The thermal state of KS 1731-260 after 14.5 years in quiescence

TL;DR

This study monitors the long-term cooling of the neutron star KS 1731-260 after 14.5 years in quiescence, using X-ray observations to understand its internal thermal properties and constrain core temperature and composition.

Contribution

It provides the first detailed long-term cooling curve analysis of KS 1731-260, constraining core temperature, crust composition, and heating processes with theoretical modeling.

Findings

Crustal cooling has likely ceased, indicating thermal equilibrium with the core.

Core temperature is constrained to approximately 9.35×10^7 K.

No need for a low thermal conductivity layer to fit the cooling data.

Abstract

Crustal cooling of accretion-heated neutron stars provides insight into the stellar interior of neutron stars. The neutron star X-ray transient, KS~1731$-$260, was in outburst for 12.5 years before returning to quiescence in 2001. We have monitored the cooling of this source since then through {\it Chandra} and {\it XMM-Newton} observations. Here, we present a 150 ks {\it Chandra} observation of KS~1731$-$260 taken in August 2015, about 14.5 years into quiescence, and 6 years after the previous observation. We find that the neutron star surface temperature is consistent with the previous observation, suggesting that crustal cooling has likely stopped and the crust has reached thermal equilibrium with the core. Using a theoretical crust thermal evolution code, we fit the observed cooling curves and constrain the core temperature (T$_c = 9.35\pm0.25\times10^7$ K), composition (Q$_{imp} = 4.4^{+2.2}_{-0.5}$) and level of extra shallow heating required (Q$_{sh} = 1.36\pm0.18$ MeV/nucleon). We find that the presence of a low thermal conductivity layer, as expected from nuclear pasta, is not required to fit the cooling curve well, but cannot be excluded either.