Current Status of Neutron Star Thermal Evolution Theories and Related Problems

摘要

Since the first detection of thermal radiation directly from the surface of neutron stars (NSs), the number of NSs with temperature detection has increased significantly. By carefully analyzing the data and comparing the results with thermal evolution theories, we now have realistic hope for exploring the nature and properties of ultrahigh density matter. Currently, the data of Vela pulsar already suggest that the core of a high mass NS includes such exotic particles as pion condensates. However, to confirm such possibilities, more such data for more NSs are needed. Fortunately, it was found recently that soft X-ray transients (SXTs) in low-mass X-ray binaries (LMXB) can provide additional such extra data when they are in quiescence. Very recently it was reported that the compact object in the supernova remnant Cassiopeia A (Cas A) is a neutron star (NS) emitting X-rays from the whole stellar surface. Moreover, the flux decreased significantly over the last 10 years. We propose a promising model where the Cas A compact remnant is a relatively large NS with a stiff equation of state (EOS) undergoing nonstandard cooling with superfluid suppression. In this model the rapid cooling of Cas A NS is naturally explained by the thermal relaxation. We demonstrate that when not only the latest data of Cas A NS but also all other existing temperature data of other isolated NSs are taken into account, this model is naturally consistent with both current physical theories and observations. Here we will review these developments, giving special emphasis on the latest exciting developments. We emphasize that future observatories such as ASTRO-H and IXO are expected to mean breakthroughs in this and related area.