Vortex Creep Heating in Neutron Star Cooling: New Insights into Thermal Evolution of Heavy Neutron Stars

TL;DR

This paper investigates how vortex creep heating influences the thermal evolution of neutron stars, especially massive ones, by extending cooling models to include internal heating mechanisms alongside standard processes.

Contribution

It introduces an extended cooling framework incorporating vortex creep heating and direct Urca processes, providing new insights into neutron star thermal evolution.

Findings

Vortex creep heating can significantly slow down neutron star cooling.

The combined effect of VCH and DUrca explains the warmth of old, massive neutron stars.

Heating mechanisms alter the expected temperature trajectories of neutron stars.

Abstract

Neutron stars provide unique laboratories for probing physics of dense nuclear matter under extreme conditions. Their thermal and luminosity evolution reflects key internal properties such as the equation of state (EoS), nucleon superfluidity and superconductivity, envelope composition, and magnetic field, and so on. Recent observations [\textit{e.g.}, V. Abramkin \textit{et al.,} ApJ \textbf{924}, 128 (2022)] have revealed unexpectedly warm old neutron stars, which cannot be explained by standard neutrino-photon cooling models. The failure of the standard cooling models implies the presence of additional internal heating mechanism. Building on the previous study [M. Fujiwara \textit{et al}., JCAP \textbf{03}, 051 (2024)], which proposed vortex creep heating (VCH) from the frictional motion of superfluid vortices as a viable mechanism, we extend the cooling framework to include both VCH and direct Urca (DUrca) processes. These are implemented in our code to explore their combined impact, particularly for massive neutron stars where DUrca operates. By varying rotational parameters ($P$, $\dot{P}$, $P_0$), EoS models (APR, BSk24), pairing gaps, and envelope compositions, we examine how heating-cooling interplay shapes the temperature evolution. Our results show that VCH can substantially mitigate the rapid cooling driven by DUrca, offering new evolutionary pathways for massive neutron stars.