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Type I bursts are transient explosions triggered by the thermonuclear burning of the accreted matter accumulated on neutron star (NS) surfaces. Among Type I bursters, several sources exhibit bursts with nearly constant recurrence times ($t_{\rm rec}$), known as “clocked bursts”. Clocked bursters serve as useful probes for the nature of low-mass X-ray binaries (LMXBs)—including accretion rate, inferred from persistent flux ($F_{\rm per}$) and its compositions—as well as the properties of NSs, such as $rp$-process, mass, radius, and temperature.
We report the discovery of clocked bursts from Circinus X-1 (hereafter Cir X-1) observed in 2023. We identified 71 bursts in total with $t_{\rm rec}$ and $F_{\rm per}$ varying across different observational epochs. At lower fluxes below 0.6 times the Eddington limit ($F_{\rm Edd}$), $t_{\rm rec}$ exhibits a clear anti-correlation with $F_{\rm per}$, as seen in most bursters. In contrast, at higher fluxes ($F_{\rm per}$ ~ 0.65$F_{\rm Edd}$), $t_{\rm rec}$ dropped by approximately half without a corresponding change in $F_{\rm per}$. We attribute this shift in burst regimes to marginally stable burning—where a fraction of the fuels burn stably while the remainder triggers bursts—occurring during the luminous phases. Aside from Cir X-1, IGR J17480−2446 is the only source known to exhibit a continuous transition between stable and unstable burning, making Cir X-1 a crucial source for understanding nuclear burst regimes.
Burst durations ($\tau_{\rm D}$) strongly depend on the hydrogen fraction in burning fuel via the $rp$-process. We found that $\tau_{\rm D}$ of Cir X-1 is a few seconds longer than those of GS 1826−24, a well-known solar-abundance system, implying the hydrogen-rich environment in this binary. This conclusion is well consistent with the extremely young age of Cir X-1 (<4600 yr) suggested by a supernova remnant associated with this system, whereas most bursters typically classified as LMXBs are much older (~$10^9$ yr). This supports the scenario where the hydrogen envelope in such a young binary has not yet been stripped (Ishii et al. to be submitted).
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