Abstract. We here discuss the possibility that the peculiar supernova SN 1998bw and the Gamma-Ray Burst GRB 980425 are associated. If the two are related, the energy radiated in γ rays is at least four orders of magnitude less than in other GRBs, although its appearance is otherwise unremarkable: this suggests that very different mechanisms may give rise to GRBs.

Abstract. Quasi-simultaneousSpectral Energy Distributions (SEDs) at different epochs of four among the most intensively observed Gamma-Ray Burst (GRB) afterglows have been constructed in order to investigate and compare the temporal evolution of their multiwavelength emission. The data suggest that generally the peak frequency moves toward lower frequencies with time. SED local maxima in the optical/IR range are present in some cases, possibly due to dust absorption within the host galaxy.

Abstract. For the case of an adiabatically expanding blast wave, which is applicable to observed gamma-ray burst (GRB) afterglows at late times, we give expressions to infer the physical properties of the afterglow from the measured spectral features.We show that enough data exist for GRB 970508 to compute unambiguously the ambient density, n = 0.03 cm −3 , and the blast wave energy per unit solid angle, E = 4 10 52 erg/4π sr. We also compute the energy density in electrons and magnetic field. We find that they are 12% and 9%, respectively, of the nucleon energy density.

Cosmic γ-ray bursts are one of the great frontiers of astrophysics today. They are a playground of relativists and observers alike. They may teach us about the death of stars and the birth of black holes, the physics in extreme conditions, and help us probe star formation in the distant and obscured universe. In this review we summarise some of the remarkable progress in this field over the past few years. While the nature of the GRB progenitors is still unsettled, it now appears likely that at least some bursts originate in explosions of very massive stars, or at least occur in or near the regions of massive star formation. The physics of the burst afterglows is reasonably well understood, and has been tested and confirmed very well by the observations. Bursts are found to be beamed, but with a broad range of jet opening angles; the mean γ-ray energies after the beaming corrections are ∼ 10 51 erg. Bursts are associated with faint ( R ∼ 25 mag) galaxies at cosmological redshifts, with z ∼ 1. The host galaxies span a range of luminosities and morphologies, but appear to be broadly typical for the normal, actively star-forming galaxy populations at comparable redshifts and magnitudes. Some of the challenges for the future include: the nature of the short bursts and possibly other types of bursts and transients; use of GRBs to probe the obscured star formation in the universe, and possibly as probes of the very early universe; and their detection as sources of high-energy particles and gravitational waves. IntroductionEver since their discovery 89 , the nature of the cosmic γ-ray bursts (GRBs) has been one of the great puzzles of science. While a complete physical explanation of this remarkable phenomenon is still not in hand, there has been a great deal of progress in this field over the last few years. Studies of GRBs are now one of the most active areas of research in astronomy, with a publication rate of ∼ 500 per year 82 , and the total number of GRB-related publications now exceeding 5000. GRBs represent a great laboratory in the sky for relativistic astrophysics, with Lorentz factors reaching Γ ∼ 10 2 − 10 3 . The pre-1997 state of affairs was summarised well in many reviews 43,98,123 . The distribution of bursts on the sky found by BATSE/CGRO is highly isotropic 109 , 1 which provided the first solid hint about their cosmological origins. After many years of speculation based on a limited observational evidence, handicapped mainly by the lack of precise and rapid positional identifications, the field was revolutionized by the BeppoSAX satellite 23 . The key was the enabling discovery of long-lived and precisely located GRB afterglows at longer wavelengths, in the X-rays 29 , optical 166 , and radio 44 , and the resulting direct determination of the cosmological distance scale to the bursts 115 . Allowing for the observational selection and coverage, GRBs are detectable at a mean rate of ∼ 10 3 per year down to the limiting fluxes of ∼ 10 −7 erg cm −2 s −1 , or fluences of ∼ 10 −6 erg cm −2 . While there mu...

Abstract. We present results from the Target of Opportunity (TOO) program to observe GRB locations with the CAM and PHOT instruments aboard the Infrared Space Observatory (ISO). Four BeppoSAX GRB error circles were observed by ISO on timescales ranging from days to months after the GRB events. These observations represent the first prompt GRB counterpart searches at far-infrared wavelengths. A list of observations made in this TOO program, along with results of CAM follow-up observations of GRB 970508, are presented. A marginal CAM (12 µm) detection of a source, consistent with the position of the optical transient of GRB 970508, is reported.