Monitoring observations of SMC X-1’s excursions (MOOSE) – II. A new excursion accompanies spin-up acceleration

Chin Ping Hu, Kristen C. Dage, William I. Clarkson, McKinley Brumback, Philip A. Charles, Daryl Haggard, Ryan C. Hickox, Tatehiro Mihara, Arash Bahramian, Rawan Karam, Wasundara Athukoralalage, Diego Altamirano, Joey Neilsen, Jamie Kennea

Research output: Contribution to journalArticlepeer-review

2 Scopus citations


SMC X-1 is a high-mass X-ray binary showing superorbital modulation with an unstable period. Previous monitoring shows three excursion events in 1996–1998, 2005–2007, and 2014–2016. The superorbital period drifts from 60 to 40 d and then evolves back during an excursion. Here, we report a new excursion event of SMC X-1 in 2020–2021, indicating that the superorbital modulation has an unpredictable, chaotic nature. We trace the spin-period evolution and find that the spin-up rate accelerated 1 yr before the onset of this new excursion, which suggests a possible inside-out process connecting the spin-up acceleration and the superorbital excursion. This results in a deviation of the spin-period residual, similar to the behaviour of the first excursion in 1996–1998. In further analysis of the pulse profile evolution, we find that the pulsed fraction shows a long-term evolution and may be connected to the superorbital excursion. These discoveries deepen the mystery of SMC X-1 because they cannot be solely interpreted by the warped-disc model. Upcoming pointed observations and theoretical studies may improve our understanding of the detailed accretion mechanisms taking place.

Original languageEnglish (US)
Pages (from-to)3436-3442
Number of pages7
JournalMonthly Notices of the Royal Astronomical Society
Issue number3
StatePublished - Apr 1 2023

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


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