A New Black Hole Mass Estimate for Obscured Active Galactic Nuclei
  We propose a new method for estimating the mass of a supermassive black hole,
applicable to obscured AGNs. This method estimates the black hole mass using
the width of the narrow core of the neutral FeKa emission line in X-rays and
the distance of its emitting region from the black hole based on the isotropic
luminosity indicator via the luminosity scaling relation. We collect the line
width data of the neutral FeKa line core for seven type-1 AGNs and seven type-2
AGNs obtained by the Chandra HETGS. Assuming the virial relation between the
locations and the velocity widths of the neutral FeKa line core and the broad
Hb emission line, the luminosity scaling relation of the neutral FeKa line core
emitting region is estimated. We find that the FWHM of the neutral FeKa line
core falls between that of the broad Balmer emission lines and the
corresponding value at the dust reverberation radius for most of the type-1
AGNs and for all of the type-2 AGNs. This suggests that significant fraction of
photons of the neutral FeKa line core originates between the outer BLR and the
inner dust torus in most cases. The black hole mass M_FeKa estimated with this
method is then compared with other black hole mass estimates, such as the broad
emission-line reverberation mass M_rev for the type-1 AGNs, the mass M_H2O
based on the H2O maser and the single-epoch mass estimate M_pol based on the
polarized broad Balmer lines for the type-2 AGNs. We find that M_FeKa is
consistent with M_rev for the most of the type-1 AGNs and with M_pol for all of
the type-2 AGNs. We also find that M_FeKa is correlated well with M_H2O for the
type-2 AGNs. These results suggest that M_FeKa is a potential indicator of the
black hole mass especially for obscured AGNs. In contrast, M_FeKa for which the
same virial factor as for M_rev and M_pol is adopted is systematically larger
than M_H2O by about a factor of about 5. (abridged)
