Crust structure and thermal evolution of neutron stars in soft X-ray transients

Abstract

Context. Thermal evolution of neutron stars in soft X-ray transients (SXTs) is sensitive to the equation of state, nucleon superfluidity, composition and structure of the crust. Comparison of observations of their crust cooling with simulations is a powerful tool of verification of theoretical models of the dense matter. Aims. We study the effect of physics input on thermal evolution of neutron stars in SXTs. In particular, we consider different modern models of the sources of deep crustal heating during accretion episodes and the effects brought about by impurities embedded in the crust during its formation. Methods. We simulate thermal structure and evolution of episodically accreting neutron stars under different assumptions on the crust composition and on the distribution of heat sources and impurities. For the nonaccreted crust, we consider the nuclear charge fluctuations that arise at crust formation. For the accreted crust, we compare different theoretical models of composition and internal heating. We also compare results of numerical simulations with observations of the crust cooling in SXT MXB 1659−29. Results. The nonaccreted part of the inner crust of a neutron star can have a layered structure, with almost pure crystalline layers interchanging with layers composed of mixtures of different nuclei. The latter layers have relatively low thermal conductivities, which affects thermal evolution of the transients. The impurity distribution in the crust strongly depends on the models of the dense matter and the crust formation scenario. The shallow heating that is needed to reach agreement between the theory and observations depends on characteristics of the crust and envelope.

Accreting neutron stars: heating of the upper layers of the inner crust

“…Accreting neutron stars (NSs) in low-mass X-ray binaries (LMXBs) are observed and actively studied for more than 50 years (Giacconi et al, 1962;Shklovsky 1967;Fujimoto et al, 1984;Yakovlev et al, 2004;Heinke et al, 2009;Wijnands, Degenaar & Page 2017;Potekhin, Chugunov & Chabrier 2019;Liu et al, 2021;Potekhin & Chabrier 2021;Fortin et al, 2021). During this time, a generally accepted picture of the basic physical processes in such systems has emerged.…”

Deep crustal heating for realistic compositions of thermonuclear ashes

“…Comparison of the NS crust cooling curves to the theoretical models of heat relaxation (Rutledge et al 2002;Shternin et al 2007;Brown & Cumming 2009;Cackett et al 2010;Page & Reddy 2013;Degenaar, Wijnands & Miller 2013;Meisel et al 2018;Parikh et al 2019;Wijngaarden et al 2020;Potekhin & Chabrier 2021), as well as modelling of the steady-state thermal configurations (Brown et al 1998;Yakovlev, Levenfish & Haensel 2003;Yakovlev et al 2004;Heinke et al 2007Heinke et al , 2009Wijnands, Degenaar & Page 2013;Beznogov & Yakovlev 2015;Han & Steiner 2017;Fortin et al 2018;Brown et al 2018;Potekhin et al 2019;Fortin et al 2021) allows one to constrain the properties of NS matter. To model the thermal evolution one needs to know the profile of energy release, the crust equation of state (EOS) and composition, which can be found by studying the chain of nuclear reactions in the crust of an accreting NS.…”

Quiescent luminosities of transiently accreting neutron stars with neutrino heating due to charged pion decay

“…A comparison of the theory of quiescent luminosity of neutron star in SXTs with observations can provide important information. Since the heating and cooling depend on neutron star properties such as deep crustal heating, shallow heating, mass, equation of state, superfluidity, composition of the stellar core and outer layers, such a comparison can help us * heleiliu@xju.edu.cn understand both the crust and core properties of neutron star [2][3][4][5][6][7].…”

“…[8][9][10][11][12]). There have been many works about the study of quiescent luminosities of transiently accreting neutron stars [4][5][6][7][16][17][18][19][20]. From these studies, a strong superfluidity in the cores of low mass stars and a fast neutrino emission in the cores of high-mass stars are needed to explain the observations.…”