Self-shadowing Effects of Slim Accretion Disks in Active Galactic Nuclei: Diverse Appearance of the Broad-line Region

TL;DR

This paper investigates how anisotropic radiation from slim accretion disks causes diverse observational appearances of the broad-line region in active galactic nuclei, linking accretion rates to BLR structure and line profiles.

Contribution

It introduces a model showing self-shadowing effects of slim disks lead to distinct BLR regions and line profiles, advancing understanding of AGN diversity.

Findings

Self-shadowing causes anisotropic radiation fields increasing with accretion rate.

Two distinct BLR regions produce different broad Hβ line components.

Different reverberation lags depend on accretion rate and viewing angle.

Abstract

Supermassive black holes in active galactic nuclei (AGNs) undergo a wide range of accretion rates, which lead to diversity of appearance. We consider the effects of anisotropic radiation from accretion disks on the broad-line region (BLR), from the Shakura-Sunyaev regime to slim disks with super-Eddington accretion rates. The geometrically thick funnel of the inner region of slim disks produces strong self-shadowing effects that lead to very strong anisotropy of the radiation field. We demonstrate that the degree of anisotropy of the radiation fields grows with increasing accretion rate. As a result of this anisotropy, BLR clouds receive different spectral energy distributions depending on their location relative to the disk, resulting in diverse observational appearance of the BLR. We show that the self-shadowing of the inner parts of the disk naturally produces two dynamically distinct regions of the BLR, depending on accretion rate. These two regions manifest themselves as kinematically distinct components of the broad H$\beta$ line profile with different line widths and fluxes, which jointly account for the Lorentzian profile generally observed in narrow-line Seyfert 1 galaxies. In the time domain, these two components are expected reverberate with different time lags with respect to the varying ionizing continuum, depending on the accretion rate and the viewing angle of the observer. The diverse appearance of the BLR due to the anisotropic ionizing energy source can be tested by reverberation mapping of H$\beta$ and other broad emission lines (e.g., \feii), providing a new tool to diagnose the structure and dynamics of the BLR. Other observational consequences of our model are also explored.