  A recent study of the X-ray source XMMUJ134736.6+173403 has revealed a strong
quasi-periodic modulation of the X-ray flux. The observation of two
quasiperiodic oscillations (QPOs) that occur on a daily timescale and exhibit a
3:1 frequency ratio strongly supports the evidence for the presence of an
active galactic nucleus black hole (AGN BH). Assuming the orbital origin of
QPOs we calculate the upper and lower limit on AGN BH mass M arriving at
$M\approx 10^7-10^9M_{\odot}$. Comparing that to mass estimates of other
sources, MMUJ134736.6+173403 appears to be the most massive source with
commensurable QPO frequencies, and its mass represents the current
observational upper limit on AGN BH mass based on QPO observations. We note it
will be crucial for the falsification of particular resonance models of QPOs
whether only single QPO with frequency that completes the harmonic sequence
3:2:1 is found in this source, or whether there is a new different pair of QPOs
with frequencies in the 3:2 ratio. The former case would agree with the
prediction of the 3:2 epicyclic resonance model and black hole mass
$M\approx(5a^2 + 8a + 8)\times 10^{7}M_{\odot}$, where a is a dimensionless BH
spin.
