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

P. S. Shternin,1 D. G. Yakovlev,1 C. O. Heinke,2 W. C. G. Ho,3 D. J. Patnaude4
1Ioffe Physical Technical Institute, Politekhnicheskaya 26, 194021 St. Petersburg, Saint-Petersburg, Russia
2Department of Physics, U. of Alberta, Room 238 CEB, 11322-89 Avenue, Edmonton, AB, T6G 2G7, Canada
3School of Mathematics, U. of Southampton, Southampton, SO17 1BJ, United Kingdom
4Smithsonian Astrophysical Observatory, Cambridge, MA 02138, USA

According to recent results of Ho and Heinke (2009) and Heinke and Ho (2010), the Cassiopeia~A supernova remnant contains a young (∼ 330 yr old) neutron star (NS) 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 Tcn(ρ) for the onset of neutron superfluidity [Tcn(ρ) should have a wide peak with maximum ∼ (7–9)× 108 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.

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