Deep crustal heating by neutrinos from the surface of accreting neutron stars

TL;DR

This paper introduces a new deep crustal heating mechanism in accreting neutron stars via neutrinos from pion decay, impacting thermal evolution models and requiring consideration of neutrino heating in neutron star cooling studies.

Contribution

It proposes a novel neutrino-based heating process from surface pion production during accretion, affecting neutron star thermal modeling.

Findings

Neutrino heating deposits 1-2 MeV per nucleon in the crust.

Neutrino heating is comparable to pycnonuclear fusion heating.

Higher crustal thermal conductivity is needed to match observations.

Abstract

We present a new mechanism for deep crustal heating in accreting neutron stars. Charged pions ($\pi^+$) are produced in nuclear collisions on the neutron star surface during active accretion and upon decay they provide a flux of neutrinos into the neutron star crust. For massive and/or compact neutron stars, neutrinos deposit $\approx 1\textrm{--} 2 \, \mathrm{MeV}$ of heat per accreted nucleon into the inner crust. The strength of neutrino heating is comparable to the previously known sources of deep crustal heating, such as from pycnonuclear fusion reactions, and is relevant for studies of cooling neutron stars. We model the thermal evolution of a transient neutron star in a low-mass X-ray binary, and in the particular case of the neutron star MXB~1659-29 we show that additional deep crustal heating requires a higher thermal conductivity for the neutron star inner crust. A better knowledge of pion production cross sections near threshold would improve the accuracy of our predictions.