NGC 5548: The AGN Energy Budget Problem and the Geometry of the Broad-Line Region and Torus

TL;DR

This study analyzes the spectral energy distribution and geometry of the broad-line region and torus in NGC 5548, revealing a non-spherical structure with significant internal reddening that impacts observed properties and resolves key AGN energy budget issues.

Contribution

It provides a detailed model of the BLR and torus geometry, including the covering factors and internal reddening, explaining observed ionization stratification and reverberation lags.

Findings

BLR covering factor is approximately 40%.

The inner BLR causes shielding that explains ionization stratification.

The geometry implies a non-spherical, hole-viewed structure.

Abstract

We consider in detail the spectral energy distribution (SED) and multi-wavelength variability of NGC5548. Comparison with the SEDs of other AGNs implies that the internal reddening of NGC5548 is E(B-V) = 0.17 mag. The extinction curve is consistent with the mean curve of other AGNs found by Gaskell & Benker, but inconsistent with an SMC-type reddening curve. Because most IR emission originates exterior to the broad-line region (BLR), the SED seen by the inner BLR is different from that seen by the outer BLR and from the earth. The most likely BLR covering factor is ~ 40% and it is not possible to get an overall BLR covering factor of less than 20%. This requires that the BLR is not spherically symmetric and that we are viewing through a hole. Line-continuum variability transfer functions are consistent with this geometry. The covering factor and geometry imply that near the equatorial plane the BLR covering approaches 100%. The spectrum seen by the outer regions of the BLR and by the torus is thus modified by the absorption in the inner BLR. This shielding solves the problem of observed BLR ionization stratification being much greater than implied by photoionization models. The BLR obscuration also removes the problem of the torus covering factor being greater than the BLR covering factor, and gives consistency with the observed fraction of obscured AGNs. The flux reduction at the torus also reduces the problem of AGN dust-reverberation lags giving sizes smaller than the dust-sublimation radii.