Longterm properties of accretion discs in X-ray Binaries - III. A search for spin-superorbital correlation in SMC X-1

TL;DR

This study investigates the potential correlation between superorbital period variations and pulsar spin changes in SMC X-1, using long-term X-ray data to explore accretion disc and pulsar behavior over 16 years.

Contribution

It provides the first analysis of simultaneous long-term changes in superorbital periods and pulsar spin in SMC X-1, suggesting a possible link between accretion flow variations and disc-pulsar dynamics.

Findings

Superorbital period showed two excursions during 1996-2011.

A superorbital period excursion coincides with a change in pulsar spin-up rate.

No definitive correlation was established, but some temporal coincidences suggest a connection.

Abstract

Thanks to long-term X-ray monitoring, a number of interacting binaries are now known to show X-ray periodicities on timescales of tens to hundreds of binary orbits. In some systems, precession of a warped accretion disc is the leading model to explain the superorbital modulation. The High Mass X-ray Binary SMC X-1 showed two excursions in superorbital period (from ~60d to ~45d) during the 1996-2011 interval, suggesting that some characteristic of the accretion disc is varying on a timescale of years. Because its behaviour as an X-ray pulsar has also been intensively monitored, SMC X-1 offers the rare chance to track changes in both the accretion disk and pulsar behaviours over the same interval. We have used archival X-ray observations of SMC X-1 to investigate whether the evolution of its superorbital variation and pulse period are correlated. We use the 16-year dataset afforded by the RXTE~All-Sky Monitor to trace the behaviour of the warped accretion disc in this system, and use published pulse-period histories to trace the behaviour of the pulsar. While we cannot claim a strong detection of correlation, the first superorbital period excursion near MJD 50,800 does coincide with structure in SMC X-1's pulse period history. Our preferred interpretation is that the superorbital period excursion coincides with a change in the long-term spin-up rate of the SMC X-1 pulsar. In this scenario, the pulsar and the accretion disc are both responding to a change in the accretion flow, which the disc itself may regulate.