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

Abstract

We present the X- and $\gamma$-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 0.83$\pm$0.16. Such a slow decay is consistent with the non-detection in our second observation, but its back-extrapolation to the time of the GRB largely underestimates the detected GRB X-ray prompt emission. In addition, the GRB prompt event shows, among the BeppoSAX-WFC detected sample, unprecedentedly high ratios of X- and gamma-ray peak fluxes (F${}_{2-10keV}$/F${}_{40-700keV}$$\sim$0.6, and F${}_{2-26keV}$/F${}_{40-700keV}$$\sim$1.6) and fluences (S${}_{2-10keV}$/S${}_{40-700keV}$$\sim$1.5 and S${}_{2-26keV}$/S${}_{40-700keV}$$\sim$2.8), making it, among the BeppoSAX arcminute-localized GRBs, the closest to the recently discovered class of Fast X-ray Transients.

Accepted for publication on Astronomy & Astrophysics, Main Journal. 14 pages, Figure 6 in color

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