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-Field Instruments (NFI) was performed on March 11 and 12, for a total elapsed time of 8.51 × 10 4 s. The wide band spectral data (0.1-200 keV) obtained with the NFI show a remarkable hard X-ray spectrum detected up to ∼ 200 keV, which can be described by a power law with photon spectral index Γ=1.96±0.04, plus a soft component which is compatible with blackbody radiation of temperature kT ∼ 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 ∼ 5 kpc. The inferred 2-10 keV X-ray luminosity is ∼ 9 × 10 35 erg s −1 at the time of the discovery.