The Lick AGN Monitoring Project: Recalibrating Single-Epoch Virial Black
  Hole Mass Estimates
  We investigate the calibration and uncertainties of black hole mass estimates
based on the single-epoch (SE) method, using homogeneous and high-quality
multi-epoch spectra obtained by the Lick Active Galactic Nucleus (AGN)
Monitoring Project for 9 local Seyfert 1 galaxies with black hole masses < 10^8
M_sun. By decomposing the spectra into their AGN and stellar components, we
study the variability of the single-epoch Hbeta line width (full width at
half-maximum intensity, FWHM_Hbeta; or dispersion, sigma_Hbeta) and of the AGN
continuum luminosity at 5100A (L_5100). From the distribution of the "virial
products" (~ FWHM_Hbeta^2 L_5100^0.5 or sigma_Hbeta^2 L_5100^0.5) measured from
SE spectra, we estimate the uncertainty due to the combined variability as ~
0.05 dex (12%). This is subdominant with respect to the total uncertainty in SE
mass estimates, which is dominated by uncertainties in the size-luminosity
relation and virial coefficient, and is estimated to be ~ 0.46 dex (factor of ~
3). By comparing the Hbeta line profile of the SE, mean, and root-mean-square
(rms) spectra, we find that the Hbeta line is broader in the mean (and SE)
spectra than in the rms spectra by ~ 0.1 dex (25%) for our sample with
FWHM_Hbeta < 3000 km/s. This result is at variance with larger mass black holes
where the difference is typically found to be much less than 0.1 dex. To
correct for this systematic difference of the Hbeta line profile, we introduce
a line-width dependent virial factor, resulting in a recalibration of SE black
hole mass estimators for low-mass AGNs.
