Speaker
Description
After sixteen years in quiescence, EXO 0748-676 unexpectedly alerted the world to its return to outburst with a bright X-ray burst in June 2024, having been increasing in brightness since March 2024. However, in stark contrast to its previous outburst (1985 - 2008), EXO 0748-676 returned to quiescence in January 2025, after roughly 10 months in outburst. In this talk, I will present a multi-wavelength summary of this surprisingly short outburst, utilising data from thirteen detectors, spanning wavelengths from radio to X-ray. Unlike the 1985 outburst, both the rise and decay are fully captured at X-ray and optical wavelengths. The outburst light curve does not exhibit the hallmarks of a typical transient low-mass X-ray binary outburst, and the short outburst duration creates additional tension with EXO 0748-676’s previous ‘quasi-persistent’ classification. Our optical monitoring reveals a skewed, asymmetric optical phase curve that is unique to false widow binaries, and a cooling phase dominated by an inside-out recession of the ionised accretion disc. X-ray observations by XMM-Newton and NICER in June 2024 revealed that EXO 0748-676 was heavily absorbed/obscured at the beginning of the outburst, and exhibited X-ray spectra with a strongly dampened soft X-ray contribution. A spectral model that includes a clumpy, high-density absorber can describe the strong obscuration, consistent with the false widow sub-classification of EXO 0748-676. The X-ray colours show that EXO 0748-676 spent a large portion of the ~10-month outburst in a low-intensity, hard state. EXO 0748-676 marginally increased in brightness towards the end of the outburst, but never entered a soft spectral state. This behaviour is strongly indicative of a failed transition outburst or the onset of a transitional evolutionary phase.