A Limit Relation between Black Hole Mass and H$β$ Width: Testing Super-Eddington Accretion in Active Galactic Nuclei

Abstract

(abbreviated) We show that there is a limit relation between the black hole mass and the width at the half maximum of H$β$ for active galactic nuclei (AGNs) with super-Eddington accretion rates. When a black hole has a super-Eddington accretion rate, the empirical relation of reverberation mapping has two possible ways. First, it reduces to a relation between the black hole mass and the size of the broad line region due to the photon trapping effects inside the accretion disk. For the Kaspi et al.'s empirical reverberation relation, we get the limit relation as $M_{\rm BH}=(2.9 - 12.6)\times 10^6M_{\odot} (\upsilon_{\rm FWHM}/10^3{\rm km s^{-1}})^{6.67}$, called as the Eddington limit. Second, the Eddington limit luminosity will be relaxed if the trapped photons can escape from the magnetized super-Eddington accretion disk via the photon bubble instability, and the size of the broad line region will be enlarged according to the empirical reverberation relation, leading to a relatively narrow width of H$β$. We call this the Begelman limit. Super-Eddington accretions in a sample composed of 164 AGNs have been searched by this limit relation. We find there are a handful of objects locate between the Eddington and Begelman limit lines, they may be candidates of super-Eddington accretors in a hybrid structure of photon trapping and photon bubble instability. The maximum width of H$β$ is in the reange of $(3.0 - 3.8)\times 10^3$ km s$^{-1}$ for the maximum mass black holes with super-Eddington accretion rates among AGNs. We suggest that this limit relation is more reliable and convenient to test whether a source is super-Eddington and useful to probe the structure of the super-Eddington accretion process.