A Multi-Wavelength Picture of the Surprisingly Short 2024/25 Outburst from EXO 0748–676

Abstract After sixteen years in quiescence, EXO 0748–676 unexpectedly returned to outburst in March 2024. However, in stark contrast to its previous outburst (1985 - 2008), EXO 0748–676 returned to quiescence in January 2025, after ∼10 months in outburst. Here, we collate data from nine observing facilities, covering wavelengths from radio to X-ray, to provide the first multi-wavelength study of the 2024/25 outburst. Unlike the 1985 outburst, both the rise and decay are fully captured at X-ray and optical wavelengths. The light curves do not exhibit the hallmarks of a canonical transient low-mass X-ray binary outburst. Instead, the X-ray and optical fluxes remain reasonably stable during the 2024/25 outburst. The wealth of optical data of EXO 0748-676 reveals a skewed optical phase curve and an irradiation-dominated optical light curve which lasts until the onset of hydrogen recombination in the outer disc at ∼11000K, triggering a rapid fade at the end of the outburst. X-ray observations by XMM-Newton and NICER revealed that EXO 0748–676 was heavily obscured early on, with spectra exhibiting a dampened soft X-ray contribution. We find that a spectral model including a clumpy, ionised absorber describes the obscuration, although the exact cause of the early-time obscuration remains uncertain. X-ray colour analysis indicates that EXO 0748–676 spent most of the ∼10 month outburst in a faint, hard state. EXO 0748-676 increased marginally in brightness towards the end of the outburst, but never entered a soft spectral state. This behaviour is characteristic of a failed transition outburst.

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Journal
Monthly Notices of the Royal Astronomical Society
Published
2026-10-30
DOI
https://doi.org/10.1093/mnras/stag2056
Primary Topic
Astrophysical Phenomena and Observations
Type
article
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article

A Multi-Wavelength Picture of the Surprisingly Short 2024/25 Outburst from EXO 0748–676

Amy H Knight, Noel Castro Segura, D. M. Russell, Payaswini Saikia et al.
2 links
Monthly Notices of the Royal Astronomical Society
Astrophysical Phenomena and Observations
article

A Multi-Wavelength Picture of the Surprisingly Short 2024/25 Outburst from EXO 0748–676

Amy H Knight, Noel Castro Segura, D. M. Russell, Payaswini Saikia, Matthew J. Middleton, Kevin Alabarta, J. van den Eijnden, M. C. Baglio, D. J. K. Buisson, Aru Beri, Lauren Rhodes, Rob Fender, Fraser Lewis, Chris Done
article en

Abstract

Abstract After sixteen years in quiescence, EXO 0748–676 unexpectedly returned to outburst in March 2024. However, in stark contrast to its previous outburst (1985 - 2008), EXO 0748–676 returned to quiescence in January 2025, after ∼10 months in outburst. Here, we collate data from nine observing facilities, covering wavelengths from radio to X-ray, to provide the first multi-wavelength study of the 2024/25 outburst. Unlike the 1985 outburst, both the rise and decay are fully captured at X-ray and optical wavelengths. The light curves do not exhibit the hallmarks of a canonical transient low-mass X-ray binary outburst. Instead, the X-ray and optical fluxes remain reasonably stable during the 2024/25 outburst. The wealth of optical data of EXO 0748-676 reveals a skewed optical phase curve and an irradiation-dominated optical light curve which lasts until the onset of hydrogen recombination in the outer disc at ∼11000K, triggering a rapid fade at the end of the outburst. X-ray observations by XMM-Newton and NICER revealed that EXO 0748–676 was heavily obscured early on, with spectra exhibiting a dampened soft X-ray contribution. We find that a spectral model including a clumpy, ionised absorber describes the obscuration, although the exact cause of the early-time obscuration remains uncertain. X-ray colour analysis indicates that EXO 0748–676 spent most of the ∼10 month outburst in a faint, hard state. EXO 0748-676 increased marginally in brightness towards the end of the outburst, but never entered a soft spectral state. This behaviour is characteristic of a failed transition outburst.

Monthly Notices of the Royal Astronomical Society
New York University Abu Dhabi (AE), Indian Institute of Astrophysics (IN), University of Cape Town (ZA), Durham University (GB), Brera Astronomical Observatory (IT), Yale University (US), University of Warwick (GB), University of Oxford (GB), Sport Wales (GB), University of Southampton (GB), Trottier Space Institute at McGill (CA), McGill University (CA), Liverpool John Moores University (GB), University of Amsterdam (NL)
National Aeronautics and Space Administration, California Institute of Technology, McGill University, New York University Abu Dhabi, Space Telescope Science Institute, Ministerio de Ciencia, Innovación y Universidades, European Space Agency, University of Leicester, European Commission, Queen's University Belfast, National Research Foundation, Canada Research Chairs, Universiteit van Amsterdam, Canada Excellence Research Chairs, Government of Canada, Nederlandse Organisatie voor Wetenschappelijk Onderzoek, Tamkeen, Jet Propulsion Laboratory, Goddard Space Flight Center, HORIZON EUROPE Framework Programme, Natural Sciences and Engineering Research Council of Canada, Science and Technology Facilities Council, Fonds de recherche du Québec – Nature et technologies, Japan Aerospace Exploration Agency, RIKEN, Division of Materials Research, Astrophysics Science Division
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