Disentangling the neighbouring pulsars SXP 15.3 and SXP 305

TL;DR

This study disentangles the overlapping X-ray emissions of two nearby pulsars, SXP 15.3 and SXP 305, using multi-wavelength observations to identify the true source of X-ray activity and analyze their orbital and disc behaviors.

Contribution

The paper provides a novel analysis of spatially close pulsars, demonstrating the origin of X-ray emission and revealing unusual optical modulation patterns.

Findings

X-ray emission primarily from SXP 15.3

Optical outbursts modulate at twice the X-ray frequency

Models suggest geometric orientation affects outburst origin

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 lightcurve from the Neil Gehrels Swift Observatory, and we investigate the origin of this emission. Using the H-alpha emission line from the Southern African Large Telescope (SALT) and photometric flux from the Optical Gravitational Lensing Experiment (OGLE) 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 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.