Rotochemical heating with a density-dependent superfluid energy gap in neutron stars

TL;DR

This paper investigates how density-dependent superfluid energy gaps affect rotochemical heating in neutron stars, comparing model predictions with observed surface temperatures to better understand superfluid properties.

Contribution

It introduces models with density-dependent superfluid gaps into rotochemical heating calculations and compares their predictions with observational data.

Findings

Density-dependent gaps influence surface temperature predictions.

Certain gap models better match the observed temperature of pulsar J0437-4715.

The study constrains superfluid gap models based on thermal observations.

Abstract

When a rotating neutron star loses angular momentum, the reduction of the centrifugal force makes it contract. This perturbs each fluid element, raising the local pressure and originating deviations from beta equilibrium, inducing reactions that release heat (rotochemical heating). This effect has previously been studied by Fern\'andez and Reisenegger for neutron stars of non-superfluid matter and by Petrovich and Reisenegger for superfluid matter, finding that the system in both cases reaches a quasi-steady state, corresponding to a partial equilibration between compression, due to the loss of angular momentum, and reactions that try to restore the equilibrium. However, Petrovich and Reisenegger assumes a constant value of the superfluid energy gap, whereas theoretical models predict density-dependent gap amplitudes, and therefore gaps that depend on the location in the star. In this work, we try to discriminate between several proposed gap models, comparing predicted surface temperatures to the value measured for the nearest millisecond pulsar, J0437-4715.