# Long-term temperature evolution of neutron stars undergoing episodic accretion outbursts

## TL;DR

This study models the long-term thermal evolution of neutron stars in transient low-mass X-ray binaries, highlighting the significant role of shallow heating in reaching thermal equilibrium and affecting observable luminosity.

## Contribution

It introduces the first detailed analysis of crustal properties, especially shallow heating, on neutron star temperature evolution over 10^5 years using the NSCool code.

## Key findings

- Shallow heating significantly influences the equilibrium core temperature.
- Replacing deep crustal heating with shallow heating results in only 2% lower core temperature.
- Shallow heating can increase quiescent luminosity beyond previous estimates.

## Abstract

Transient neutron star (NS) LMXBs undergo episodes of accretion, alternated with quiescent periods. During an accretion outburst, the NS heats up due to exothermic accretion-induced processes taking place in the crust. Besides the long-known deep crustal heating of nuclear origin, a likely non-nuclear source of heat, dubbed 'shallow heating', is present at lower densities. Most of the accretion-induced heat slowly diffuses into the core on a timescale of years. Over many outburst cycles, a state of equilibrium is reached when the core temperature is high enough that the heating and cooling (photon and neutrino emission) processes are in balance. We investigate how stellar characteristics and outburst properties affect the long-term temperature evolution of a transiently accreting NS. For the first time the effects of crustal properties are considered, particularly that of shallow heating. Using our code NSCool, we tracked the thermal evolution of a NS undergoing outbursts over a period of $10^5$ yr. The outburst sequence is based on the regular outbursts observed from Aql X-1. For each model, we calculated the timescale over which equilibrium was reached and we present these timescales along with the temperature and luminosity parameters of the equilibrium state. We find that shallow heating significantly contributes to the equilibrium state. Increasing its strength raises the equilibrium core temperature. We find that if deep crustal heating is replaced by shallow heating alone, the core would still heat up, reaching only a 2% lower equilibrium core temperature. Deep crustal heating may therefore not be vital to the heating of the core. Additionally, shallow heating can increase the quiescent luminosity to values higher than previously expected.