An X-ray and optical study of the outbursting behaviour of the SMC Be X-ray binary SXP 91.1

I M Monageng,V A Mcbride,A Udalski,L J Townsend,M J Coe,P D Roche,J A Kennea,P A Evans,D A H Buckley

doi:10.1093/mnras/stz2262

Abstract

ABSTRACT In this paper we report on the optical and X-ray behaviour of the Be X-ray binary, SXP 91.1, during a recent type I outburst. We monitored the outburst using the Neil Gehrels Swift Observatory. These data were supported by optical data from the Southern African Large Telescope and the Optical Gravitational Lensing Experiment (OGLE) to show the circumstellar disc activity. Matter from this disc accretes on to the neutron star, giving rise to the X-ray outburst as seen in the synchronous evolution of the optical and X-ray light curves. Using data taken with OGLE we show that the circumstellar disc has exhibited stable behaviour over two decades. A positive correlation is seen between the colour and magnitude from the OGLE and massive compact halo object observations, which indicates that the disc is orientated at relatively low-inclination angles. From the OGLE and Swift data, we demonstrate that the system has shown relative phase offsets that have persisted for many years. The spin period derivative is seen to be at maximum spin-up at phases when the mass accretion rate is at maximum. We show that the neutron star in SXP 91.1 is an unusual member of its class in the sense that it has had a consistent spin period derivative over many years, with the average spin-up rate being one of the highest for known Small Magellanic Cloud pulsars. The most recent measurements of the spin-up rate reveal higher values than the global trend, which is attributed to the recent mass accretion event leading to the current outburst.

Highlights

High mass X-ray binaries (HMXBs) comprise a massive early-type star (O or B spectral type) and a compact object
The intensities from the lightcurves follow each other as they rise and decline. It is well-known that the optical photometric variability in BeXRBs is attributed to changes in the circumstellar disc (e.g. Rajoelimanana et al 2011). This suggests that the disc grew large enough in size for matter to be accreted by the neutron star (NS) resulting in the enhanced X-ray activity which peaks while the disc is still growing, before shrinking
The long-term Optical Gravitational Lensing Experiment (OGLE) I-band lightcurve (Fig. 2) shows stable behaviour, with peaks separated by the orbital period with similar amplitudes ( 0.1 mag) over a time period of ∼9 years

Summary

Introduction

High mass X-ray binaries (HMXBs) comprise a massive early-type star (O or B spectral type) and a compact object (neutron star or black hole). By convention, they are divided up into two sub-groups on the basis of their luminosity class: supergiant X-ray binaries (luminosity class I and II) and Be X-ray binaries (III, IV and V). Schmidtke et al (2004) reported an 88.25 day orbital period from MACHO V and R data This period was later refined by Bird et al (2012) to 88.37 ± 0.03d using a larger OGLE data base.

Results

Discussion

Conclusion