Crustal Emission and the Quiescent Spectrum of the Neutron Star in KS 1731−260

Abstract

The type-I X-ray bursting low mass X-ray binary KS 1731-260 was recently detected for the first time in quiescence by Wijnands et al., following a τoutburst≈13 yr outburst which ended in Feb 2001. We show that the emission area radius for a H atmosphere spectrum (possibly with a hard power-law component that dominates the emission above 3.5 keV) is consistent with that observed from other quiescent neutron star transients, R∞=23 +30 -15(d/8 kpc) km, and examine possible IR counterparts for KS 1731-260. Unlike all other known transient neutron stars (NS), the duration of this recent (and the only observed) outburst is as long as the thermal diffusion time of the crust. The large amount of heat deposited by reactions in the crust will have heated the crust to temperatures much higher than the equilibrium core temperature. As a result, the thermal luminosity currently observed from the neutron star is dominated not by the core, but by the crust. This scenario implies that the mean outburst recurrence timescale found by Wijnands et al. (∼ 200 yr) is a lower limit. Moreover, because the thermal emission is dominated by the hot crust, the level and the time evolution of quiescent luminosity is determined mostly by the amount of heat deposited in the crust during the most recent outburst (for which reasonable constraints on the mass accretion rate exist), and is only weakly sensitive to t he core temperature. Using estimates of the outburst mass accretion rate, our calculations of the quiescent flux i mmediately following the end of the outburst agree with the observed quiescent flux to within a factor of a few. In this paper, we present simulations of the evolution of the quiescent lightcurve for different scenarios of the c rust microphysics, and demonstrate that monitoring observations (with currently flying instruments) spanning fr om 1‐30 yr can measure the crust cooling timescale and the total amount of heat stored in the crust. These quantitie s have not been directly measured for any neutron star. This makes KS 1731-260 a unique laboratory for studying the thermal properties of the crust by monitoring the luminosity over the next few years to decades. Subject headings: stars: atmospheres — stars: individual (KS 1731-260) — stars: neutron — x-rays: binaries