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 (NFIs) was performed on March 11 and 12, for a total elapsed time of 8.51 × 10⁴ s. The wide-band spectral data (0.1-200 keV) obtained with the NFIs 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 × 10³⁵ ergs s^-1 at the time of the discovery.