Supermassive Black Holes with High Accretion Rates in Active Galactic Nuclei. IV. H${\hat{I}}^2$ Time Lags and Implications for Super-Eddington Accretion

Abstract

We have completed two years of photometric and spectroscopic monitoring of a large number of active galactic nuclei (AGNs) with very high accretion rates. In this paper, we report on the result of the second phase of the campaign, during 2013--2014, and the measurements of five new H${\hat{I}}^2$ time lags out of eight monitored AGNs. All five objects were identified as super-Eddington accreting massive black holes (SEAMBHs). The highest measured accretion rates for the objects in this campaign are $\dot{\mathcal{M}}\gtrsim 200$, where $\dot{\mathcal{M}}={\dot{M}}_{\bullet }/L_{\mathrm{Edd}}{c}^{-2}$, ${\dot{M}}_{\bullet }$ is the mass accretion rates, $L_{\mathrm{Edd}}$ is the Eddington luminosity and $c$ is the speed of light. We find that the H${\hat{I}}^2$ time lags in SEAMBHs are significantly shorter than those measured in sub-Eddington AGNs, and the deviations increase with increasing accretion rates. Thus, the relationship between broad-line region size ($R_{{}_{\mathrm{H}{\hat{\mathrm{I}}}^2}}$) and optical luminosity at 5100Ã , $R_{{}_{\mathrm{H}{\hat{\mathrm{I}}}^2}}-L_{5100}$, requires accretion rate as an additional parameter. We propose that much of the effect may be due to the strong anisotropy of the emitted slim-disk radiation. Scaling $R_{{}_{\mathrm{H}{\hat{\mathrm{I}}}^2}}$ by the gravitational radius of the black hole, we define a new radius-mass parameter ($Y$) and show that it saturates at a critical accretion rate of ${\dot{\mathcal{M}}}_c=6\sim 30$, indicating a transition from thin to slim accretion disk and a saturated luminosity of the slim disks. The parameter $Y$ is a very useful probe for understanding the various types of accretion onto massive black holes. We briefly comment on implications to the general population of super-Eddington AGNs in the universe and applications to cosmology.

53 pages, 12 figures, 7 tables, accepted for publication in The Astrophysical Journal