Session X3: Neutron Star Physics

The cooling of neutron star transients

Accretion of matter from a stellar companion compresses the crust of a neutron star and induces reactions that heat its interior. The temperature in the crust is set by balancing this nuclear heating with thermal radiation from the surface and neutrino emission from the crust and core. As a result, the crust is hotter than the core. Many neutron stars accrete intermittently; when the accretion halts, the crust cools. Recent observations of KS~1731$-$260 and MXB~1659$-$29 show a roughly exponential decline in the luminosity following an accretion outburst, consistent with this cooling. Moreover, the transient 1H~1905$+$000 has a very low quiescent luminosity, $<10^{31}$ergs~s$^{-1}$. In this talk, I describe what these observations tell us about the physics of the neutron star crust and core. In particular, I explore how the thermal timescale depends on the equation of state of the neutron star core and the constraints on the strength of the neutrino emissivity. Our models of the heating and cooling in the neutron star crust incorporate new calculations of electron capture rates in the outer crust. We find that the heat deposition in the outer crust is substantially larger than previous estimates. In addition, our models allow us to compute the heating in the outer crust for a wide range of possible crust compositions.   

Observations of Cooling Neutron Star Transients

Observations of Soft X--ray Transients (SXTs) with the X--ray satellites {\it Chandra} and XMM--{\it Newton} turned out to have a profound impact on the study of cooling neutron stars. Model fits especially to those cases where the accretion history is well known, such as quasi--persistent sources (sources that have accreted matter at a high rate for $\sim$10 years) provide good test grounds for theoretical models. I will review the observations of quiescent neutron star SXTs. In particular, I will discuss the neutron star SXT with the lowest quiescent luminosity, 1H~1905+000, and focus on the implications of our deep 300 ksec Chandra observation of this source on the neutron star EoS and the relation between the quiescent luminosity of neutron stars and black holes.