Hybrid cooling of the Cassiopeia A neutron star

TL;DR

This paper introduces a hybrid cooling model for the Cassiopeia A neutron star, combining superfluidity and direct Urca processes, which better matches observed rapid cooling than previous models.

Contribution

It proposes a more complex cooling scenario involving both superfluid neutrons and direct Urca processes, improving the fit to observational data.

Findings

Excellent agreement with observations using the hybrid model

Specific neutron star mass and radius estimates for different equations of state

Relates cooling behavior to the star's equation of state and mass

Abstract

The observed rapid cooling of the neutron star Cassiopeia A is usually interpreted as being caused by transitions of neutrons and protons in the star's core from the normal state to the superfluid and superconducting state. However, this so-called "minimal" cooling paradigm faces the problem of numerically simulating the observed anomalously fast drop in the neutron star surface temperature using theoretical neutrino energy losses from superfluid neutrons. As a solution to this problem, I propose a somewhat more complex cooling model, in which, in addition to superfluid neutrons, direct Urca processes from a very small central part of the neutron star core are also involved. Numerical simulations of the cooling trajectory in this scenario show excellent agreement with observations of the Cassiopeia A neutron star. The proposed cooling scenario unambiguously relates the used equation of state and the mass of the neutron star. For a neutron star constructed according to BSk25 equation of state, the most appropriate are the mass $M=1.62M_{\sun}$ and the radius $R=12.36$ km. If BSk24 equation of state is used, then the most suitable solution is $M=1.60M_{\sun}$ and $R=12.55$ km.