Single-Epoch Black Hole Mass Estimators For Broad-Line Active Galactic
  Nuclei: Recalibrating Hbeta with A New Approach
  Based on an updated Hbeta reverberation mapping (RM) sample of 44 nearby
active galactic nuclei (AGN), we propose a novel approach for black hole (BH)
mass estimation using two filtered luminosities computed from single-epoch (SE)
AGN spectra around the Hbeta region. We found that the two optimal-filter
luminosities extract virial information (size and virial velocity of the broad
line region, BLR) from the spectra, justifying their usage in this empirical BH
mass estimator. The major advantages of this new recipe over traditional SE BH
mass estimators utilizing continuum luminosity and broad line width are: 1) it
has a smaller intrinsic scatter of 0.28 dex calibrated against RM masses; 2) it
is extremely simple to use in practice, without any need to decompose the
spectrum; 3) it produces unambiguous and highly repeatable results even with
low signal-to-noise spectra. The combination of the two luminosities can also
cancel out, to some extent, systematic luminosity errors potentially introduced
by uncertainties in distance or flux calibration. In addition, we recalibrated
the traditional SE mass estimators using broad Hbeta full width at half maximum
(FWHM) and monochromatic continuum luminosity at 5100A (L_5100). We found that
using the best-fit slopes on FWHM and L_5100 (derived from fitting the BLR
radius-luminosity relation and the correlation between rms line dispersion and
SE FWHM, respectively) rather than simple assumptions (e.g., 0.5 for L_5100 and
2 for FWHM) leads to more precise SE mass estimates, improving the intrinsic
scatter from 0.41 dex to 0.36 dex with respect to the RM masses. We compared
different estimators and discussed their applications to the Sloan Digital Sky
Survey (SDSS) quasar sample. Due to the limitations of the current RM sample,
application of any SE recipe calibrated against RM masses to distant quasars
should be treated with caution.
