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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BibTeX
@misc{knight2026multiwavelength,
title = {{A Multi-Wavelength Picture of the Surprisingly Short 2024/25 Outburst from EXO 0748-676}},
author = {Amy H. Knight and Lauren Rhodes and Noel Castro Segura and Douglas J. K. Buisson and Aru Beri and David M. Russell and Kevin Alabarta and Maria C. Baglio and Jakob van den Eijnden and Matthew J. Middleton and Payaswini Saikia and Chris Done and Rob Fender and Fraser Lewis},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.00495},
eprint = {2610.00495},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.00495},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - A Multi-Wavelength Picture of the Surprisingly Short 2024/25 Outburst from EXO 0748-676
AU - Amy H. Knight
AU - Lauren Rhodes
AU - Noel Castro Segura
AU - Douglas J. K. Buisson
AU - Aru Beri
AU - David M. Russell
AU - Kevin Alabarta
AU - Maria C. Baglio
AU - Jakob van den Eijnden
AU - Matthew J. Middleton
AU - Payaswini Saikia
AU - Chris Done
AU - Rob Fender
AU - Fraser Lewis
PY - 2026
DA - 2026/10/01
PB - arXiv
DO - 10.48550/arxiv.2610.00495
UR - https://arxiv.org/abs/2610.00495
AB - 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.
ER -