Abstract
Disc reverberation mapping of a handful of nearby active galactic nuclei
(AGNs) suggests accretion disc sizes that are a factor of a few too
large for their luminosities, apparently at odds with the standard
model. Here, we investigate the likely contribution to the measured
delay signature of diffuse continuum emission arising from broad-line
region gas. We start by constructing spherically symmetric pressure-law
BLR models (i.e. P(r)∝r-s) that approximately reproduce
the observed emission line fluxes of the strong UV-optical emission
lines in the best-studied source, NGC 5548. We then determine the
contribution of the diffuse continuum to the measured continuum flux and
inter-band delays, accounting for the observed variability behaviour of
the ionizing nuclear continuum. Those pressure-law models that
approximately reproduce the observed emission-line luminosities
unavoidably produce substantial diffuse continuum emission. This causes
a significant contamination of the disc reverberation signature (i.e.
wavelength-dependent continuum delays). Qualitatively, the diffuse
continuum delay signatures produced by our models resemble that observed
for NGC 5548, including the deviation of the lag spectrum above that of
a simple power law in wavelength, short-ward of the Balmer and Paschen
jumps. Furthermore, for reasonable estimates of the BLR covering
fraction, the delay induced by diffuse continuum emission causes
elevated inter-band delays over the entire UV-optical regime; for these
pressure-law models, there are no `disc-dominated' wavelength intervals.
Thus, the diffuse continuum contribution must be taken into account in
order to correctly infer AGN accretion disc sizes based on inter-band
continuum delays.
| Originalsprog | Engelsk |
|---|---|
| Tidsskrift | Monthly Notices of the Royal Astronomical Society |
| Vol/bind | 481 |
| Udgave nummer | 1 |
| Sider (fra-til) | 533-554 |
| ISSN | 0035-8711 |
| DOI | |
| Status | Udgivet - 21 nov. 2018 |