Long term optical variability of bright X-ray point sources in
  elliptical galaxies
  We present long term optical variability studies of bright X-ray sources in
four nearby elliptical galaxies with {\it Chandra} Advanced CCD Imaging
Spectrometer array (ACIS-S) and {\it Hubble Space Telescope (HST)} Advanced
Camera for Surveys observations. Out of the 46 bright (X-ray counts $> 60$)
sources that are in the common field of view of the {\it Chandra} and {\it HST}
observations, 34 of them have potential optical counterparts, while the rest of
them are optically dark. After taking into account of systematic errors,
estimated using the field optical sources as reference, we find that four of
the X-ray sources (three in NGC1399 and one in NGC1427) have variable optical
counterparts at a high significance level. The X-ray luminosities of these
source are $\sim 10^{38}$ $\rm ergs~s^{-1}$ and are also variable on similar
time-scales. The optical variability implies that the optical emission is
associated with the X-ray source itself rather than being the integrated light
from a host globular cluster. For one source the change in optical magnitude is
$> 0.3$, which is one of the highest reported for this class of X-ray sources
and this suggests that the optical variability is induced by the X-ray
activity. However, the optically variable sources in NGC1399 have been reported
to have blue colours ($g - z > 1$). All four sources have been detected in the
infra-red (IR) by {\it Spitzer} as point sources, and their ratio of $5.8$ to
$3.6 \mu m$ flux are $> 0.63$ indicating that their IR spectra are like those
of Active Galactic Nuclei (AGN). While spectroscopic confirmation is required,
it is likely that all four sources are background AGNs. We find none of the
X-ray sources having optical/IR colours different from AGNs, to be optically
variable.
