reveal that the sources continued to change in flux density. The relatively large flux density variations at 92 cm, compared to those at higher frequencies, and the inverted spectra in the frequency range from 325-38O MHz make the sources somewhat unusual. Because the sources were already detected at 5 GHz in 1986 most, if not all, of the radio emission is probably associated with activity in Active Galactic Nuclei in distant galaxies.

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Two variable radio sources near the position of GRB 940301

We report on the detection of a large amplification of the flux density of PSR B0655+64 at 325 MHz (a factor of similar to 43) that lasted about one hour. To the best of our knowledge such a large amplification has not been reported before. The phenomenon is restricted to a very narrow bandwidth (50 % decorrelation half-width of 3.6 +/- 0.4 MHz). The decorrelation bandwidth and characteristic time scale of the variation are in agreement with the values expected for diffractive interstellar scintillation. However, the observed amplification of the flux density of PSR B0655+64 is rather large. The observations suggest that amplifications of a factor up to 8 are not uncommon for PSR 0655+64 at 92 cm. The extreme flux density amplification of PSR 0655+64 might be explained in terms of caustics. We consider it unlikely that this event can be explained by intrinsic variability.

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J vanParadijs

Papers

Transient optical emission from the error box of the gamma-ray burst of 28 February 1997

Two variable radio sources near the position of GRB 940301

Unusual increase in the 325 mhz flux density of psr b0655+64