Gamma-ray bursts (GRBs) are the most brilliant objects in the Universe but efforts to estimate the total energy released in the explosion -- a crucial physical quantity -- have been stymied by their unknown geometry: spheres or cones. We report on a comprehensive analysis of GRB afterglows and derive their conical opening angles. We find that the gamma-ray energy release, corrected for geometry, is narrowly clustered around 5x10**50 erg. We draw three conclusions. First, the central engines of GRBs release energies that are comparable to ordinary supernovae, suggesting a connection. Second, the wide variation in fluence and luminosity of GRBs is due entirely to a distribution of opening angles. Third, only a small fraction of GRBs are visible to a given observer and the true GRB rate is at least a factor of 500 times larger than the observed rate.Comment: Nature, submitte

The discovery of the unusual supernova SN1998bw, and its possible association with the ␥-ray burst GRB 980425 1-3 , provide new insights into the explosion mechanism of very massive stars and the origin of some classes of ␥-ray bursts. Optical spectra indicate that SN1998bw is a type Ic supernova 3,4 , but its peak luminosity is unusually high compared with typical type Ic supernovae 3 . Here we report our findings that the optical spectra

Nature © Macmillan Publishers Ltd 19988 amounts of 56 Ni (ϳ0.7 solar masses) have to be synthesized in the explosion 16 ; the large energy and 56 Ni mass would be unprecedented for a core-collapse supernova.If one accepts the possibility that GRB980425 and SN1998bw are associated, one must conclude that GRB980425 is a rare type of GRB, and SN1998bw is a rare type of supernova. The radio properties 8,9 of SN1998bw show the peculiar nature of this event independent of whether or not it is associated with GRB980425.The consequence of an association is that the ␥-ray peak luminosity of GRB980425 is L ␥ ¼ ð5:5 Ϯ 0:7Þ ϫ 10 46 erg s −1 (in the 24-1,820 keV band) and its total ␥-ray energy budget is (8:1 ϫ 1:0Þ ϫ 10 47 erg. These values are much smaller than those of 'normal' GRBs which have peak luminosities of up to 10 52 erg s −1 and total energies 5 up to several times 10 53 erg. This implies that very different mechanisms can produce GRBs which cannot be distinguished on the basis of their ␥-ray properties, and that models explaining GRB980425/SN1998bw are unlikely to apply to 'normal' GRBs and vice versa. Ⅺ

We have calculated synchrotron spectra of relativistic blast waves, and find predicted characteristic frequencies that are more than an order of magnitude different from previous calculations. 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 = 3 × 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 and thus confirm for the first time that both are close to but below equipartition.For GRB 971214, we discuss the break found in its spectrum by Ramaprakash et al. (1998). It can be interpreted either as the peak frequency or as the cooling frequency; both interpretations have some problems, but on balance the break is more likely to be the cooling frequency. Even when we assume this, our ignorance of the selfabsorption frequency and presence or absence of beaming make it impossible to constrain the physical parameters of GRB 971214 very well.

We present and discuss the photometric and spectroscopic evolution of the peculiar SN 1998bw, associated with GRB 980425, through an analysis of optical and near-IR data collected at ESOÈLa Silla. The spectroscopic data, spanning the period from day [9 to day ]376 (relative to B maximum), have shown that this supernova (SN) was unprecedented, although somewhat similar to SN 1997ef. Maximum expansion velocities as high as 3 ] 104 km s~1 to some extent mask its resemblance to other Type Ic SNe. At intermediate phases, between photospheric and fully nebular, the expansion velocities (D104 km s~1) remained exceptionally high compared to those of other recorded core-collapse SNe at a similar phase. The mild linear polarization detected at early epochs suggests the presence of asymmetry in the emitting material. The degree of asymmetry, however, cannot be decoded from these measurements alone. The He I 1.083 and 2.058 km lines are identiÐed, and He is suggested to lie in an outer region of the envelope. The temporal behavior of the Ñuxes and proÐles of emission lines of Mg I] j4571, [O I] jj6300, 6364, and a feature ascribed to Fe are traced to stimulate future modeling work. The uniqueness of SN 1998bw became less obvious once it entered the fully nebular phase (after 1 yr), when it was very similar to other Type Ib/cÈIIb objects, such as the Type Ib SN 1996N and the Type IIb SN 1993J, even though SN 1998bw was 1.4 mag brighter than SN 1993J and 3 mag brighter than SN 1996N at a comparable phase. The late-phase optical photometry, which extends up to 403 days after B maximum, shows that the SN luminosity declined exponentially but substantially faster than the decay rate of 56Co. The ultraviolet-optical-infrared bolometric light curve, constructed using all available optical data and the early JHK photometry presented in this work, shows a slight Ñattening starting on about day ]300. Since no clear evidence of ejecta-wind interaction was found in the late-time spectroscopy (see also the work of Sollerman and coworkers), this may be due to the contribution of the positrons since most c-rays escape thermalization at this phase. A contribution from the superposed H II region cannot, however, be excluded.

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We analyze a complete sample of gray burst afterglows and find X-ray evidence for high column densities of gas around them. The column densities are in the range 10 22-10 23 cm Ϫ2 , which is right around the average column density of Galactic giant molecular clouds. We also estimate the cloud sizes to be 10-30 pc, implying masses տ10 5. This strongly suggests that gray bursts lie within star-forming regions and therefore argues M , against neutron star mergers and for collapses of massive stars as their sources. The optical extinctions, however, are 10-100 times smaller than expected from the high column densities. This confirms theoretical findings that the early hard radiation from gray bursts and their afterglows can destroy the dust in their environment, thus carving a path for the afterglow light out of the molecular cloud. Because of the self-created low extinction and location in star-forming regions, we expect gray bursts to provide a relatively unbiased sample of high-redshift star formation. Thus, they may help resolve what is the typical environment of high-redshift star formation.

We present B-, V -, J-, H-, K-, and K@-band observations of the optical transient (OT) associ-R C -, I C -, ated with GRB 970228, based on a reanalysis of previously used images and unpublished data. In order to minimize calibration di †erences, we have collected and analyzed most of the photometry and consistently determined the magnitude of the OT relative to a set of secondary Ðeld stars. We conÐrm our earlier Ðnding that the early decay of the light curves (before 1997 March 6) was faster than that at intermediate times (between 1997 March 6 and April 7). At late times the light curves resume a fast decay (after 1997 April 7). The early-time observations of GRB 970228 are consistent with relativistic blast-wave models, but the intermediate-and late-time observations are hard to understand in this framework. The observations are well explained by an initial power-law decay with a \ [1.51^0.06 modiÐed at later times by a Type Ic supernova light curve. Together with the evidence for GRB 980326 and GRB 980425, this gives further support for the idea that at least some c-ray bursts are associated with a possibly rare type of supernova.