RENEWED ACTIVITY FROM THE X-RAY TRANSIENT SAXJ 1810.8-2609 WITH INTEGRAL

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 x 10(36) erg s(-1) in the 20-100 keV energy range.

This low outburst luminosity and the long-term time-average accretion rate of similar to 5 x 10(-12) M-circle dot yr(-1) 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) similar to 23-30 keV and an optical depth of tau similar to 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 (approximate to 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 approximate to 3.8 x 10(38) erg s(-1). The observed recurrence time of similar to 1.2 days and the ratio of the total energy emitted in the persistent flux to that emitted in the bursts (alpha similar to 73) allow us to conclude that the burst fuel was composed by mixed hydrogen and helium with X >= 0.4.