  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.

Solar and Stellar Astrophysics