Geminga: A Cooling Superfluid Neutron Star

We show that Geminga's surface temperature can be understood by both types of neutrino cooling scenarios, i.e, slow neutrino cooling by the modified Urca process or fast neutrino cooling by the direct Urca process or by some exotic matter, and thus does not allow us to discriminate between these two competing schemes. However, for both types of scenarios, agreement with the observed temperature can only be obtained if BARYON PAIRING IS PRESENT IN MOST, IF NOT ALL, OF THE CORE OF THE STAR. We also comment on the recent temperature estimates for PSR 0656+14 and PSR 1055-52, which pertain to the same photon cooling era. We argue though that observational evidence for the slow neutrino cooling model (the ``standard'' model) is in fact very dim and that the interpretation of the surface temperature of all neutron stars could be done with a reasonable theoretical a priori within the fast neutrino cooling scenarios only. In this case, Geminga, PSR 0656+14, and PSR 1055-52 all show evidence of baryon pairing down to their very centers. xxx----To be published in Ap.J. (June 10 isue).---xxxComment: 26 pages, AAS LaTeX, CAL-54

Pairing and the Cooling of Neutron Stars

In this review, I present a brief summary of the impact of nucleon pairing at supra-nuclear densities on the cooling of neutron stars. I also describe how the recent observation of the cooling of the neutron star in the supernova remnant Cassiopeia A may provide us with the first direct evidence for the occurrence of such pairing. It also implies a size of the neutron 3P-F2 energy gap of the order of 0.1 MeV.Comment: Contributed chapter in "50 Years of Nuclear BCS", edited by R. A. Broglia and V. Zelevinsk

The Geminga neutron star: Evidence for nucleon superfluidity at very high density

A comparison of the recent age and temperature estimates of the Geminga neutron star with cooling models is presented. This star is already in the photon cooling era and it is shown that its temperature can be understood within both the slow and fast neutrino emission scenarios and consequently will not allow discrimination between these two scenarios. However in both cases agreement of the theoretical cooling curves with the observed temperature depends crucially on the presence of nucleon pairing in most, if not all, of the core

Surface temperature of a magnetized neutron star and interpretation of the ROSAT data. II

We complete our study of pulsars' non-uniform surface temperature and of its effects on their soft X-ray thermal emission. Our previous work had shown that, due to gravitational lensing, dipolar fields cannot reproduce the strong pulsations observed in Vela, Geminga, PSR 0656+14, and PSR 1055-52. Assuming a standard neutron star mass of 1.4 Msol, we show here that the inclusion of a quadrupolar component, if it is suitably oriented, is sufficient to increase substantially the pulsed fraction, Pf, up to, or above, the observed values if the stellar radius is 13 km or even 10 km. For models with a radius of 7 km the maximum pulsed fraction obtainable with (isotropic) blackbody emission is of the order of 15% for orthogonal rotators (Vela, Geminga and PSR 1055-52) and only 5% for an inclined rotator as PSR 0656+14. Given the observed values this indicates that the neutron stars in Geminga and PSR 0656+14 have radii significantly larger than 7 km and, given the very specific quadrupole components required to increase Pf, even radii of the order of 10 km may be unlikely in all four cases. We confirm our previous finding that the pulsed fraction always increases with photon energy, below about 1 keV, when blackbody emission is used and show that it is due to the hardenning of the blackbody spectrum with increasing temperature. The observed decrease of pulsed fraction may thus suggest that the emitted spectrum softens with increasing temperature. Finally, we apply our model to reassess the magnetic field effect on the outer boundary condition used in neutron star cooling models and show that, in contradistinction to several previous claims, it is very small and most probably results in a slight reduction of the heat flow through the envelope.Comment: 17 pages with 8 figures. Uses AASTeX v4.0 macro. Submitted to Ap.

Cooling of Accretion-Heated Neutron Stars

We present a brief, observational review about the study of the cooling behaviour of accretion-heated neutron stars and the inferences about the neutron-star crust and core that have been obtained from these studies. Accretion of matter during outbursts can heat the crust out of thermal equilibrium with the core and after the accretion episodes are over, the crust will cool down until crust-core equilibrium is restored. We discuss the observed properties of the crust cooling sources and what has been learned about the physics of neutron-star crusts. We also briefly discuss those systems that have been observed long after their outbursts were over, i.e, during times when the crust and core are expected to be in thermal equilibrium. The surface temperature is then a direct probe for the core temperature. By comparing the expected temperatures based on estimates of the accretion history of the targets with the observed ones, the physics of neutron-star cores can be investigated. Finally, we discuss similar studies performed for strongly magnetized neutron stars in which the magnetic field might play an important role in the heating and cooling of the neutron stars.Comment: Has appeared in Journal of Astrophysics and Astronomy special issue on 'Physics of Neutron Stars and Related Objects', celebrating the 75th birth-year of G. Srinivasan. In case of missing sources and/or references in the tables, please contact the first author and they will be included in updated versions of this revie