http://arxiv.org/abs/1906.02554
Transiently accreting neutron stars (NSs) in LMXBs undergo episodes of accretion, alternated with periods of quiescence. During an accretion outburst, the NS heats up due to exothermic accretion-induced processes taking place in the crust. Most of this heat slowly diffuses into the stellar core on a thermal diffusion time scale of years. Over many accretion outburst cycles, an equilibrium state is reached when the core temperature is high enough that the heating and cooling (photon and neutrino emission) processes are in balance with each other. Here we present a parameter study to investigate how stellar characteristics and outburst properties affect the long-term temperature evolution of a transiently accreting neutron star. For the first time, not only the effects of the core, but also the crustal properties are considered, in particular that of shallow heating. We track the thermal evolution of a NS undergoing accretion outbursts over a period of $10^5$ yr using our code NSCool. The outburst sequence is based on the regular outbursts observed from the NS X-ray transient Aql X-1. We calculate for each model the time scale over which equilibrium is reached and present these along with the temperature and luminosity parameters of the equilibrium state. We find that shallow heating significant contributes to the equilibrium state. Increasing its strength raises the equilibrium core temperature, while the depth of the shallow heat has a smaller effect on the long-term equilibrium state. 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 than if the star was only heated by deep crustal heating. Deep crustal heating may therefore not be vital to the heating of the core. Additionally, shallow heating can increase the quiescence luminosity to values higher than previously expected.
L. Ootes, R. Wijnands and D. Page
Fri, 7 Jun 19
14/49
Comments: 20 pages, 18 figures, 2 tables, submitted to A&A