Thermal State of Transiently Accreting Neutron Stars

We study thermal states of transiently accreting neutron stars (with mean accretion rates Ṁ ∼ 10 − 10 M⊙ yr ) determined by the deep crustal heating of accreted matter sinking into stellar interiors. We formalize a direct correspondence of this problem to the problem of cooling neutron stars. Using a simple toy model we analyze the most important factors which affect the thermal states of accreting stars: a strong superfluidity in the cores of low-mass stars and a fast neutrino emission (in nucleon, pion-condensed, kaon-condensed, or quark phases of dense matter) in the cores of high-mass stars. We briefly compare the results with the observations of soft X-ray transients in quiescence. If the upper limit on the quiescent thermal luminosity of the neutron star in SAX J1808.4–3658 (Campana et al. 2002) is associated with the deep crustal heating, it favors the model of nucleon neutron-star cores with switched-on direct Urca process.