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

Abstract

Neutron stars in low-mass X-ray binaries are thought to be heated up by accretion-induced exothermic nuclear reactions in the crust. The energy release and the location of the heating sources are important ingredients of the thermal evolution models. Here we present thermodynamically consistent calculations of the energy release in three zones of the stellar crust: at the outer-inner crust interface, in the upper layers of the inner crust (up to the density ρ ≤ 2 × 1012 g cm−3), and in the underlying crustal layers. We consider three representative models of thermonuclear ashes (Superburst, Extreme rp, and Kepler ashes). The energy release in each zone is parameterized by the pressure at the outer-inner crust interface, Poi, which encodes all uncertainties related to the physics of the deepest inner-crust layers. Our calculations allow us to set new theoretical lower limits on the net energy release (per accreted baryon): Q ≳ 0.28 MeV for Extreme rp ashes and Q ≳ 0.43 − 0.51 MeV for Superburst and Kepler ashes. Our results can be directly incorporated into numerical codes and provide an opportunity to constrain Poi by comparing thermal evolution models of accreting neutron stars with observations.

Thermodynamically Consistent Equation of State for an Accreted Neutron Star Crust

“…Thus, the approach discussed in this subsection is likely applicable to accreted crust with a multicomponent composition of initial ashes. Note, however, that in the latter case, noticeable fraction of the heat can be released in the intermediate layers of the inner crust [29,30], so that the constraint should be applied to the residual part of the heat release, which also can be estimated within the AMT approach [29,30].…”

“…For numerical illustration (Section IV) we limit ourselves to a pure 56 Fe ash composition (see Refs. [28][29][30] for multicomponent models, which, however, are limited to not-too-deep crustal layers) and apply the recently suggested CLD model with proton shell effects added on top (CLD+sh model, [31]). Using this model, we constrain the pressure P oi in Section V. In Section VI, we analyze heat release at the innermost regions of inner crust and, in Section VII, present a heuristic energy-based approach to predict FAC properties for more refined nuclear physics models.…”

Constraining Accreted Neutron Star Crust Shallow Heating with the Inferred Depth of Carbon Ignition in X-ray Superbursts

“…The remaining nuclear physics uncertainties that were not investigated in this work include 𝑒 − -capture heating and deep crustal heating, where the latter is primarily dependent on the crustcore transition pressure and hence the dense-matter equation-ofstate (Shchechilin et al 2021(Shchechilin et al , 2022. Neither of these heat sources would remove the need for shallow heating, but they do impact the accreting neutron star thermal profile and therefore likely have some influence on the shallow heating constraints inferred from 𝑦 ign (Cooper et al 2009).…”

Impacts of the Direct Urca and Superfluidity inside a Neutron Star on Type I X-Ray Bursts and X-Ray Superbursts

“…For = -M 4 9  without the DU process, CF88 and TK21 mostly match with the observations of 4U 1820−30, but for T18, Δt sb is lower by 1-2 orders of magnitude. Considering also the necessity of the hot regime implying high -M 9  , although the uncertainties of crustal heating are crucial (e.g., Shchechilin et al 2021Shchechilin et al , 2022, T18 seems to be unpreferred. Many previous studies of thermal evolution of accreting NSs consider the shallow heating that is necessary for explaining some hot accreting NSs (Brown & Cumming 2009;Deibel et al 2015;Waterhouse et al 2016;Ootes et al 2016Ootes et al , 2018, though their physical mechanism has been still unknown and some candidates have been investigated (e.g., Fattoyev et al 2018;Liu et al 2021b for neutrino heating scenario due to charged pion decay).…”