BASS XXV: DR2 Broad-line Based Black Hole Mass Estimates and Biases from
  Obscuration
  We present measurements of broad emission lines and virial estimates of
supermassive black hole masses ($M_{BH}$) for a large sample of ultra-hard
X-ray selected active galactic nuclei (AGNs) as part of the second data release
of the BAT AGN Spectroscopic Survey (BASS/DR2). Our catalog includes $M_{BH}$
estimates for a total 689 AGNs, determined from the H$\alpha$, H$\beta$,
$MgII\lambda2798$, and/or $CIV\lambda1549$ broad emission lines. The core
sample includes a total of 512 AGNs drawn from the 70-month Swift/BAT all-sky
catalog. We also provide measurements for 177 additional AGNs that are drawn
from deeper Swift/BAT survey data. We study the links between $M_{BH}$
estimates and line-of-sight obscuration measured from X-ray spectral analysis.
We find that broad H$\alpha$ emission lines in obscured AGNs ($\log (N_{\rm
H}/{\rm cm}^{-2})> 22.0$) are on average a factor of $8.0_{-2.4}^{+4.1}$
weaker, relative to ultra-hard X-ray emission, and about $35_{-12}^{~+7}$\%
narrower than in unobscured sources (i.e., $\log (N_{\rm H}/{\rm cm}^{-2}) <
21.5$). This indicates that the innermost part of the broad-line region is
preferentially absorbed. Consequently, current single-epoch $M_{BH}$
prescriptions result in severely underestimated ($>$1 dex) masses for Type 1.9
sources (AGNs with broad H$\alpha$ but no broad H$\beta$) and/or sources with
$\log (N_{\rm H}/{\rm cm}^{-2}) > 22.0$. We provide simple multiplicative
corrections for the observed luminosity and width of the broad H$\alpha$
component ($L[{\rm b}{\rm H}\alpha]$ and FWHM[bH$\alpha$]) in such sources to
account for this effect, and to (partially) remedy $M_{BH}$ estimates for Type
1.9 objects. As key ingredient of BASS/DR2, our work provides the community
with the data needed to further study powerful AGNs in the low-redshift
Universe.
