Searching for Quasi-periodic Oscillations in Active Galactic Nuclei of
  the Chandra Deep Field South
  Recent X-ray observations have revealed growing evidence of quasi-periodic
oscillation (QPO) in the light curve of active galactic nuclei (AGNs), which
may serve as a useful probe of black hole physics. In this work, we present a
systematic search for X-ray QPOs among ~ 1000 AGNs of the Chandra Deep Field
South (CDF-S) in a homogeneous fashion. Dividing the 7-Ms Chandra observations
into four epochs, we search for periodic signals that are persistent throughout
any of these epochs, using two independent methods: Lomb-Scargle periodogram
and Gregory-Loredo Algorithm. No statistically significant periodic signal is
found with either method on any of the four epochs. Our extensive simulations
of source light curves suggest that this non-detection is primarily due to a
moderate sensitivity of the CDF-S data in QPO detection. Using the
simulation-predicted detection efficiency, we are able to provide a meaningful
constraint on the intrinsic occurrence rate of persistent QPOs, < (15-20) %,
provided that they share a similar power spectral density with a handful of
currently known AGN QPOs. The true intrinsic occurrence rate might be
significantly below this upper limit, however, given the non-detection among
the CDF-S sources. Our additional search for short-lived QPOs that are only
detected over a small subset of all observations results in two candidates, one
in source XID 643 at a period of ~ 13273 s and the other in source XID 876 at a
period of ~ 7065 s.
