Searching for quasi-periodic oscillations in active galactic nuclei of the Chandra Deep Field South

Abstract

ABSTRACT 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, $\lt (15\!-\!20)\,{{\mathrm{ per}\,\mathrm{ cent}}}$, 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 ∼13 273 s and the other in source XID 876 at a period of ∼7065 s.