It is important that we report the new observation. In particular, we discuss the dependence of cooling curves on the poorly known efficiency of neutrino emission due to Cooper pair formation (CPF) process and the possibility to interpret observations of all cooling stars by one model of superdense matter. However, the two papers are different in details and can be regarded as complementary. (2010) who proposed similar explanation of the Cas A NS observations. When this Letter was nearly completed, we became aware of the paper by Page et al. We interpret them as a manifestation of neutron superfluidity in the Cas A NS. These results are confirmed by new observations we report below. They interpret it as direct observation of Cas A NS cooling, the phenomenon which has never been observed before for any isolated NS. It cools via neutrino emission from the stellar core its neutrino luminosity is not very different from that provided by the modified Urca process.įollowing Ho & Heinke (2009), Heinke & Ho (2010) analysed Chandra observations of the Cas A NS during 10 yr and found a steady decline of T s by about 4 per cent. The authors concluded that the Cas A NS has already reached the stage of internal thermal relaxation. (2011) compared these observations with the NS cooling theory. It is the youngest in the family of observed cooling NSs.

It emits thermal radiation from the entire surface and has the surface temperature typical for an isolated NS. These parameters indicate that the compact source is an NS with the carbon atmosphere. The gravitational mass of the object, as inferred from the fits, is M≈ 1.3–2 M ⊙, circumferential radius R≈ 8–15 km and the non-redshifted effective surface temperature T s∼ 2 × 10 6 K ( Yakovlev et al. Recently Ho & Heinke (2009) have shown that the observed spectrum is successfully fitted taking a carbon atmosphere model with a low magnetic field ( B≲ 10 11 G). The fits of the observed X-ray spectrum with magnetized or non-magnetized hydrogen atmosphere models or with blackbody spectrum revealed too small size of the emission region (could be hotspots on NS surface although no pulsations have been observed, e.g. (2001), and Pavlov & Luna (2009), but its nature has been uncertain. The compact central source was identified in first light Chandra X-ray observations ( Tananbaum 1999) and studied by Pavlov et al. The Cas A age is reliably estimated as t≈ 330 ± 20 yr from observations of the remnant expansion ( Fesen et al. The distance to the remnant is d= 3.4 +0.3 −0.1 kpc ( Reed et al. We analyse observations of the NS in the supernova remnant Cassiopeia A (Cas A).

Pethick 1992 Yakovlev & Pethick 2004 Page, Geppert & Weber 2006 Page et al. One can explore these properties by studying the cooling of isolated NSs (see e.g. Haensel, Potekhin & Yakovlev 2007 Lattimer & Prakash 2007).

It is well known that neutron star (NS) cores contain superdense matter whose properties are still uncertain (see e.g. This is serious evidence for nucleon superfluidity in NS cores that comes from observations of cooling NSs.ĭense matter, equation of state, neutrinos, stars: neutron, supernovae: individual: Cassiopeia A, X-rays: stars 1 Introduction 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 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). 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. We report a new (2010 November) Chandra observation which confirms the previously reported decline rate. 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.

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