We report the detection of a large number of optical bursts in the Low Mass
X-ray Binary (LMXB) EXO 0748-676 simultaneous with the thermonuclear X-ray
bursts. The X-ray and the optical bursts are detected in a long observation of
this source with the XMM-Newton observatory. This has increased the number of
thermonuclear X-ray bursts in the LMXBs with simultaneous optical detection by
several factors. The optical bursts are found to have a linear rise followed by
a slow, somewhat exponential decay. Most of the optical bursts have longer rise
and decay timescale compared to the corresponding X-ray bursts. We have
determined the X-ray and optical excess photon counts in the bursts that allow
us to look at the optical to X-ray burst fluence ratio for each burst and the
ratio as a function of the X-ray burst intensity and as a function of the
orbital phase. The delay between the onset of the X-ray bursts and the onset of
the optical bursts have also been measured and is found to have an average
value of 3.25 seconds. We do not find any convincing evidence of orbital phase
dependence of the following parameters: X-ray to optical delay, rise time of
the optical bursts, and optical to X-ray burst intensity ratio as would be
expected if the optical bursts were produced by reprocessing from the surface
of the secondary star that is facing the compact star. On the other hand, if
the optical bursts are produced by reprocessing of the X-rays in the accretion
disk, the onset of the bursts is not expected to have a sharp, linear shape as
is observed in a few of the bursts in EXO 0748-676. We emphasise the fact that
simultaneous optical observations of the X-ray bursts in multiple wavelength
bands will enable further detailed investigations of the reprocessing
phenomena, including any non-linear effect of the X-ray irradiation.Comment: 12 pares, 5 figures (Submitted to the Bulletin of the Astronomical
Society of India