Simultaneous optical and X-ray detection of a thermonuclear burst in the 2024 outburst of EXO 0748−676

ABSTRACT The neutron star low-mass X-ray binary, EXO 0748−676, recently returned to outburst after a $\sim\!\! 16$ yr-long quiescence. Since its return, there has been a global effort to capture the previously unseen rise of the source and to understand its somewhat early return to outburst, as it is typical for a source to spend longer in quiescence than in outburst. Here, we report on the simultaneous optical and X-ray detection of a type I X-ray burst, captured by XMM–Newton during a director’s discretionary time observation on 2024 June 30. The data show three X-ray eclipses consistent with the known ephemeris and one type I X-ray burst at 60492.309 MJD. The X-ray burst is reprocessed into the optical band and captured by XMM–Newton’s Optical Monitor during a 4399 s exposure with the B filter in image $+$ fast mode. We determine that the optical peak lags the X-ray peak by $4.46 \pm 1.71$ s. The optical and X-ray rise times are similar, but the optical decay time-scale is shorter than the X-ray decay time-scale. The reprocessing site is likely within a few light-seconds of the X-ray emitting region, so the companion star, accretion disc, and ablated material are all plausible.