Axions from cooling compact stars: pair-breaking processes

TL;DR

This paper calculates axion emission from superfluid neutron stars during cooling, establishing bounds on axion properties based on the balance with neutrino cooling.

Contribution

It introduces a novel calculation of axion emission via pair-breaking processes in superfluid stars, deriving new bounds on axion decay constant and mass.

Findings

Lower bound on axion decay constant: f_a > 6×10^9 T_{c9}^{-1} GeV

Upper bound on axion mass: m_a < 10^{-3} T_{c9} eV

Axion cooling is constrained by neutrino cooling in neutron stars.

Abstract

Once formed in a supernova explosion, a neutron star cools rapidly via neutrino emission during the first 10^4-10^5 yr of its life-time. Here we compute the axion emission rate from baryonic components of a star at temperatures below their respective critical temperatures T_c for normal-superfluid phase transition. The axion production is driven by a charge neutral weak process, associated with Cooper pair breaking and recombination. The requirement that the axion cooling does not overshadow the neutrino cooling puts a lower bound on the axion decay constant f_a > 6 10^9 T_{c9}^{-1} GeV, with T_{c9} = T_c/10^9 K. This translates into a upper bound on the axion mass m_a < 10^{-3} T_{c9} eV.