GCN Circular 317

L. Amati, F. Frontera (ITESRE/CNR, Bologna), E. Costa, M. Feroci 
(IAS/CNR, Rome), on behalf of the BeppoSAX/GRBM team, report:

GRB990510 was detected by the BeppoSAX Gamma Ray Burst Monitor (see
also IAUC #7160) as a bright, complex gamma-ray burst composed by two well
separated and multi-peaked pulses with a total duration of about 75 s. The
GRB fluence in the 40-700 keV energy range is (1.9 +/- 0.2) x 10E-5 erg
cmE-2, while its peak flux (integrated on 1 s, same energy band) is (2.4
+/- 0.2) x 10E-6 erg cmE-2 sE-1.  For comparison, the peak flux in the
2-28 keV range as measured by the BeppoSAX/WFC N. 2 (IAUC #7160) is 
1.4 x 10E-7 erg cmE-2 sE-1. The fluence of the event is the highest,
among the BeppoSAX localized events, after GRB990123, GRB980329, and
GRB970111. The burst shows typical hard-to soft spectral evolution with an
average power law photon index of  1.8 +/- 0.1 in the 70-650 keV energy
band.

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