  We use optical and near-infrared spectroscopy to observe rest-UV emission
lines and estimate the black hole mass of WISEA J224607.56-052634.9
(W2246-0526) at z = 4.601, the most luminous hot dust-obscured galaxy yet
discovered by WISE. From the broad component of the MgII-2799A emission line,
we measure a black hole mass of log (M_BH/M_sun) = 9.6 +- 0.4. The broad
CIV-1549A line is asymmetric and significantly blue-shifted. The derived M_BH
from the blueshift-corrected broad CIV line width agrees with the MgII result.
From direct measurement using a well-sampled SED, the bolometric luminosity is
3.6 * 10^14 L_sun. The corresponding Eddington ratio for W2246-0526 is
lambda_Edd = L_AGN / L_Edd = 2.8. This high Eddington ratio may reach the level
where the luminosity is saturating due to photon trapping in the accretion
flow, and be insensitive to the mass accretion rate. In this case, the M_BH
growth rate in W2246-0526 would exceed the apparent accretion rate derived from
the observed luminosity.
