Constraining the properties of neutron star crusts with the transient low-mass X-ray binary Aql X-1

TL;DR

This study uses X-ray observations of Aql X-1 to support the crust cooling model for neutron star thermal emission, constraining shallow heat release and emphasizing the importance of post-outburst monitoring.

Contribution

It provides the first detailed analysis of multiple outbursts to constrain shallow heating properties in neutron star crusts using thermal evolution modeling.

Findings

Crust cooling explains the quiescent X-ray emission in Aql X-1.

Shallow heat release of 1.2-3.2 MeV/nucleon fits observational data.

Monitoring post-outburst can resolve uncertainties about shallow heating.

Abstract

Aql X-1 is a prolific transient neutron star low-mass X-ray binary that exhibits an accretion outburst approximately once every year. Whether the thermal X-rays detected in intervening quiescent episodes are the result of cooling of the neutron star or due to continued low-level accretion remains unclear. In this work we use Swift data obtained after the long and bright 2011 and 2013 outbursts, as well as the short and faint 2015 outburst, to investigate the hypothesis that cooling of the accretion-heated neutron star crust dominates the quiescent thermal emission in Aql X-1. We demonstrate that the X-ray light curves and measured neutron star surface temperatures are consistent with the expectations of the crust cooling paradigm. By using a thermal evolution code, we find that ~1.2-3.2 MeV/nucleon of shallow heat release describes the observational data well, depending on the assumed mass-accretion rate and temperature of the stellar core. We find no evidence for varying strengths of this shallow heating after different outbursts, but this could be due to limitations of the data. We argue that monitoring Aql X-1 for up to ~1 year after future outbursts can be a powerful tool to break model degeneracies and solve open questions about the magnitude, depth and origin of shallow heating in neutron star crusts.