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

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 ∼60 d to ∼45 d) 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.

M51 ULX-7: superorbital periodicity and constraints on the neutron star magnetic field

“…We must also note that several authors have proposed the precession of a radiation-driven warped disk as the origin of super-orbital variability in X-ray binaries (e.g. Pringle 1996;Wijers & Pringle 1999;Ogilvie & Dubus 2001;Clarkson et al 2003;Dage et al 2019). This scenario -which has also been proposed to interpret similar temporal characteristics in ULXs M82 X-2 (Kong et al 2016) and SS 433 (Foulkes et al 2010) -cannot be ruled out for M51 ULX-7.…”

Observing the Transient Pulsations of SMC X-1 with NuSTAR

“…In order to synthesize these results, we propose that the pulsing region was observed emerging from behind absorbing material. Given that Epochs I and II took place near the end of the low state, as illustrated in Figure 1 and that the super-orbital period has been attributed to a warped precessing accretion disk (Wojdowski et al 1998;Clarkson et al 2003;Dage et al 2018), the absorbing material obscuring the pulsing region is likely part of the accretion disk. In short, the warped accretion disk absorbs and scatters the pulsed emission from the neutron star, leading to the absence of detected pulsations in the low state and a gradual turn-on of pulsations as the disk moves out of the line of sight.…”

Monitoring observations of SMC X-1’s excursions (MOOSE)–I. Programme description and initial high state spectral results

“…SMC X-1 is also well known for its super-orbital period, which is typically measured close to 60 days, but can take excursions to periods as low as 45 days (Clarkson et al 2003a;Dage et al 2019;Hu et al 2019). The characterisation and physical origins of the superorbital period (𝑃 𝑠𝑢 𝑝 ) modulation have been the subject of many studies over the last few decades (see, e.g.…”