Hz and assuming a 1/$M_{BH}$ scaling of frequencies, $f_{HFQPO}\sim 3\times 10^{-3} (M_{BH}/ 10^{6} M_\sun)^{-1}$ Hz [@abram], corresponding to timescales greater than 400s for AGN.

While this parallel between AGN and XRBs seems promising, no claim of an X-ray quasi-period in an AGN has been found to be statistically robust. @vaub remark that a major source of false detections arise from assuming an inappropriate background noise power spectrum. X-ray variations of AGN have intrinsically red noise power spectra (i.e. the power spectra have a continuum resembling a power law with a steep slope; Press 1978), however many purported QPOs in AGN are compared against an assumed background of white noise (i.e. Poisson photon noise or a flat spectrum). For example, in a $\sim$5 day ASCA observation of IRAS 18325-5926 the significance of the candidate periodicity was estimated with white noise [@iwa98]. After including red noise in the periodogram fitting, @vau found that the candidate periodicity was no longer significant at the 95$\%$ level. @fiore also claimed high ($>99\%$) significance peaks in NGC 4151, however, after fitting red noise and Poisson photon noise components of the spectrum @vaub showed that the significances of the QPOs fell below the 95$\%$ confidence level. It 