Abstract
Disk reverberation mapping of a handful of nearby AGN suggest accretion disk sizes which are a factor 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., $P(r)\propto r^{-s}$) 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 disk 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 `disk-dominated' wavelength intervals. Thus, the diffuse continuum contribution must be taken into account in order to correctly infer AGN accretion disk sizes based on inter-band continuum delays.
Highlights
The immense energy outputs of active galactic nuclei (AGN) are fueled by gas accretion onto supermassive black holes (e.g., Lynden-Bell & Rees 1971), which reside at the centres of most - perhaps all - massive galaxies (e.g., Kormendy & Gebhardt 2001)
We investigate the likely contribution to the measured delay signature of diffuse continuum emission arising from broad line region gas
For reasonable estimates of the broad line region (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 ‘disk-dominated’ wavelength intervals
Summary
The immense energy outputs of active galactic nuclei (AGN) are fueled by gas accretion onto supermassive black holes (e.g., Lynden-Bell & Rees 1971), which reside at the centres of most - perhaps all - massive galaxies (e.g., Kormendy & Gebhardt 2001). Supermassive black holes in the local Universe probably achieved their current masses through bouts of AGN activity. The details of these accretion episodes are not well-understood. The in-falling gas is thought to form an accretion disk that heats up and emits much of the observed continuum radiation; Shakura & Sunyaev (1973) develop the standard αdisk model of steady-state disk accretion, originally in the context of X-ray binaries. The nuclear continuum in turn excites fast moving gas (clouds) near the central engine, which emit the observed broad emission lines (BEL); these clouds reside in the so-called broad line region (BLR). On larger c 0000 The Authors scales, the nuclear continuum excites the dusty obscuring structure (e.g., Antonucci 1993) to produce the infrared dust emission feature (e.g., Barvainis 1987), and excites gas in the host galaxy bulge to produce the narrow emission lines, the kinematics of which are dominated by the host galaxy potential (e.g., Nelson & Whittle 1996)
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