Axion Emission from Proton Cooper Pairs in Neutron Stars

TL;DR

This paper analyzes axion emission from proton Cooper pairs in neutron stars, confirming previous models and highlighting significant deviations affecting axion constraints from neutron star cooling data.

Contribution

It re-derives axion emissivity from proton pairs, clarifies discrepancies with recent studies, and assesses the impact of neutron-star equations of state on emission estimates.

Findings

Re-derivation confirms consistency with most literature.

Discrepancy with recent study leads to over an order-of-magnitude difference.

Uncertainty from equations of state does not resolve the emissivity discrepancy.

Abstract

We investigate axion emission from singlet proton Cooper pairs in neutron stars, a process that dominates axion emission in young neutron stars in the KSVZ model. By re-deriving its emissivity, we confirm consistency with most existing literature, except for a recent study that exhibits a different dependence on the effective mass. This discrepancy results in more than an order-of-magnitude deviation in emissivity, significantly impacting constraints on the KSVZ axion from the cooling observations of the Cassiopeia A neutron star. Furthermore, we examine uncertainties arising from neutron-star equations of state and their role in the discrepancy, finding that the large deviation persists regardless of the choice of equations of state.