Impacts of the direct URCA and Superfluidity inside a Neutron Star on Type-I X-Ray Bursts and X-Ray Superbursts

TL;DR

This paper studies how neutrino cooling mechanisms, especially the direct Urca process and nucleon superfluidity, affect the light curves and recurrence times of type-I X-ray bursts and superbursts in neutron stars.

Contribution

It investigates the effects of the direct Urca process and nucleon superfluidity on neutron star cooling and X-ray burst properties using a general-relativistic stellar-evolution model.

Findings

Direct Urca process increases recurrence time and peak luminosity of bursts.

Nucleon superfluidity can suppress the cooling effects of the Urca process.

Model comparisons with observed burst data show significant effects within certain parameters.

Abstract

We investigate the impacts of neutrino cooling mechanism inside the neutron star (NS) core on the light curves of type-I X-ray bursts and X-ray superbursts. From several observations of NS thermal evolution, physical processes of fast neutrino cooling, such as the direct Urca (DU) process, are indicated. They significantly decrease the surface temperature of NSs, though the cooling effect could be suppressed by nucleon superfluidity. In the present study, focusing on the DU process and nucleon superfluidity, we investigate the effects of NS cooling on the X-ray bursts using a general-relativistic stellar-evolution code. We find that the DU process leads find the longer recurrence time and the higher peak luminosity, which could be obstructed by the neutrons superfluidity. We also apply our burst models to the comparison with {\it Clocked burster} GS 1826$-$24, and to the recurrence time of superburst triggered by carbon ignition. These effects are significant within a certain range of binary parameters and uncertainty of the NS equation of state.