Long-term properties of accretion discs in X-ray binaries - III. A search for spin-superorbital correlation in SMC X-1

Abstract

Due to long-term X-ray monitoring, a number of interacting binaries are now known to show X-ray periodicities on time-scales 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 ~60 d to ~45 d) during the 1996-2011 interval, suggesting that some characteristic of the accretion disc is varying on a time-scale 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 disc 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 data set 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.