The kilo-second variability of X-ray sources in nearby galaxies
  Chandra observations of 17 nearby galaxies were analysed and 166 bright
sources with X-ray counts > 100, were chosen for temporal analysis. Fractional
root mean square variability amplitudes were estimated for lightcurves binned
at ~ 4 ksec and of length ~ 40 ksec. While there are nine ultra-luminous X-ray
sources (ULXs) with unabsorbed luminosity (in 0.3-8.0 keV band) L > 10^39 erg/s
in the sample for which the fractional r.m.s variability is constrained to be <
10%, only two of them show variability. One of the variable ULXs exhibits a
secular transition and has a ultra-soft spectrum with temperature ~ 0.3 keV
while the other is a rapidly varying source in NGC 0628, which has been
previously compared to the Galactic micro-quasar GRS1915+105. These results
seem to indicate that ULXs are typically not highly variable in ksec
time-scales, except for some ultra-soft ones. Among the relatively low
luminosity sources (L ~ 10^38 erg/s) we find five of them to be variable. Apart
from an earlier known source in NGC 1569, we identify a source in NGC 2403,
which exhibits persistent high amplitude fluctuations.The variability of the
sources in general, do not seem to be correlated with hardness,which indicates
that they may not be due to variations in any absorbing material, but instead
could reflect inner accretion disk instabilities.
