B. Paul, Archana M. and Lakshmi Saripalli
Abstract.
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.
Keywords: accretion -- accretion discs -- binaries: eclipsing -- binaries: general --
stars: individual: EXO 0748-676 -- stars: neutron -- X-rays: stars
Raman Research Institute, Sadashivnagar, C. V. Raman Avenue, Bangalore 560080, India