# Cooling of neutron stars in soft X-ray transients

## TL;DR

This paper models the thermal states of neutron stars in soft X-ray transients, analyzing how deep crustal heating and various physical parameters influence their quiescent luminosity, and compares predictions with observations.

## Contribution

It introduces a comprehensive model of neutron star cooling in SXRTs considering multiple physical factors and performs a statistical analysis of uncertainties to interpret observational data.

## Key findings

- Direct Urca process is necessary for the most massive neutron stars.
- Small superfluid gaps are favored by observational data.
- Models match observed luminosities of SXRTs.

## Abstract

Thermal states of neutron stars in soft X-ray transients (SXRTs) are thought to be determined by "deep crustal heating" in the accreted matter that drives the quiescent luminosity and cooling via emission of photons and neutrinos from the interior. In this study, we assume a global thermal steady-state of the transient system and calculate the heating curves (quiescent surface luminosity as a function of mean accretion rate) predicted from theoretical models, taking into account variations in the equations of state, superfluidity gaps, thickness of the light element layer and a phenomenological description of the direct Urca threshold. We further provide a statistical analysis on the uncertainties in these parameters, and compare the overall results with observations of several SXRTs, in particular the two sources containing the coldest (SAX J1808.4-3658) and the hottest (Aql X-1) neutron stars. Interpretation of the observational data indicates that the direct Urca process is required for the most massive stars and also suggests small superfluid gaps.