Urca cooling pairs in the neutron star ocean and their effect on superbursts

TL;DR

This paper investigates how Urca pairs in the neutron star ocean influence thermal regulation and superburst ignition depths, revealing that Urca cooling lowers ocean temperatures but has limited impact on superburst luminosity.

Contribution

It identifies Urca pairs in the neutron star ocean and demonstrates their role in cooling and shifting superburst ignition depths.

Findings

Urca pairs are present throughout the neutron star ocean.

Urca cooling reduces the ocean's steady-state temperature during accretion.

Superburst peak luminosity exceeds Urca neutrino luminosity, minimally affecting light curves.

Abstract

An accretion outburst onto a neutron star deposits hydrogen-rich and/or helium-rich material into the neutron star's envelope. Thermonuclear burning of accreted material robustly produces Urca pairs\textrm{---}pairs of nuclei that undergo cycles of $e^-$-capture and $\beta ^-$-decay. The strong $T^5$ dependence of the Urca cooling neutrino luminosity means that Urca pairs in the neutron star interior potentially remove heat from accretion-driven nuclear reactions. In this study, we identify Urca pairs in the neutron star's ocean \textrm{---} a plasma of ions and electrons overlaying the neutron star crust \textrm{---} and demonstrate that Urca cooling occurs at all depths in the ocean. We find that Urca pairs in the ocean and crust lower the ocean's steady-state temperature during an accretion outburst and that unstable carbon ignition, which is thought to trigger superbursts, occurs deeper than it would otherwise. Cooling superburst light curves, however, are only marginally impacted by cooling from Urca pairs because the superburst peak radiative luminosity $L_{\rm peak}$ is always much greater than the Urca pair neutrino luminosity $L_{\nu}$ in the hot post-superburst ocean.