Crust Cooling of Soft X-Ray Transients—the Uncertainties of Shallow Heating

Abstract

Crust cooling of soft x-ray transients have been observed after outbursts, an additional shallow heating during accretion in outburst is needed to explain the quiescent light curve. However, shallow heating is significantly different between sources and even within one source between different outbursts, the shallow heat source is unknown yet. Using the open source code ``dStar" which solves the fully general relativistic heat diffusion equation for the crust, we investigate the effect of magnitude and depth of shallow heating on crust cooling and find that some exception sources (Swift J174805.3-244637, MAXI J0556-332 during outburst II and GRO J1750-27) in which the shallow heating may inactive could be explained by a deeper shallow heating mechanism. We compare our results with that get from previous works and find that the shallow heating is model dependent. Besides, the effect of mass and radius of neutron star on shallow heating are studied, it shows that the more compact the star, the less the shallow heating will be needed to fit the crust cooling light curves.

The Impacts of Neutron Star Structure and Base Heating on Type I X-Ray Bursts and Code Comparison

“…The deep crust heating theory suggests that the generated heat may be larger, up to Q b = 2 MeV u −1 (Haensel & Zdunik 1990;Haensel 2003Haensel , 2008; although most of the heating in the deep crust is conducted into the core and carried off by neutrinos, a considerable amount of local heating will occur, which may increase Q b . A yet-unknown shallow heating may also increase Q b (Brown & Cumming 2009;Deibel et al 2015;Lu et al 2022). On the other hand, Q b may be reduced by the competing effect of neutrino cooling (Cumming & Macbeth 2006); the Urca neutrino cooling process in the outer crust may also complicate the estimation of Q b (Schatz et al 2014).…”