Simultaneous optical and X-ray bursts from 4U/MXB 1636-53

We report the detection of five coincident optical/X-ray bursts from 4U/MXB 1636-53. The optical observations were made with the Danish 1.5 m telescope at the European Southern Observatory (E.S.O.) and the X-ray observations were made with the Hakucho X-ray observatory. We have analyzed these bursts in terms of a simple general mathematical model, in which the optical emission (both during and between the bursts) is that of a blackbody and the result of thermally reprocessed X-ray. One expects that the optical burst is a delayed and sneared verison of the X-ray burst. We estimate that the optical radiative delay is smaller than the observed values, which we believe are therefore mainly due to the geometry of the system. For the delay and smearing of the optical burst and the maximum temperature of the reprocessing region, we find values of approx.2.5 s, 1.5 1t-sec, the mass of the Roche lobe filling companion star is