Abstract
Understanding the nature of the γ-ray burst phenomenon is one of the outstanding problems of modern astrophysics. The identification of counterparts at optical wavelengths is considered a crucial factor for determining the origin of these events. Here we report the detection and temporal properties of a variable optical source, which has been identified1,2 as the counterpart of the X-ray transient associated with the γ-ray burst GRB970508 (ref. 3). The initial optical images were obtained only 5.8 hours after the initial γ-ray burst, after which the optical source was observed to brighten over the next two days before declining in luminosity with a t−1 power law. The decline in brightness follows a form predicted by many relativistic fireball models4,5,6,7 for γ-ray bursts, although the initial rise does not appear to be compatible with the simplest of these models. The observed fluence of the source at visible wavelengths over the period spanned by our observations is ⩾4.6 × 10−8 erg cm−2, about 3% of the fluence of the γ-ray burst itself.
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Acknowledgements
We thank the staff of Palomar Observatory for their assistance during our observing runs, and the entire BeppoSAX team for their efforts in providing accurate burst positions as rapidly as possible. This work was supported in part by the NSF, NASA, the Bressler Foundation, the Norris Foundation and Caltech.
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Djorgovski, S., Metzger, M., Kulkarni, S. et al. The optical counterpart to the γ-ray burst GRB970508. Nature 387, 876–878 (1997). https://doi.org/10.1038/43126
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DOI: https://doi.org/10.1038/43126
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