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

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\pm 0.25\times 10^7$ K), composition (Q${}_{imp}=4.4_{-0.5}^{+2.2}$) and level of extra shallow heating required (Q${}_{sh}=1.36\pm 0.18$ 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.

7 pages, 6 figures, 1 table, accepted to ApJ

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Cited by in corpus (6)

• Cooling of Accretion-Heated Neutron Stars
• Superfluidity and Superconductivity in Neutron Stars
• A lower limit on the heat capacity of the neutron star core
• Experimental constraints on shallow heating in accreting neutron-star crusts
• Constraining the properties of dense neutron star cores: The case of the low-mass X-ray binary HETE J1900.1-2455
• The Impact of Neutron Transfer Reactions on Heating and Cooling of Accreted Neutron Star Crusts