3 Conclusions

The blast wave model of gamma-ray burst afterglows has proved quite robust in providing a consistent overall interpretation of the major features of these objects at various frequencies. The "standard model" of afterglows, involving four spectral slopes and three breaks, is quite useful in understanding "snaphsot" multiwavelength spectra of afterglows. However, the constraints on the angle-integrated energy, especially at $\gamma$-ray energies, are not strong, and beaming effects remain uncertain. Some caution is required in interpreting the observations on the basis of the simple standard model. For instance, if one integrates the flux over all angles visible to the observer, the contributions from different angles lead to a considerable rounding-off of the spectral shoulders, so that breaks cannot be easily located unless the spectral sampling is dense and continuous, both in frequency and in time. Some of the observed light curves with humps, e.g. in GRB 970508, require "post-standard" model features (i.e. beyond those assumed in the standard model), such as either non-uniform injection episodes or anisotropic outflows. Time-dependent multiwavelength fits of this and other bursts also seem to indicate that the parameters characterizing the shock physics change with time. A relatively brief (1-100 s), probably modulated energy input appears the likeliest interpretation for most bursts. This can provide an explanation both for the highly variable $\gamma$-ray light curves and for late glitches in the afterglow decays.

There has been significant progress in understanding how gamma-ray bursts can arise in fireballs produced by brief events depositing a large amount of energy in a small volume, and in deriving the generic properties of the ensuing long wavelength afterglows. There still remain a number of mysteries, especially concerning the identity of their progenitors, the nature of the triggering mechanism, the transport of the energy and the time scales involved. However, independently of the details of the gamma-ray burst central engine, even if beaming reduces their total energy requirements, these objects are the most extreme phenomena that we know about in high energy astrophysics, and may provide useful beacons for probing the universe at $z \mathrel{\hbox{\rlap{\lower.55ex \hbox {$\sim$}} \kern-.3em \raise.4ex \hbox{$\gt$}}}5$. With new experiments coming on-line in the near future, there is every prospect for continued and vigorous developments both in the observational and theoretical understanding of these fascinating objects.

Acknowledgements

I am grateful to Martin Rees for stimulating collaborations on this subject, as well as to Ralph Wijers, Hara Papathanassiou and Alin Panaitescu. This research is supported in part by NASA NAG5-2857