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

Abstract

According to recent results of Ho & Heinke, the Cassiopeia A supernova remnant contains a young (≈330-yr-old) neutron star (NS) which has carbon atmosphere and shows notable decline of the effective surface temperature. We report a new (2010 November) 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 cn (ρ) for the onset of neutron superfluidity [T cn (ρ) should have a wide peak with maximum ≈ (7-9) × 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.

Neutron Stars—Cooling and Transport

“…A spectacular example is the neutron star CXO J232327.9+584842 in the Cassiopeia A supernova remnant, dubbed Cas A NS, which shows an unexpectedly appreciable temperature decline during several years (Heinke and Ho 2010;Elshamouty et al 2013) (but see Posselt et al 2013 for tentative alternative interpretations of the observations). This decline can be comfortably explained by the PBF emission (Page et al 2011;Shternin et al 2011; see also Ho et al 2015 for a recent analysis including modern observational data).…”

Magnetic field evolution in superconducting neutron stars

“…Anderson & Itoh (1975) first proposed that the neutron star's dynamical evolution during and after a glitch could be explained by the weak viscous properties of a superfluid component that is coupled to the crust. Moreover, recent spectral analyses of the neutron star in the supernova remnant Cassiopeia A indicate that the surface temperature of this young object decreases faster than one would expect from standard cooling models (Page et al 2011;Shternin et al 2011). The rapid cooling could be explained by enhanced neutrino emission, resulting from the onset of neutron superfluidity and proton superconductivity in the core.…”

Sources of Gravitational Waves: Theory and Observations

“…Neutron-star equation of state: By determining the EoS of NSs we can infer the composition and structure of NS cores, which has remained largely unknown nearly half-a-century after the discovery of the first pulsar, although astronomical observations have begun to indicate hints of neutron superfluidity in the core [386,387]. Tidal interaction in compact binaries, where one or both of the companions is a NS, depends on the EoS.…”