Cooling of neutron stars in soft X-ray transients with realistic crust composition

TL;DR

This study models neutron star crust cooling in soft X-ray transients using advanced nuclear evolution models, comparing traditional and modern theories, and highlights the need for additional factors to match observations.

Contribution

It introduces a thermodynamically consistent model of accreted crust composition and compares its predictions with traditional models against observational data.

Findings

Both models can explain the thermal evolution with modifications.

Additional factors like shallow heating are necessary for accurate fits.

Modern models are comparable to traditional ones in explaining observations.

Abstract

Thermal radiation of neutron stars in soft X-ray transients (SXTs) in a quiescent state is believed to be powered by the heat deposited in the stellar crust due to nuclear reactions during accretion. Confronting observations of this radiation with simulations helps to verify theoretical models of the dense matter in neutron stars. We simulate the thermal evolution of the SXTs with theoretical models of the equation of state and composition of the accreted crust. The new family of such models were recently developed within a thermodynamically consistent approach by modeling the nuclear evolution of an accreted matter as it sinks toward the stellar center, starting from representative thermonuclear ash compositions. The crust cooling curves computed with the traditional and modern theory are compared with observations of SXTs MXB 1659-29 and IGR J17480-2446. We show that the new and traditional models of the accreted neutron star crusts are similar in their capability to explain the thermal evolution of neutron stars in SXTs. Both kinds of models require inclusion of additional ingredients not supplied by the current theory, such as the shallow heating and variation of thermal conductivity, to fit observations.