Continuum reverberation mapping and a new lag-luminosity relationship for AGN

TL;DR

This paper investigates the origin of continuum lags in AGN, presenting evidence that diffuse gas emission from the broad line region explains lag spectra and introduces a new lag-luminosity relationship, challenging the disk irradiation model.

Contribution

The study provides spectral evidence linking lag spectra to broad line region gas emission and proposes a new lag-luminosity relation for AGN, emphasizing the role of diffuse emission.

Findings

Lag spectra show spectral signatures of diffuse gas emission.

A new lag-luminosity relationship for AGN is established.

Diffuse emission from BLR clouds explains lag normalization and shape.

Abstract

High cadence, high quality observations of active galactic nuclei (AGN) clearly show continuum variations with lags, relative to the shortest observed variable UV continuum, that increase with wavelength ("lag spectra"). These have been attributed to the irradiation and heating of the central accretion disk by the central X-ray emitting corona. An alternative explanation, connecting the observed lag-spectra to line and continuum emission from gas in the broad line region (BLR), has also been proposed. In this paper I show the clear spectral signature of the time-dependent diffuse gas emission in the lag-spectrum of 6 AGN. I also show a new lag-luminosity relationship for 9 objects which is a scaled down version of the well known lag(H-beta)-L(5100A) relationship in AGN. The shape of the lag-spectrum, and its normalization, are entirely consisted with diffuse emission from radiation pressure supported clouds in a BLR with a covering factor of about 0.2. While some contributions to the continuum lag from the irradiated disk cannot be excluded, there is no need for this explanation.