2008ApJ...679..182E

Quasi-periodic signals have yielded important constraints on the masses of black holes in galactic X-ray binaries, and here we extend this to active galactic nuclei (AGNs). We employ a wavelet technique to analyze 19 observations of 10 AGNs obtained with the XMM-Newton EPIC pn camera. We report the detection of a candidate 3.3 ks quasi period in 3C 273. If this period represents an orbital timescale originating near a last stable orbit of 3R_s_, it implies a central black hole mass of 7.3x10⁶ M_☉. For a maximally rotating black hole with a last stable orbit of 0.6R_s, a central black hole mass of 8.1x10⁷ M_☉ is implied. Both of these estimates are substantially lower than previous reverberation-mapping results, which place the central black hole mass of 3C 273 at about 2.35x10⁸ M_☉. Assuming that this reverberation mass is correct, the X-ray quasi period would be caused by a higher order oscillatory mode of the accretion disk.