SAX J1810.8-2609: A new hard X-ray bursting transient

The transient X-ray source SAX J1810.8 - 2609 was discovered on 1998 March 10 with the Wide Field Cameras on board the BeppoSAX satellite, while observing the Galactic bulge in the 2-28 keV energy range. On March 11 a strong type I X-ray burst was detected with evidence of photospheric radius expansion. A follow-up target of opportunity observation with the narrow held instruments (NFIs) was performed on March 11 and 12, for a total elapsed time of 8.51 x 10(4) s. The wide-band spectral data (0.1-200 keV) obtained with the NFIs show a remarkable hard X-ray spectrum detected up to similar to 200 keV, which can be described by a power law with photon spectral index Gamma = 1.96 +/- 0.04, plus a soft component which is compatible with blackbody radiation of temperature kT similar to 0.5 keV. The detection of the type I X-ray burst is a strong indication that the compact object is a neutron star in a low-mass X-ray binary system. Assuming standard burst parameters and attributing the photospheric radius expansion to near-Eddington luminosity, we estimate a distance of similar to 5 kpc. The inferred 2-10 keV X-ray luminosity is similar to 9 x 10(35) ergs s(-1) at the time of the discovery.