Univ Tubingen, Inst Astron & Astrophys, Abt Astron, D-72076 Tubingen, Germany
; Chinese Acad Sci, Inst High Energy Phys, High Energy Astrophys Lab, Beijing 100039, Peoples R China
We show that there is a limit relation between the black hole mass (M-BH) and the width at half-maximum (v(FWHM)) of Hbeta for active galactic nuclei (AGNs) with super-Eddington accretion rates. When a black hole has a super-Eddington accretion rate, the empirical relation derived from reverberation mapping can be applied in two possible ways. First, it reduces to a relation between the black hole mass and the size of the broad-line region because of photon-trapping effects inside the accretion disk. For the empirical reverberation relation of Kaspi et al., we obtain the limit relation M-BH=(2.9-12.6)x10(6) [v(FWHM)/(10(3) km s(-1))](6.67) M-circle dot, called 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 Hbeta width. We call this the Begelman limit. Using this limit relation, we searched 164 AGNs for super-Eddington accretion. We find that most of them are well confined by the Eddington limit relation that is, most have sub-Eddington accretion rates-but there are a handful of objects located between the Eddington and Begelman limit lines; they may be candidate super-Eddington accretors in a hybrid structure of photon trapping and photon bubble instability. The maximum Hbeta width is in the range (3.0-3.8)x10(3) km s(-1) for the most massive black holes with super-Eddington accretion rates among AGNs. We suggest that the FWHM(Hbeta)-M-BH relation is a reliable and convenient method to test whether a source is super-Eddington and useful to probe the structure of the super-Eddington accretion process.