Multiwavelength Observations of EXO 0748–676. I. Reprocessing of X‐Ray Bursts

We present the first high time-resolution simultaneous X-ray, ultraviolet,and optical observations of X-ray bursts in UY Vol, the optical counterpart ofthe low mass X-ray binary EXO 0748-676, obtained with RXTE, HST, and Gemini-S.Strong reprocessed signals are present in the ultraviolet (a factor of 4) andoptical (a factor of 2.5). These signals are lagged with respect to the X-raysand appear significantly smeared as well. The addition of far-ultravioletcoverage for one burst, together with the high quality of the dataset, allowmuch tighter constraints upon the temperature and geometry of the reprocessingregion than previously possible. A single-zone black body reprocessing modelfor this burst suggests a rise in temperatures during the burst from 18,000 to35,000K and an emitting area comparable to that expected for the disk and/orirradiated companion star. The lags, a mean of 4.0s and range of 2.5s, areconsistent with those expected within the binary. The single-zone black bodymodel cannot reproduce the ratio of optical to ultraviolet flux during theburst, however. The discrepancy, corresponding to underpredicting the opticalby more than a factor of two, seems too large to explain with deviations from alocal black body spectrum and more likely indicates that a range ofreprocessing temperatures are required, as would be expected, with coolerregions not contributing to the UV. Comparable results are derived from otherbursts, and in particular the lag and smearing both appear shorter when thecompanion star is on the near side of the disk as predicted. The burst observedby HST also yielded a spectrum of the reprocessed light. [Abridged]