TITLE: GCN GRB OBSERVATION REPORT NUMBER: 317 SUBJECT: GRB 990510: BeppoSAX/GRBM observations DATE: 99/05/11 19:35:50 GMT FROM: Filippo Frontera at ITESRE CNR 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. This message is citeable.