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

## TL;DR

This study investigates the optical and X-ray outburst behavior of the SMC Be X-ray binary SXP 91.1, revealing stable disc activity, phase offsets, and an unusually high and consistent neutron star spin-up rate during recent accretion events.

## Contribution

It provides a detailed multi-wavelength analysis of SXP 91.1's outbursts, highlighting its stable disc behavior and unique neutron star spin evolution compared to similar systems.

## Key findings

- Disc has been stable over two decades.
- Positive correlation between colour and magnitude indicates low inclination.
- Consistent high spin-up rate over many years.

## 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 (SALT) and the Optical Gravitational Lensing Experiment (OGLE) to show the circumstellar disc activity. Matter from this disc accretes onto the neutron star, giving rise to the X-ray outburst as seen in the synchronous evolution of the optical and X-ray lightcurves. 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 MACHO 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 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 SMC 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.