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In this paper we report on the optical and X-ray behaviour of the Be X-raybinary, SXP 91.1, during a recent type I outburst. We monitored the outburstusing the Neil Gehrels Swift Observatory. These data were supported by opticaldata from the Southern African Large Telescope (SALT) and the OpticalGravitational Lensing Experiment (OGLE) to show the circumstellar discactivity. Matter from this disc accretes onto the neutron star, giving rise tothe X-ray outburst as seen in the synchronous evolution of the optical andX-ray lightcurves. Using data taken with OGLE we show that the circumstellardisc has exhibited stable behaviour over two decades. A positive correlation isseen between the colour and magnitude from the OGLE and MACHO observations,which indicates that the disc is orientated at relatively low inclinationangles. From the OGLE and Swift data, we demonstrate that the system has shownrelative phase offsets that have persisted for many years. The spin periodderivative is seen to be at maximum spin-up at phases when the mass accretionrate is at maximum. We show that the neutron star in SXP 91.1 is an unusualmember of its class in that it has had a consistent spin period derivative overmany years, with the average spin-up rate being one of the highest for knownSMC pulsars. The most recent measurements of the spin-up rate reveal highervalues than the global trend, which is attributed to the recent mass accretionevent leading to the current outburst.

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