Extending virial black hole mass estimates to low-luminosity or obscured AGN: the cases of NGC 4395 and MCG -01-24-012

In the last decade, using single epoch (SE) virial based spectroscopic optical observations, it has been possible to measure the black hole (BH) mass on large type 1 active galactic nuclei (AGN) samples. However this kind of measurements cannot be applied on those obscured type 2 and/or low-luminosity AGN where the nuclear component does not dominate in the optical. We have derived new SE relationships, based on the full width at half-maximum and luminosity of the broad-line region component of the Paβ emission line and/or the hard X-ray luminosity in the 14-195 keV band, which have the prospect of better working with low luminosity or obscured AGN. The SE relationships have been calibrated in the 105-109 M⊙ mass range, using a sample of AGN, whose BH masses have been previously measured using reverberation mapping techniques. Our tightest relationship between the reverberation-based BH mass and the SE virial product has an intrinsic spread of 0.20 dex. Thanks to these SE relations, in agreement with previous estimates, we have measured a BH mass of M_BH = 1.7^{+1.3}_{-0.7} × 10^5 M⊙ for the low luminosity, type 1, AGN NGC 4395 (one of the smallest active galactic BH known). We also measured, for the first time, a BH mass of M_BH = 1.5^{+1.1}_{-0.6} × 10^7 M⊙ for the Seyfert 2 galaxy MCG -01-24-012.