A new black-hole mass scaling relation based on Coronal lines

Determining the mass of the supermassive black hole (BH) at the centre of active galactic nuclei (AGN) via reverberation mapping (RM) is time consuming and cannot be applied to large samples of sources. However, methods employing single-epoch spectrum for the same purpose have become very popular due to their convenience but require a well-calibrated relationship. We present a new approach to determine the black hole mass (M_BH) using coronal lines (CLs). To this aim, bona-fide M_BH from RV for ~30 AGN and their corresponding integrated emission line fluxes of CLs such as [FeVII] 608.7 nm and [SiVI] 1963 nm normalized to their nearest HI emission, were employed. We found a scaling relation of the form log(M_BH) = 6.40 - 1.99 * log ([SiVI]/Br_gamma), over the M_BH mass interval 10^6-10^8 solar mass, with dispersion 0.47 dex. Under a central source photoionization scenario, the ratio [SiVI]/Br_gamma traces the AGN ionizing continuum. Following on the thin accretion disc approximation and after surveying a basic parameter space for CLs production, we found that a key parameter driving this anti-correlation is the effective temperature of the accretion disc, this being sampled by the CL gas and in line with the thin accretion disc prediction T_disc ~ M_BH^{-1/4}. In the light of the copious amount of data generated by the JWST, this new scaling relationship can be applied to large samples of AGN and possibly expanded to other CLs detected in the mid-infrared region.