GRB 990704: The most X-ray rich BeppoSAX gamma-ray burst

¹ Istituto Astrofisica Spaziale, C.N.R., Area di Tor Vergata, Via Fosso del Cavaliere 100, 00133 Roma, Italy
² Osservatorio Astronomico di Roma, Via Frascati 33, 00040 Monteporzio Catone, Italy
³ Space Research Organization in the Netherlands, Sorbonnelaan 2, 3584, CA Utrecht, The Netherlands
⁴ Istituto Tecnologie e Studio Radiazioni Extraterrestri, CNR, Via Gobetti 101, 40129 Bologna, Italy
⁵ Dipartimento di Fisica, Università di Ferrara, Via Paradiso 12, 44100 Ferrara, Italy
⁶ Osservatorio Astronomico di Trieste, Via G.B. Tiepolo 11, 34131, Trieste, Italy
⁷ Istituto di Fisica Cosmica ed Applicazioni dell'Informatica, CNR, Via Ugo La Malfa 153, 90138 Palermo, Italy

Corresponding author: M. Feroci, feroci@ias.rm.cnr.it

Received: 30 March 2001
Accepted: 22 August 2001

Abstract

We present the X- and γ-ray detection of GRB 990704 and the discovery and study of its X-ray afterglow, 1SAX J1219.5-0350. Two pointed BeppoSAX observations with the narrow field instruments were performed on this source, separated in time by one week. The decay of the X-ray flux within the first observation appears unusually slow, being best-fit by a power law with negative index . Such a slow decay is consistent with the non-detection in our second observation, but its back-extrapolation to the time of the GRB largely underestimates the detected GRB X-ray prompt emission. In addition, the GRB prompt event shows, among the BeppoSAX-WFC detected sample, unprecedentedly high ratios of X- and gamma-ray peak fluxes (F_{2-10 keV}/, and F_{2-26 keV}/) and fluences (S_{2-10 keV}/ and S_{2-26 keV}/), making it, among the BeppoSAX arcminute-localized GRBs, the closest to the recently discovered class of Fast X-ray Transients.