Cassiopeia A and direct URCA cooling

TL;DR

This paper models neutron star cooling, especially Cas A's rapid temperature decline, using a microscopic nuclear equation of state with direct Urca processes and pairing gaps, suggesting strong Urca processes are plausible.

Contribution

It demonstrates that specific nuclear pairing gaps and the inclusion of direct Urca processes can explain Cas A's cooling behavior within a microscopic nuclear framework.

Findings

Large proton $^1S_0$ gaps are necessary.

Small neutron $^3PF_2$ gaps are compatible.

Strong direct Urca processes cannot be ruled out.

Abstract

We model neutron star cooling, in particular the current rapid cooldown of the neutron star Cas A, with a microscopic nuclear equation of state featuring strong direct Urca processes and using compatible nuclear pairing gaps as well as effective masses. Several scenarios are possible to explain the features of Cas A, but only large and extended proton $^1S_0$ gaps and small neutron $^3PF_2$ gaps are able to accommodate also the major part of the complete current cooling data. We conclude that the possibility of strong direct Urca processes cannot be excluded from the cooling analysis.