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, Lx = 1.5 × 10³² 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 M ≈ 10⁻² M⊙yr⁻¹, in conflict with its observed outburst activity corresponding to a mass accretion rate that is an order of magnitude larger (M ≈ 10⁻¹¹M⊙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.United States. National Aeronautics and Space Administration (Grant NNX16AE12G