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

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.

12 pages, 8 figures, 3 Tables

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• NinjaSat monitoring of Type-I X-ray bursts from the clocked burster SRGA J144459.2$-$604207
• The Impacts of Neutron-Star Structure and Base Heating on Type I X-Ray Bursts and Code Comparison
• Gravitational wave asteroseismology of accreting neutron stars in a steady state
• Constraining Accreted Neutron Star Crust Shallow Heating with the Inferred Depth of Carbon Ignition in X-ray Superbursts
• New ${}^{63}$Ga(p,$\gamma$)${}^{64}$Ge and ${}^{64}$Ge(p,$\gamma$)${}^{65}$As reaction rates corresponding to the temperature regime of thermonuclear X-ray bursts