The faint neutron star soft X-ray transient SAX J1810.8-2609 in quiescence

We present the analysis of a 35-ksec-long Chandra observation of the neutron star soft X-ray transient (SXT) SAX J1810.8-2609. We detect three sources in the field of view. The position of one of them is consistent with the location of the ROSAT error circle of SAX J1810.8-2609. The accurate Chandra position of that source coincides with the position of the proposed optical counterpart, strengthening the identification as the counterpart. We detected the neutron star SXT system in quiescence at an unabsorbed luminosity of ~1 ¥ 1032 erg s-1 (assuming a distance of 4.9 kpc). This luminosity is at the low end of quiescent luminosities found in other neutron star SXTs. This renders support to the existence of a group of faint soft X-ray transients of which the accreting millisecond X-ray pulsar SAX J1808.4-3658 is the most prominent member. The quiescent spectrum of SAX J1810.8-2609 is well-fitted with an absorbed power law with a photon index of 3.3 +/- 0.5. With a value of 3.3 ¥ 1021 cm-2, the galactic absorption is consistent with the value derived in outburst. Because the spectra of quiescent neutron star SXTs are often fitted with an absorbed blackbody or neutron star atmosphere plus power-law model, we also fitted the spectrum using those fitting functions. Both models provide a good fit to the data. If cooling of the neutron star core and/or crust is responsible for the soft part of the spectrum, the time-averaged mass accretion rate must have been very low (~5.7 ¥ 10-13 Msolar yr-1 assuming standard core cooling only) or the neutron star must be massive. We also discuss the possibility that the thermal spectral component in neutron stars in quiescence is produced by residual accretion.