Neutron star crust cooling in KS 1731-260: the influence of accretion outburst variability on the crustal temperature evolution

TL;DR

This study introduces a detailed crust cooling model for neutron stars that incorporates observed outburst variability, revealing how fluctuations influence the thermal evolution and parameter estimation of the crust.

Contribution

The paper presents the first crust cooling model that accounts for detailed outburst variability based on observed light curves, improving understanding of neutron star crust thermal evolution.

Findings

Outburst variability affects early crust cooling phases.

Long-term outburst fluctuations have mild impact on cooling.

Final months' variability significantly influences initial cooling curve.

Abstract

Using a theoretical model, we track the thermal evolution of a cooling neutron star crust after an accretion induced heating period with the goal of constraining the crustal parameters. We present for the first time a crust cooling model $-\text{ } NSCool\text{ } -$ that takes into account detailed variability during the full outburst based on the observed light curve. We apply our model to KS 1731-260. The source was in outburst for $\sim$12 years during which it was observed to undergo variations on both long (years) and short (days-weeks) timescales. Our results show that KS 1731-260 does not reach a steady state profile during the outburst due to fluctuations in the derived accretion rate. Additionally, long time-scale outburst variability mildly affects the complete crust cooling phase, while variations in the final months of the outburst strongly influence the first $\sim$40 days of the calculated cooling curve. We discuss the consequences for estimates of the neutron star crust parameters, and argue that detailed modelling of the final phase of the outburst is key to constraining the origin of the shallow heat source.