Axion mass limit from observations of the neutron star in Cassiopeia A

Lev B Leinson

doi:10.1088/1475-7516/2014/08/031

Abstract

Direct Chandra observations of a surface temperature of isolated neutron star in Cassiopeia A (Cas A NS) and its cooling scenario which has been recently simultaneously suggested by several scientific teams put stringent constraints on poorly known properties of the superfluid neutron star core. It was found also that the thermal energy losses from Cas A NS are approximately twice more intensive than it can be explained by the neutrino emission. We use these unique data and well-defined cooling scenario to estimate the strength of KSVZ axion interactions with neutrons. We speculate that enlarged energy losses occur owing to emission of axions from superfluid core of the neutron star. If the axion and neutrino losses are comparable we find cn2ma2∼ 5.7× 10-6 eV2, where ma is the axion mass, and cn is the effective Peccei-Quinn charge of the neutron. (Given the QCD uncertainties of the hadronic axion models, the dimensionless constant cn could range from -0.05 to 0.14.)

Highlights

The neutrino pair emission caused by recombination of thermally broken Cooper pairs [34, 35] occurs through neutral weak currents generated by spin fluctuations of the nucleons [36, 37]
The dominating energy losses occur owing to the pair breaking and formation (PBF) neutrino radiation from triplet pairing of neutrons, while the proton superfluidity quenches the other neutrino reactions which efficiently operate in normal nucleonic systems
The dominant axion emission from a hot neutron star core is caused by spin fluctuations of non-relativistic neutrons

Summary

Introduction

The neutrino pair emission caused by recombination of thermally broken Cooper pairs [34, 35] occurs through neutral weak currents generated by spin fluctuations of the nucleons [36, 37]. Neutrino and axion energy losses from superfluid NS core Since the neutrino emission occurs mainly owing to neutron spin fluctuations, the part of the interaction Hamiltonian relevant for PBF processes is (we use natural units, kB = 1): Hνn

Results

Conclusion