Quiescent thermal emission from neutron stars in LMXBs

Abstract

The quiescent thermal emission from neutron stars in low mass X-ray binaries after active periods of intense activity in x-rays (outbursts) has been monitored. The theoretical modeling of the thermal relaxation of the neutron star crust may be used to establish constraints on the crust and envelope composition and transport properties, depending on the astrophysical scenarios assumed. We perform numerical simulations of the neutron star crust thermal evolution and compare them with inferred surface temperatures for five sources: MXB 1659-29, KS 1731-260, EXO 0748-676, XTE J1701-462 and IGR J17480-2446. We also present stationary envelope models to be used as a boundary condition for the crustal cooling models. We obtain a relation between the mass accretion rate and the temperature reached at the crust-envelope interface at the end of the active phase that accounts for early observations and reduces the number of free parameters of the problem. With this relation we are also able to set constraints to the envelope composition depending on the accretion mass rate. We find that the evolution of MXB 1659-29, KS 1731-260 and EXO 0748-676 can be well described within a deep crustal cooling scenario. Conversely, we find that other two sources can only be explained with models beyond crustal cooling. For the peculiar emission of XTE J1701-462 we propose alternative scenarios like residual accretion during quiescence, additional heat sources in the outer crust and/or thermal isolation of the inner crust due to a buried magnetic field. We also explain the very recent reported temperature of IGR J17480-2446 with an extra heat deposition in the outer crust coming from shallow sources.