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

TL;DR

This paper reports on the 2025 near-Eddington X-ray outburst of SXP 31.0, a Be X-ray binary in the SMC, highlighting its unusual double outburst, the nature of its surrounding halo, and multi-wavelength observations over 200 days.

Contribution

It provides the first multi-fibre IFU observations of the halo around SXP 31.0, identifying it as likely a coincidental HII region, and documents the rare double Type II outbursts in 2025.

Findings

The halo is probably a coincidental HII region.

SXP 31.0 underwent two bright Type II outbursts in 2025.

The outbursts approached the Eddington luminosity of 10^38 erg/s.

Abstract

SXP 31.0 is an X-ray source in the Small Magellanic Cloud (SMC) 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 31s. In 2025 a new X-ray outburst phase began with the source exhibiting a luminosities approaching the Eddington limit of 10^38 erg/s. 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 multi-fibre Integrated Flux Unit (IFU) observations, which identify this emission as probably a coincidental HII region. The X-ray, UV & optical data cover a period of ~200d and reveal that the source underwent two bright, back-to-back, Type II outbursts in 2025 - a rare occurrence for any BeXRB system.