Thermal Emission in the Quiescent Neutron Star SAX J1810.8-2609

We have observed the neutron star low-mass X-ray binary SAX J1810.8-2609 in quiescence with XMM-Newton. SAX J1810.8-2609 is one of the faintest non-pulsing neutron star low-mass X-ray binaries in quiescence and previously only had upper limits on its quiescent thermal emission. We found SAX J1810.8-2609 at the same 0.5-10 keV, unabsorbed luminosity as the previous quiescent observation in 2003, {L}_{{X}}=1.5× {10}³² erg s^-1. We show that the spectrum requires both thermal and nonthermal components, each contributing approximately half the total emission. The low neutron star luminosity suggests a time-averaged outburst accretion rate of \dot{M}≈ {10}^-12 M _⊙ yr^-1, in conflict with its observed outburst activity corresponding to a mass accretion rate that is an order of magnitude larger (\dot{M}≈ {10}^-11 M _⊙ yr^-1). Our observation designates SAX J1810.8-2609 more firmly as a member of a population of faint quiescent neutron star LMXBs whose quiescent thermal luminosity is not aligned with standard cooling models.