A cooling neutron star crust after recurrent outbursts: Modelling the accretion outburst history of Aql X-1

TL;DR

This study models the thermal evolution of the neutron star in Aql X-1 over 23 outbursts from 1996 to 2015, revealing that previous outbursts influence subsequent cooling and that shallow heating varies between outbursts.

Contribution

First detailed collective modeling of multiple outbursts in Aql X-1, demonstrating variable shallow heating and the impact of outburst history on neutron star crust cooling.

Findings

Data fit well when envelope composition and shallow heating vary between outbursts

Shallow heating properties are not constant across outbursts

Heating during one outburst affects subsequent cooling curves

Abstract

With our neutron star crust cooling code {\tt NSCool} we track the thermal evolution of the neutron star in Aql X-1 over the full accretion outburst history from 1996 until 2015. For the first time, we model many outbursts (23 outbursts were detected) collectively and in great detail. This allows us to investigate the influence of previous outbursts on the internal temperature evolution and to test different neutron star crust cooling scenarios. Aql X-1 is an ideal test source for this purpose, because it shows frequent, short outbursts and thermally dominated quiescence spectra. The source goes into outburst roughly once a year for a few months. Assuming that the quiescent {\it Swift}/XRT observations of Aql X-1 can be explained within the crust cooling scenario (Waterhouse et al. 2016), we find three main conclusions. Firstly, the data are well reproduced by our model if the envelope composition and shallow heating parameters are allowed to change between outbursts. This is not the case if both shallow heating parameters (strength and depth) are tied throughout all accretion episodes, supporting earlier results that the properties of the shallow heating mechanism are not constant between outbursts. Second, from our models shallow heating could not be connected to one specific spectral state during outburst. Third, and most importantly, we find that the neutron star in Aql X-1 does not have enough time between outbursts to cool down to crust-core equilibrium and that heating during one outburst influences the cooling curves of the next.