Speaker
Description
SMC X-1 is a high-mass X-ray binary exhibiting superorbital modulation with an unstable period. Monitoring carried out by RXTE/ASM, Swift/BAT, and MAXI revealed excursion events in 1996–1998, 2005–2007, and 2014–2016, during which the superorbital period drifted from ~60 days to ~40 days. These events occurred at intervals of about 3,200 days. However, recent observations suggest that a new excursion occurred in 2020–2021, and the overall superorbital period shows a long-term decreasing trend. We further trace the spin-period evolution of SMC X-1 using MAXI and find that the spin-up rate accelerated one year before the onset of this new excursion, suggesting a possible inside-out process connecting the spin-up acceleration and the superorbital excursion. Further analysis of the pulse profile evolution reveals that the pulsed fraction exhibits long-term changes that may be connected to the superorbital excursion. These discoveries deepen the mystery of SMC X-1, as they cannot be solely interpreted by the warped-disc model. In 2024, we carried out a series of NinjaSat observations monitoring the spin frequency evolution. This regular monitoring reveals detailed spin-period evolution within a single superorbital cycle and may enhance our understanding of the underlying accretion mechanisms.
