Cooling of the crust in the neutron star low-mass X-ray binary MXB 1659-29

TL;DR

This study observes the cooling process of a neutron star crust in MXB 1659-29 over 6.6 years, determining a relaxation timescale and core temperature, confirming crustal thermal relaxation after outbursts.

Contribution

It provides extended monitoring data of crust cooling in MXB 1659-29, establishing the thermal relaxation timescale and core temperature with model-independent results.

Findings

Crust has thermally relaxed after 6.6 years.

Cooling timescale is approximately 465 days.

Core temperature estimated between 3.5E7 and 8.3E7 K.

Abstract

In quasi-persistent neutron star transients, long outbursts cause the neutron star crust to be heated out of thermal equilibrium with the rest of the star. During quiescence, the crust then cools back down. Such crustal cooling has been observed in two quasi-persistent sources: KS 1731-260 and MXB 1659-29. Here we present an additional Chandra observation of MXB 1659-29 in quiescence, which extends the baseline of monitoring to 6.6 yr after the end of the outburst. This new observation strongly suggests that the crust has thermally relaxed, with the temperature remaining consistent over 1000 days. Fitting the temperature cooling curve with an exponential plus constant model we determine an e-folding timescale of 465 +/- 25 days, with the crust cooling to a constant surface temperature of kT = 54 +/- 2 eV (assuming D=10 kpc). From this, we infer a core temperature in the range 3.5E7-8.3E7 K (assuming D=10 kpc), with the uncertainty due to the surface composition. Importantly, we tested two neutron star atmosphere models as well as a blackbody model, and found that the thermal relaxation time of the crust is independent of the chosen model and the assumed distance.