A new black hole mass estimate for obscured active galactic nuclei

Abstract

We propose a new method for estimating the mass of a supermassive black hole, applicable to obscured active galactic nuclei (AGNs). This method estimates the black hole mass using the width of the narrow core of the neutral FeKα 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. Assuming the virial relation between the locations and the velocity widths of the neutral FeK α line core and the broad Hβ emission line, the luminosity scaling relation of the neutral FeKα line core emitting region is estimated. We find that the velocity width of the neutral FeKα line core falls between that of the broad Balmer emission lines and the corresponding value at the dust reverberation radius for most of the target AGNs. The black hole mass MBH,FeKα estimated with this method is then compared with other black hole mass estimates, such as the broad emission-line reverberation mass MBH,rev for type 1 AGNs, the mass MBH,H2O based on the H2O maser, and the single-epoch mass estimate MBH,pol based on the polarized broad Balmer lines for type 2 AGNs. We find that MBH,FeKα is consistent with MBH,rev and MBH,pol, and find that MBH,FeKα correlates well with MBH,H2O. These results suggest that MBH,FeKα is a potential indicator of the black hole mass for obscured AGNs. In contrast, MBH,FeKα is systematically larger than MBH,H2O by about a factor of 5, and the possible origins are discussed.