Intervening BLR Clouds' Effects on Optical/UV Spectrum

TL;DR

This paper explores how intervening broad line region clouds can produce absorption features in the optical/UV spectra of AGNs, affecting our understanding of their emission and ionization processes.

Contribution

It introduces the concept of intervening BLR clouds affecting optical/UV spectra and predicts specific absorption features, expanding the understanding of AGN spectral variability.

Findings

Ensemble of clouds causes a depression between Lyman limit and Lyα.

Absorption features can reveal cloud covering factors.

Intervening clouds may be common in AGNs.

Abstract

Recent x-ray observations of Mrk 766 suggest that broad emission line region clouds cross our line of sight and produce variable x-ray absorption. Here we investigate what optical/ultraviolet spectroscopic features would be produced by such "Intervening BLR Clouds" (IBC) crossing our line of sight to the accretion disk, the source of the optical/UV continuum. Although the emission spectrum produced by intervening clouds is identical to the standard BLR model, they may produce absorption features on the optical or UV continuum. Single clouds will have little effect on the optical/UV spectrum because BLR clouds are likely to be much smaller than the accretion disk. This is unlike the X-ray case, where the radiation source is considerably smaller. However, an ensemble of intervening clouds will produce spectroscopic features in the FUV including a strong depression between the Lyman limit and Ly{\alpha}. The amount of the depression will indicate the line-of-sight covering factor of clouds, an unknown quantity that is important for the ionization of the intergalactic medium and the energy budget of AGN. Comparison with observations suggests that the SED of Mrk 766 may be affected by intervening BLR clouds and IBC may exist in most of AGNs.