2009ApJ...693..333F

We report on the results of International Gamma-Ray Astrophysics Laboratory (INTEGRAL) observations of the neutron star low-mass X-ray binary SAX J1810.8-2609 during its latest active phase in 2007 August. The current outburst is the first one since 1998 and the derived luminosity is 1.1-2.6 x10³⁶ erg/s in the 20-100 keV energy range. This low outburst luminosity and the long-term time-average accretion rate of ∼5x10^–12 M_☉/yr suggest that SAX J1810.8-2609 is a faint soft X-ray transient. During the flux increase, spectra are consistent with a thermal Comptonization model with a temperature plasma of kT_e∼ 23-30 keV and an optical depth of τ∼ 1.2-1.5, independent of the luminosity of the system. This is a typical low hard spectral state for which the X-ray emission is attributed to the upscattering of soft seed photons by a hot, optically thin electron plasma. During the decay, spectra have a different shape, the high energy tail being compatible with a single power law. This confirm similar behavior observed by BeppoSAX during the previous outburst, with the absence of visible cutoff in the hard X-ray spectrum. INTEGRAL/JEM-X instrument observed four X-ray bursts in Fall 2007. The first one has the highest peak flux (~3.5 crab in 3-25 keV) giving an upper limit to the distance of the source of about 5.7 kpc, for a L_Edd~ 3.8x10³⁸ erg/s. The observed recurrence time of ∼ 1.2 days and the ratio of the total energy emitted in the persistent flux to that emitted in the bursts (α∼ 73) allow us to conclude that the burst fuel was composed by mixed hydrogen and helium with X ≥ 0.4.