Neutron star cooling after deep crustal heating in the X-ray transient KS 1731–260

Abstract

We simulate the cooling of the neutron star in the X‐ray transient KS 1731−260 after the source returned to quiescence in 2001 from a long (≳12.5 yr) outburst state. We show that the cooling can be explained assuming that the crust underwent deep heating during the outburst stage. In our best theoretical scenario the neutron star has no enhanced neutrino emission in the core, and its crust is thin, superfluid, and has the normal thermal conductivity. The thermal afterburst crust–core relaxation in the star may not be over.

Mapping Crustal Heating With the Cooling Light Curves of Quasi-Persistent Transients

“…Its recurrence time is unknown, although it has been quiescent since then. So long as the recurrence time is longer than the thermal relaxation time of the crust (as determined by Shternin et al 2007 and confirmed here), our results are not sensitive to the recurrence time since we are using the core temperature as a parameter in our fit. MXB 1659−29 has had two outbursts since being discovered in 1976 (Lewin et al 1976).…”

“…The thermal time also depends on the thermal conductivity, which changes from being set by phonon scattering in the outer crust to impurity scattering in the inner crust (top panel, dashed line). Electron-electron scattering, although included in our calculations, is not a significant component of the total thermal conductivity (Shternin et al 2007), and we do not show it in Fig. 6.…”

“…Although the break in the lightcurve occurs on the same timescale, the calculation with deep heating only is too faint to match observations. We then increased the accretion rate toṀ = 3.5 × 10 17 g s −1 , which gives a crust heating rate comparable to that used by Shternin et al (2007). Because the higher temperature decreases the thermal conductivity, we set Q imp = 0 to compensate.…”

“…Measurement of Q imp in the crust would therefore give an important test of the composition of rp-process ashes and their subsequent evolution as they are compressed towards nuclear density. Shternin et al (2007) compared time-dependent calculations of deep crustal heating and subsequent cooling to the observations of KS 1731−260. They found that a low value of thermal conductivity corresponding to an amorphous crust could be ruled out because it would give cooling on a much longer timescale than observed.…”

Constraining the properties of neutron star crusts with the transient low-mass X-ray binary Aql X-1

“…For example, the heating and subsequent cooling of the neutron star in response to an accretion outburst can be used to deduce valuable information about the thermal properties of its solid crust, as well as the superfluid properties of its liquid core (e.g., Rutledge et al 2002b;Shternin et al 2007;Brown & Cumming 2009;Page & Reddy 2013;Turlione et al 2015).…”