We report the discovery of a \sim 1.5 \times 10 ^ { -4 } Hz ( \sim 2 hr ) X-ray quasi-periodic oscillation ( QPO ) in the active galaxy MS 2254.9–3712 , using a \sim 70 ks XMM-Newton observation .
The QPO is significantly detected ( \sim 3.3 \sigma ) in the 1.2 - 5.0 keV band only , connecting its origin with the primary X-ray power-law continuum .
We detect a highly coherent soft lag between the 0.3 - 0.7 keV and 1.2 - 5.0 keV energy bands at the QPO frequency and at a frequency band in a 3:2 ratio , strongly suggesting the presence of a QPO harmonic .
An iron K \alpha reverberation lag is found at the harmonic frequency , indicating the reflecting material subtends some angle to the primary continuum , which is modulated by the QPO mechanism .
Frequency resolved spectroscopy reveals the QPO and harmonic to have a hard energy dependence .
These properties of the QPO variability , together with the current black hole mass estimate , \hbox { $M _ { \mathrm { BH } } $ } \sim 4 \times 10 ^ { 6 } \hbox { $ \thinspace M _ { \odot } $ } , are consistent with the QPO originating from the same process as the high frequency QPO phenomenon observed in black hole X-ray binaries .
Principle component analysis reveals the spectral variability in MS 2254.9–3712 is similar to that of the active galaxy RE J1034+396 , a source which also displays an X-ray QPO .
This suggests a distinct spectral variability pattern for accreting black holes when in a state where QPOs are present .