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The discovery of the unusual supernova SN1998bw, and its possible association with the γ-ray burst GRB 980425 1 , 2 , 3 , provide new insights into the explosion mechanism of very massive stars and the origin of some classes of γ-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 and the light curve of SN1998bw can be well reproduced by an extremely energetic explosion of a massive star composed mainly of carbon and oxygen (having lost its hydrogen and helium envelopes). The kinetic energy of the ejecta is as large as +(2–5)× 10 52  erg, more than ten times that of previously observed supernovae. This type of supernova could therefore be termed ‘hypernova’. The extremely large energy suggests the existence of a new mechanism of massive star explosion that can also produce the relativistic shocks necessary to generate the observed γ-rays.

For almost a quarter of a century 1 , 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 bursts 3 are explained naturally if they lie at cosmological distances (a few Gpc) 4 , but there is a countervailing view that they are relatively local objects 5 , 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 burst 6,7 . The optical transient appears to be associated with a faint galaxy 7,8 , suggesting that the burst occurred in that galaxy and thus that γ-ray bursts in general lie at cosmological distance.

The origin of γ-ray bursts has been one of the great unsolved mysteries in high-energy astrophysics for almost 30 years. The recent discovery of fading sources at X-ray 1 and optical 2,3 wavelengths coincident with the location of the γ-ray burst GRB970228 therefore provides an unprecedented opportunity to probe the nature of these high-energy events. The optical counterpart appears to be a transient point source embedded in a region of extended nebulosity 3–6 , the latter having been tentatively identified as a high-redshift galaxy 3 . This would seem to favour models that place γ-ray bursts at cosmological distances, although a range of mechanisms for producing the bursts is still allowed. A crucial piece of information for distinguishing between such models is how the brightness of the optical counterpart evolves with time. Here we re-evaluate the existing photometry of the optical counterpart of GRB970228 to construct an optical light curve for the transient event. We find that between 21 hours and six days after the burst, the R-band brightness decreased by a factor of ∼40, with any subsequent decrease in brightness occurring at a much slower rate. As the point source faded, it also became redder. The initial behaviour of the source appears to be consistent with the 'fireball' model 7 , but the subsequent decrease in the rate of fading may prove harder to explain.

The discovery of afterglows associated with γ-ray bursts at X-ray 1 , optical 2 and radio 3 wavelengths and the measurement of the redshifts of some of these events 4 , 5 has established that γ-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 γ-ray burst GRB980425, the light curve of which was very different from that of previous optical afterglows associated with γ-ray bursts. The optical transient is located in a spiral arm of the galaxy ESO184-G82, which has a redshift velocity of only 2,550 km s −1 ( ref. 6 ). Its optical spectrum and location indicate that it is a very luminous supernova 7 , which has been identified as SN1998bw. If this supernova and GRB980425 are indeed associated, the energy radiated in γ-rays is at least four orders of magnitude less than in other γ-ray bursts, although its appearance was otherwise unremarkable: this indicates that very different mechanisms can give rise to γ-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 8 , 9 .

Although more than 2,000 astronomical γ-ray bursts (GRBs) have been detected, and numerous models proposed to explain their occurrence 1 , they have remained enigmatic owing to the lack of an obvious counterpart at other wavelengths 2–5 . The recent ground-based detection 6,7 of a transient optical source in the vicinity of GRB970228 (refs 8–11) may therefore have provided a breakthrough. The optical counterpart appears to be embedded in an extended source which, if a galaxy as has been suggested 7,12 , would lend weight to those models that place GRBs at cosmological distances. Here we report observations using the Hubble Space Telescope of the transient counterpart and extended source 26 and 39 days after the initial γ-ray outburst. We find that the counterpart has faded since the initial detection (and continues to fade), but the extended source exhibits no significant change in brightness between the two dates of the observations reported here. The size and apparent constancy of the extended source imply that it is extragalactic, but its faintness makes a definitive statement about its nature difficult. Nevertheless, the decay profile of the transient source is consistent with a popular impulsive-fireball model 13 , which assumes a merger between two neutron stars in a distant galaxy.

Gamma-ray bursts (GRBs) are thought to arise when an extremely relativistic outflow of particles from a massive explosion (the nature of which is still unclear) interacts with material surrounding the site of the explosion. Observations of the evolving changes in emission at many wavelengths allow us to investigate the origin of the photons, and so potentially determine the nature of the explosion. Here we report the results of γ-ray, optical, infrared, submillimetre, millimetre and radio observations of the burst GRB990123 and its afterglow. Our interpretation of the data indicates that the initial and afterglow emissions are associated with three distinct regions in the fireball. The peak flux of the afterglow, one day after the burst, has a lower frequency than observed for other bursts; this explains the short-lived radio emission. We suggest that the differences between bursts reflect variations in the magnetic-field strength in the afterglow-emitting regions.

The recent discovery of fading sources at X-ray and optical wavelengths coincident with the location of gamma-ray bursts has granted an opportunity to probe the nature of high-energy events. A study finds that the initial behavior of such sources appear to be consistent with the \"fireball' model.

The discovery of the unusual supernova SN1998bw, and its possible association with the g-ray burst GRB 980425, provide new insights into the explosion mechanism of very massive stars and the origin of some classes of g-ray bursts. Optical spectra indicate that SN1998bw is a type Ic supernova,, but its peak luminosity is unusually high compared with typical type Ic supernovae. Here we report our findings that the optical spectra and the light curve of SN1998bw can be well reproduced by an extremely energetic explosion of a massive star composed mainly of carbon and oxygen (having lost its hydrogen and helium envelopes). The kinetic energy of the ejecta is as large as +(2-5) 1052erg, more than ten times that of previously observed supernovae. This type of supernova could therefore be termed 'hypernova'. The extremely large energy suggests the existence of a new mechanism of massive star explosion that can also produce the relativistic shocks necessary to generate the observed g-rays.

The discovery of afterglows associated with g-ray bursts at X-ray, optical and radio wavelengths and the measurement of the redshifts of some of these events, has established that g-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 g-ray burst GRB980425, the light curve of which was very different from that of previous optical afterglows associated with g-ray bursts. The optical transient is located in a spiral arm of the galaxy ESO184-G82, which has a redshift velocity of only 2,550kms super(-1) (ref. 6). 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 g-rays is at least four orders of magnitude less than in other g-ray bursts, although its appearance was otherwise unremarkable: this indicates that very different mechanisms can give rise to g-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,.

The origin of -ray bursts has been one of the great unsolved mysteries in high-energy astrophysics for almost 30 years. The recent discovery of fading sources at X-ray1 and optical 2,3 wavelengths coincident with the location of the -ray burst GRB970228 therefore provides an unprecedented opportunity to probe the nature of these high-energy events. The optical counterpart appears to be a transient point source embedded in a region of extended nebulosity 3-6 , the latter having been tentatively identified as a high-redshift galaxy3 . This would seem to favour models that place -ray bursts at cosmological distances, although a range of mechanisms for producing the bursts is still allowed. A crucial piece of information for distinguishing between such models is how the brightness of the optical counterpart evolves with time. Here we re-evaluate the existing photometry of the optical counterpart of GRB970228 to construct an optical light curve for the transient event. We find that between 21 hours and six days after the burst, the R-band brightness decreased by a factor of 40, with any subsequent decrease in brightness occurring at a much slower rate. As the point source faded, it also became redder. The initial behaviour of the source appears to be consistent with the 'fireball' model7 , but the subsequent decrease in the rate of fading may prove harder to explain.