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We report the discovery of a $\sim 1.5 \times 10^{-4}$ Hz ($\sim 2$ hr) X-rayquasi-periodic oscillation (QPO) in the active galaxy MS 2254.9-3712, using a$\sim 70$ ks XMM-Newton observation. The QPO is significantly detected ($\sim3.3 \sigma$) in the $1.2 - 5.0$ keV band only, connecting its origin with theprimary X-ray power-law continuum. We detect a highly coherent soft lag betweenthe $0.3 - 0.7$ keV and $1.2 - 5.0$ keV energy bands at the QPO frequency andat a frequency band in a 3:2 ratio, strongly suggesting the presence of a QPOharmonic. An iron K$\alpha$ reverberation lag is found at the harmonicfrequency, indicating the reflecting material subtends some angle to theprimary continuum, which is modulated by the QPO mechanism. Frequency resolvedspectroscopy reveals the QPO and harmonic to have a hard energy dependence.These properties of the QPO variability, together with the current black holemass estimate, $M_{\rm bh} \sim 4 \times 10^{6} M_{\rm sun}$, are consistentwith the QPO originating from the same process as the high frequency QPOphenomenon observed in black hole X-ray binaries. Principle component analysisreveals the spectral variability in MS 2254.9-3712 is similar to that of theactive galaxy RE J1034+396, a source which also displays an X-ray QPO. Thissuggests a distinct spectral variability pattern for accreting black holes whenin a state where QPOs are present.

Discovery of a ~2 hr high frequency X-ray QPO and iron K alpha reverberation in the active galaxy MS 2254.9-3712