Cooling neutron star in the Cassiopeia~A supernova remnant: Evidence for superfluidity in the core

TL;DR

Recent observations of the neutron star in Cassiopeia A show a temperature decline consistent with the onset of neutron superfluidity in the core, providing strong evidence for superfluidity in neutron star interiors.

Contribution

This paper confirms the temperature decline of the Cassiopeia A neutron star and interprets it as evidence for neutron superfluidity, constraining core properties and neutrino emission processes.

Findings

Neutron star temperature decline is consistent with superfluid transition.

Constraints on neutron superfluidity onset temperature and density dependence.

Evidence for nucleon superfluidity from cooling neutron star observations.

Abstract

According to recent results of Ho & Heinke (2009) and Heinke & Ho (2010), the Cassiopeia A supernova remnant contains a young neutron star which has carbon atmosphere and shows noticeable decline of the effective surface temperature. We report a new (November 2010) Chandra observation which confirms the previously reported decline rate. The decline is naturally explained if neutrons have recently become superfluid (in triplet-state) in the NS core, producing a splash of neutrino emission due to Cooper pair formation (CPF) process that currently accelerates the cooling. This scenario puts stringent constraints on poorly known properties of NS cores: on density dependence of the temperature $T_\mathrm{cn}(\rho)$ for the onset of neutron superfluidity [$T_\mathrm{cn}(\rho)$ should have a wide peak with maximum $\approx (7-9)\times 10^8$ K], on the reduction factor $q$ of CPF process by collective effects in superfluid matter ($q > 0.4$), and on the intensity of neutrino emission before the onset of neutron superfluidity (30--100 times weaker than the standard modified Urca process). This is serious evidence for nucleon superfluidity in NS cores that comes from observations of cooling NSs.