Disentangling the neighbouring pulsars SXP 15.3 and SXP 305

Abstract

ABSTRACT SXP 15.3 and SXP 305 are two Be X-ray binaries in the Small Magellanic Cloud that are spatially separated by ∼7 arcsec. The small separation between these sources has, in the past, resulted in confusion about the origin of the emission from the combined region. We present long-term optical and X-ray monitoring results of both sources, where we study the historic and recent behaviour. In particular, from data collected as part of the S-CUBED project we see repeating X-ray outbursts from the combined region of the two sources in the recent light curve from the Neil Gehrels Swift Observatory, and we investigate the origin of this emission. Using the Hα emission line from the Southern African Large Telescope and photometric flux from the Optical Gravitational Lensing Experiment to study the changes in the size and structure of the Be disc, we demonstrate that the X-ray emission likely originates from SXP 15.3. Timing analysis reveals unusual behaviour, where the optical outburst profile shows modulation at approximately twice the frequency of the X-ray outbursts. We consider either of these periodicities being the true orbital period in SXP 15.3 and propose models based on the geometric orientations of the Be disc and neutron star to explain the physical origin of the outbursts.