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

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 $0.83 \pm 0.16$. 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 (F2-10 keV/$F_{\rm 40-700 keV}\sim0.6$, and F2-26 keV/$F_{\rm 40-700 keV}\sim1.6$) and fluences (S2-10 keV/$S_{\rm 40-700 keV}\sim1.5$ and S2-26 keV/$S_{\rm 40-700 keV}\sim2.8$), making it, among the BeppoSAX arcminute-localized GRBs, the closest to the recently discovered class of Fast X-ray Transients.