Logo
ScienceStack
Table of Contents
Fetching ...
Sign In

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

Malcolm J. Coe, Thomas M. Gaudin, Itumeleng M. Monageng, Jamie A. Kennea, David A. H. Buckley, Andrzej Udalski, Phil A. Evans, Sabyasachi Chattopadhyay

TL;DR

SXP 31.0, a Be X-ray binary in the Small Magellanic Cloud with a 31 s spin period, underwent a rare near-Eddington double outburst in 2025 after ~26 years of quiescence. By combining Swift XRT/UVOT, OGLE, and SALT observations, the study resolves two back-to-back Type II outbursts, revealing a persistent soft X-ray excess and a disc-driven optical/UV evolution, with peak luminosities near $L_X \sim 10^{38}$ erg s$^{-1}$ and a second peak around $4.5\times10^{37}$ erg s$^{-1}$. The multi-wavelength analysis shows the Be disc expanding to interact with the neutron star at periastron, while the surrounding H$\alpha$ halo is shown to be an unrelated HII region via IFU spectroscopy. OGLE photometry indicates long-term disc growth and a 90.53-day orbital modulation, linking optical variability to disc–NS interactions. Together, these results illuminate Be disc dynamics, disc–neutron star coupling, and the complex environments of BeXRBs in the SMC.

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.

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

TL;DR

SXP 31.0, a Be X-ray binary in the Small Magellanic Cloud with a 31 s spin period, underwent a rare near-Eddington double outburst in 2025 after ~26 years of quiescence. By combining Swift XRT/UVOT, OGLE, and SALT observations, the study resolves two back-to-back Type II outbursts, revealing a persistent soft X-ray excess and a disc-driven optical/UV evolution, with peak luminosities near erg s and a second peak around erg s. The multi-wavelength analysis shows the Be disc expanding to interact with the neutron star at periastron, while the surrounding H halo is shown to be an unrelated HII region via IFU spectroscopy. OGLE photometry indicates long-term disc growth and a 90.53-day orbital modulation, linking optical variability to disc–NS interactions. Together, these results illuminate Be disc dynamics, disc–neutron star coupling, and the complex environments of BeXRBs in the SMC.

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.
Paper Structure (18 sections, 3 equations, 15 figures, 3 tables)

This paper contains 18 sections, 3 equations, 15 figures, 3 tables.

Table of Contents

  1. Introduction
  2. Observations
  3. Swift Observations
  4. XRT Observations
  5. UVOT Observations
  6. Optical photometry
  7. OGLE
  8. Period analysis
  9. SALT spectroscopic observations
  10. SALT Imaging observations
  11. Discussion
  12. The X-ray outburst trigger
  13. The X-ray luminosity evolution of the outburst
  14. An unusual Type II outburst pair
  15. Be circumstellar disc
  16. ...and 3 more sections

Figures (15)

  • Figure 1: Full OGLE IV and Swift light curves for SXP 31.0 spanning the $\sim$9 years duration of the S-CUBED survey.
  • Figure 2: Examples of typical 0.3-10 keV XRT spectra obtained by Swift during the outbursts of SXP 31.0 plotted alongside the best-fit absorbed power law + blackbody model and the residuals to the fit. Panel A: A typical spectrum for the first outburst, obtained by Swift on 2025 June 11. Panel B: A typical spectrum for the second outburst, obtained by Swift on 2025 November 17.
  • Figure 3: The variability of the best-fitting XRT spectral model parameters for SXP 31.0. Each component of the best-fitting absorbed power law plus thermal blackbody model is shown except for the column density, which was fixed at $1.8 \times 20^{21}$ cm$^{-2}$. Gray bars represent periods when SXP 31.0 was not observable by Swift due to Earth limb constraint.
  • Figure 4: Multiwavelength light curve spanning the entire duration of the 2025 outburst for SXP 31.0. OGLE and Swift UVOT band light curves are plotted alongside the 0.3-10 keV XRT count rate of the source, showing that the optical/UV brightness of the source appears to change with the X-ray count rate.
  • Figure 5: Average flux for SXP 31.0 during the 2025 outburst in each of the Swift UVOT photometric bands as well as in the OGLE I-band. The variability is plotted for each band as a box-and-whisker plot. Also plotted is a PHOENIX model spectrum of a standard B1V star scaled to the median flux of the uvw2-band for comparison.
  • ...and 10 more figures