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

TL;DR

This study reports a new superorbital excursion in SMC X-1, linking it to spin-up acceleration and pulse profile changes, revealing the chaotic nature of its modulation and challenging existing warped disc models.

Contribution

It identifies a new superorbital excursion in SMC X-1 and links it to prior spin-up acceleration, providing insights into the complex accretion processes.

Findings

A new superorbital excursion occurred in 2020-2021.

Spin-up rate accelerated before the excursion, indicating a possible causal link.

Pulse fraction shows long-term evolution connected to superorbital changes.

Abstract

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 days to <40 days 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 one year 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.