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 À1 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 $1 mcrab ($2 ; 10 À11 ergs cm À2 s À1 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 high-energy transients, with rapid data downlink and uplink available through the NASA TDRSS system. Swift transient data will be rapidly distributed to the astronomical community, and all interested observers are encouraged to participate in follow-up measurements. A Guest Investigator program for the mission will provide funding for community involvement. Innovations from the Swift program applicable to the future include (1) a large-area gamma-ray detector using the new CdZnTe detectors, (2) an autonomous rapid-slewing spacecraft, (3) a multiwavelength payload combining optical, X-ray, and gamma-ray instruments, (4) an observing program coordinated with other ground-based and space-based observatories, and (5) immediate multiwavelength data flow to the community. The mission is currently funded for 2 yr of operations, and the spacecraft will have a lifetime to orbital decay of $8 yr.

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The 'event' that triggers a gamma ray burst cannot last for more than a few seconds. This is, however, long compared with the dynamical timescale of a compact stellar-mass object (∼ 10 −3 seconds). Energy is assumed to be released as an outflow with high mean lorentz factor Γ. But a compact stellar-mass collapse or merger is, realistically, likely to generate a mass (or energy) flux that is unsteady on some timescales in the range 10 −3 -10 seconds. If Γ fluctuates by a factor of ∼ 2 around its mean value, relative motions within the outflowing material will themselves (in the comoving frame) be relativistic, and can give rise to internal shocks. For Γ ∼ 10 2 , the resultant dissipation occurs outside the 'photosphere' and can convert a substantial fraction of the overall outflow energy into non-thermal radiation. This suggests a mechanism for cosmological bursts that demands less extreme assumptions (in respect of Γ-values, freedom from baryonic contamination, etc) than earlier proposals.

Abstract. Gamma-ray bursts are the most luminous explosions in the Universe, and their origin and mechanism are the focus of intense research and debate. More than three decades after their discovery, and after pioneering breakthroughs from space and ground experiments, their study is entering a new phase with the recently launched Swift satellite. The interplay between these observations and theoretical models of the prompt gamma ray burst and its afterglow is reviewed.

We discuss the evolution of cosmological gamma-ray burst remnants, consisting of the cooling and expanding fireball ejecta together with any swept-up external matter, after the gamma-ray event. We show that significant optical emission is predicted which should be measurable for timescales of hours after the event, and in some cases radio emission may be expected days to weeks after the event. The flux at optical, X-ray and other long wavelengths decays as a power of time, and the initial value of the flux or magnitude, as well as the value of the time-decay exponent, should help to distinguish between possible types of dissipative fireball models.