A wind-based unification model for NGC 5548: spectral holidays, non-disk emission, and implications for changing-look quasars

TL;DR

This paper proposes a wind-based model to explain spectral holidays, non-disk emission, and changing-look phenomena in NGC 5548, emphasizing the role of equatorial disk winds as obscurers affecting emission lines and ionizing radiation.

Contribution

It introduces a unifying wind model that explains spectral holidays and changing-look behavior through variable density disk winds acting as obscurers.

Findings

Disk winds can cause spectral holidays by changing their density.

Obscurers can be transparent or translucent, affecting emission lines.

Dense winds may lead to Seyfert 2 and changing-look AGN states.

Abstract

The 180-day Space Telescope and Optical Reverberation Mapping campaign on NGC 5548 discovered an anomalous period, the broad-line region (BLR) holiday, in which the emission lines decorrelated from the continuum variations. This is important since the correlation between the continuum-flux variations and the emission-line response is the basic assumption for black hole (BH) mass determinations through reverberation mapping. During the BLR holiday, the high-ionization intrinsic absorption lines also decorrelated from the continuum as a result of variable covering factor of the line of sight (LOS) obscurer. The emission lines are not confined to the LOS, so this does not explain the BLR holiday. If the LOS obscurer is a disk wind, its streamlines must extend down to the plane of the disk and the base of the wind would lie between the BH and the BLR, forming an equatorial obscurer. This obscurer can be transparent to ionizing radiation, or can be translucent, blocking only parts of the SED, depending on its density. An emission-line holiday is produced if the wind density increases only slightly above its transparent state. Both obscurers are parts of the same wind, so they can have associated behavior in a way that explains both holidays. A very dense wind would block nearly all ionizing radiation, producing a Seyfert 2 and possibly providing a contributor to the changing-look AGN phenomenon. Disk winds are very common and we propose that the equatorial obscurers are too, but mostly in a transparent state.