Abstract We investigate the impacts of the 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 to a longer recurrence time and higher peak luminosity, which could be obstructed by the neutrons’ superfluidity. We also apply our burst models to the comparison with Clocked burster GS 1826−24, and to the recurrence time of a superburst triggered by carbon ignition. These effects are significant within a certain range of binary parameters and the uncertainty of the NS equation of state.
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This content will become publicly available on June 1, 2024
The Impacts of Neutron Star Structure and Base Heating on Type I X-Ray Bursts and Code Comparison
Abstract Type I X-ray bursts are rapidly brightening phenomena triggered by thermonuclear burning on the accreting layers of a neutron star (NS). The light curves represent the physical properties of NSs and the nuclear reactions on the proton-rich nuclei. The numerical treatments of the accreting NS and physics of the NS interior are not established, which shows uncertainty in modeling for observed X-ray light curves. In this study, we investigate theoretical X-ray burst models compared with burst light curves with GS 1826-24 observations. We focus on the impacts of the NS mass and radius and base heating on the NS surface using the MESA code. We find a monotonic correlation between the NS mass and the parameters of the light curve. The higher the mass, the longer the recurrence time and the greater the peak luminosity. While the larger the radius, the longer the recurrence time, the peak luminosity remains nearly constant. In the case of increasing base heating, both the recurrence time and peak luminosity decrease. We also examine the above results with a different numerical code, HERES , based on general relativity and consider the central NS. We find that the burst rate, energy, and strength are almost the same in two X-ray burst codes by adjusting the base heat parameter in MESA (the relative errors ≲5%), while the duration and rise times are significantly different between (the relative error is possibly ∼50%). The peak luminosity and the e-folding time change irregularly between two codes for different accretion rates.
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- Award ID(s):
- 1927130
- NSF-PAR ID:
- 10464923
- Date Published:
- Journal Name:
- The Astrophysical Journal
- Volume:
- 950
- Issue:
- 2
- ISSN:
- 0004-637X
- Page Range / eLocation ID:
- 110
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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