Simultaneous Optical and X-ray Detection of a Thermonuclear Burst in the 2024 Outburst of EXO 0748-676

TL;DR

This paper reports the first simultaneous optical and X-ray detection of a thermonuclear burst in the 2024 outburst of EXO 0748-676, providing insights into burst reprocessing and emission regions.

Contribution

It presents the first concurrent optical and X-ray observations of a type I burst in EXO 0748-676 during its 2024 outburst, revealing timing and decay characteristics.

Findings

Optical peak lags X-ray peak by 4.46 seconds.

Optical decay is shorter than X-ray decay.

Reprocessing likely occurs within a few light seconds of the X-ray source.

Abstract

The neutron star low-mass X-ray binary, EXO 0748--676, recently returned to outburst after a $\sim$ 16 year-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 DDT observation on 30th June 2024. The data show 3 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 $\pm$ 1.71s. The optical and X-ray rise times are similar, but the optical decay timescale is shorter than the X-ray decay timescale. 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.