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 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.

We simulate the cooling of the neutron star in the X-ray transient KS 1731−260 after the source returned to quiescence in 2001 from a long (≳12.5 yr)

We explore the thermal state of the neutron star in the Cassiopeia A supernova remnant using the recent result of Ho & Heinke that the thermal ra

We calculate the shear viscosity $\eta \approx {\eta }_{\mathrm{e}\mu }+{\eta }_{\mathrm{n}}$ in a neutron star core composed of nucleons, electrons, and muons (${\eta }_{\mathrm{e}\mu }$ being the electron-muon viscosity, mediated by collisions of electrons and muons with charged particles, and ${\eta }_{\mathrm{n}}$ the neutron viscosity, mediated by neutron-neutron and neutron-proton collisions). Deriving ${\eta }_{\mathrm{e}\mu }$, we take into account the Landau damping in collisions of electrons and muons with charged particles via the exchange of transverse plasmons. It lowers ${\eta }_{\mathrm{e}\mu }$ and leads to the nonstandard temperature behavior ${\eta }_{\mathrm{e}\mu }\propto T^{-5/3}$. The viscosity ${\eta }_{\mathrm{n}}$ is calculated taking into account that in-medium effects modify nucleon effective masses in dense matter. Both viscosities, ${\eta }_{\mathrm{e}\mu }$ and ${\eta }_{\mathrm{n}}$, can be important, and both are calculated including the effects of proton superfluidity. They are presented in the form valid for any equation of state of nucleon dense matter. We analyze the density and temperature dependence of $\eta$ for different equations of state in neutron star cores, and compare $\eta$ with the bulk viscosity in the core and with the shear viscosity in the crust.

The thermal evolution of young neutron stars (NSs) reflects the neutrino emission properties of their cores. Heinke & Ho measured a 3.6% ± 0.6% decay in the surface temperature of the Cassiopeia A (Cas A) NS between 2000 and 2009, using archival data from the Chandra X-ray Observatory ACIS-S detector in Graded mode. Page et al. and Shternin et al. attributed this decay to enhanced neutrino emission from a superfluid neutron transition in the core. Here we test this decline, combining analysis of the Cas A NS using all Chandra X-ray detectors and modes (HRC-S, HRC-I, ACIS-I, ACIS-S in Faint mode, and ACIS-S in Graded mode) and adding a 2012 May ACIS-S Graded mode observation, using the most current calibrations (CALDB 4.5.5.1). We measure the temperature changes from each detector separately and test for systematic effects due to the nearby filaments of the supernova remnant. We find a 0.92%-2.0% decay over 10 yr in the effective temperature, inferred from HRC-S data, depending on the choice of source and background extraction regions, with a best-fit decay of 1.0% ± 0.7%. In comparison, the ACIS-S Graded data indicate a temperature decay of 3.1%-5.0% over 10 yr, with a best-fit decay of 3.5% ± 0.4%. Shallower observations using the other detectors yield temperature decays of 2.6% ± 1.9% (ACIS-I), 2.1% ± 1.0% (HRC-I), and 2.1% ± 1.9% (ACIS-S Faint mode) over 10 yr. Our best estimate indicates a decline of 2.9% ± 0.5%stat ± 1.0 sys% over 10 yr. The complexity of the bright and varying supernova remnant background makes a definitive interpretation of archival Cas A Chandra observations difficult. A temperature decline of 1%-3.5% over 10 yr would indicate extraordinarily fast cooling of the NS that can be regulated by superfluidity of nucleons in the stellar core.