Afterburst thermal relaxation in neutron star crusts

TL;DR

This paper investigates the thermal relaxation process in neutron star crusts after internal heating events, considering various physical conditions and their impact on observable thermal signals.

Contribution

It provides a comprehensive analysis of long-term thermal relaxation in neutron star crusts, including effects of superfluidity, crust thickness, and core processes, enhancing interpretation of astrophysical observations.

Findings

Superfluidity shortens thermal relaxation time.

Deep crustal heating influences long-term thermal evolution.

Core processes affect the emergence of thermal waves.

Abstract

We study thermal relaxation in a neutron star after internal heating events (outbursts) in the crust. We consider thin and thick spherically symmetric heaters, superfluid and non-superfluid crusts, stars with open and forbidden direct Urca processes in their cores. In particular, we analyze long-term thermal relaxation after deep crustal heating produced by nuclear transformations in fully or partly accreted crusts of transiently accreting neutron stars. This long-term relaxation has a typical relaxation time and an overall finite duration time for the crust to thermally equilibrate with the core. Neutron superfluidity in the inner crust greatly affects the relaxation if the heater is located in the inner crust. It shortens and unifies the time of emergence of thermal wave from the heater to the surface. This is important for the interpretation of observed outbursts of magnetars and transiently accreting neutron stars in quasi-persistent low-mass X-ray binaries.