Radio/X-Ray Luminosity Relation for X-Ray–Bright Galactic Nuclei: Implications for Weighing Supermassive Black Holes

Optically thin and geometrically thick accretion flows are known to beresponsible for the observed radio/X-ray luminosity relation of the X-raybright galactic nuclei. It has also been suggested that supermassive black holemasses can be estimated from measurements of the core radio luminosity and theX-ray luminosity by using the advection-dominated accretion flow (ADAF) model.In this study we increase the number of data available by compiling theradio/X-ray fluxes and the mass in published literatures, and compare theobserved ratio of the luminosities with predictions from various models ofoptically thin accretion flows. Semi-analytically derived relations of theluminosities are presented in cases of the standard ADAF model and modifiedADAF models, in which a truncation of inner parts of the flows and windscausing a reduction of the infalling matter are included. We show that theobserved relation can be used indeed to estimate the supermassive black holemass, provided that properties of such accretion flows are known. Havinginvestigated sensitivities of the method on modifications of the 'standard'ADAF model, we find that a general trend of model predictions from the'standard' ADAF, the truncated ADAF and the 'windy' ADAF are somewhatindistinguishable. We also find, however, that the extreme case of the windymodel is inconsistent with currently available observational data, unlessmicrophysics parameters are to be substantially changed. High resolution radioobservations, however, are required to avoid the contamination of non-diskcomponents, such as, a jet component, which, otherwise, results in theover-estimated SMBH mass.Comment: 22 pages, 3 eps figures, accepted to A