SXP 31.0 – the 2025 near-Eddington double X-ray outburst after 26 years of quiescence

ABSTRACT SXP 31.0 is an X-ray source in the Small Magellanic Cloud that was first identified as a Be X-ray Binary (BeXRB) system when it went into X-ray outbusrst in 1998. It is now known to consist of an OBe main sequence star and a neutron star with a spin period of 31 s. In 2025 a new X-ray outburst phase began with the source exhibiting a luminosities approaching the Eddington limit of $10^{38}$ erg s−1. Unusually, H $\alpha$ images show it has a surrounding halo whose nature has not been clear. In this paper, we report new observations of this halo, including the first multifibre Integrated Flux Unit observations, which identify this emission as probably a coincidental H ii region. The X-ray, UV, and optical data cover a period of $\sim$200 d and reveal that the source underwent two bright, back-to-back, Type II outbursts in 2025 – a rare occurrence for any BeXRB system.