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Description
X-ray bursts are thermonuclear explosions that occur when accreted material on the surface of a neutron star reaches critical conditions. Superbursts are exceptionally energetic events, lasting several hours or longer and releasing more than two orders of magnitude more energy than ordinary X-ray bursts. They are thought to involve carbon burning. Previous studies have suggested that the fuel mass inferred from standard thermal cooling models exceeds the mass estimated to have been supplied by accretion. However, because only one superburst had been detected from each source during the observational period, it has been difficult to quantitatively investigate the origin of this discrepancy.
In this study, we investigate this issue using sources with two or more detected superbursts. We analyzed approximately 31 years of long-term monitoring data from RXTE/ASM, Swift/BAT, and MAXI/GSC for the low-mass X-ray binaries Aql X-1 and 4U 1608–52. The total accreted mass between superbursts was estimated by assuming that the persistent bolometric flux is proportional to the mass-accretion rate and integrating the persistent emission over time. Instrument count rates were converted to bolometric fluxes using conversion factors that depend on the spectral state. For MAXI/GSC, we additionally evaluated fluxes from spectra integrated over selected intervals. The inferred accreted masses are $(0.4–0.8)\times10^{25}$ g for Aql X-1 and $(1.2–1.8)\times10^{25}$ g for 4U 1608–52. In contrast, cooling-model fits to the superburst light curves yield burned fuel masses of $(1.1–2.0)\times10^{25}$ g and $(8–10)\times10^{25}$ g, respectively, assuming burning over the entire neutron-star surface. Thus, the fuel mass inferred from the cooling model exceeds that supplied by accretion between superbursts. This result suggests that conventional cooling models may need to be revisited. Possible interpretations include localized burning on the neutron-star surface and a carbon reaction rate enhanced relative to the standard theoretical value.
| Category | Observation |
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