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Astron. Astrophys. 329, 906-910 (1998)

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The first X-ray localization of a [FORMULA]-ray burst by BeppoSAX
and its fast spectral evolution

L. Piro 1, J. Heise 2, R. Jager 2, E. Costa 1, F. Frontera 3, M. Feroci 1, J.M. Muller 2, 5, L. Amati 1, 6, M.N. Cinti 1, D. Dal Fiume 4, L. Nicastro 4, M. Orlandini 4 and G. Pizzichini 4

1 Istituto Astrofisica Spaziale, C.N.R., Via E. Fermi 21, I-00044 Frascati, Italy
2 Space Research Organization in the Netherlands, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
3 Dip. Fisica, Universita` Ferrara, Via Paradiso 12, Ferrara, Italy
4 Istituto TeSRE, C.N.R., Via Gobetti 101, I-40129 Bologna, Italy
5 Beppo-SAX Scientific Data Center, Via Corcolle 19, I-00131 Roma, Italy
6 Istituto astronomico, Universita` "La Sapienza", Via Lancisi 29, Roma, Italy

Received 1 July 1997 / Accepted 21 August 1997


In this paper we present the observations performed by the BeppoSAX Gamma-Ray Burst Monitor (GRBM) and Wide Field Cameras (WFC) of GB960720. We derive a precise localization (3 arcmin radius) and fast broad band (2-700 keV) spectral evolution of the event. A search in the catalogues at all wavelengths in the error box yields a unique outstanding source: the bright radio quasar 4C 49.29. Although the probability of finding such a source by chance is very low ([FORMULA]), the absence of similar counterparts in other small error boxes suggests a chance occurrence. We also find that the duration-energy relationship for bursts previously observed above 25 keV (Fenimore et al. 1995) extends down to 1.5 keV. This result suggests that the same radiation mechanism is operating from X-rays to gamma-rays and is in agreement with radiative cooling by synchotron emission. A fast evolution of the spectrum is found, in which the ratio of X- to gamma-ray intensities varies over three orders of magnitude. Furthermore, the spectrum in the initial phase of the event betrays the presence of an optically thick source rapidly evolving in a thin configuration. No other class of sources in the universe shows such a fast and extreme evolution. These results pose new and tighter constraints on theoretical models for gamma-ray bursts.

Key words: gamma-ray burst – quasar: individual: 4C 49.29

Send offprint requests to: L. Piro (piro@alpha1.ias.fra.cnr.it)

© European Southern Observatory (ESO) 1998

Online publication: December 16, 1997