Quantifying the diffuse continuum contribution of BLR Clouds to AGN Continuum Inter-band Delays

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) ∝ r −s ) that approximately reproduce the observed emission line fluxes of the strong UVoptical emission-lines in the best-studied source, NGC 5548. We then determine the contribution of the diffuse continuum to the measured continuum flux and interband 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.

OUP accepted manuscript

“…Instead, the X-ray spectra are consistent with the inner disc being replaced by a hot flow, and the UV spectra and variability are consistent with being produced by reprocessing in dense gas in the broad line region as first suggested by Korista & Goad (2001). This is now strongly supported by more recent work (Chelouche, Nuñez & Kaspi 2018;Lawther et al 2018). It remains to be seen whether similar campaigns on objects at higher L/L Edd will give results which connect better to expectations of standard disc models or whether continuum reverberation from inner BLR scales always dominates the UV variability.…”

“…Similar results are also seen in the sparser data of Chelouche, Nuñez & Kaspi (2018), where they derive a spectrum of the slowly reverberating component in the optical and identify this with the BLR. Lawther et al (2018) show predictions for the diffuse BLR flux for clouds of different densities. It is mainly composed of a series of radiative recombination continuamost obviously at the Balmer and Paschen limit -as well as free-free continua (as was suggested by Barvainis 1987!).…”

Stratified disc wind models for the AGN broad-line region: ultraviolet, optical, and X-ray properties

“…The ionizing flux-density (or φ H -n H ) plane is a commonly used parameter space for photoionization modelling of AGN broad emission lines (e.g. BFKD95; Korista et al 1997;Hamann et al 2002;Baldwin et al 2004;Leighly & Casebeer 2007;Ruff et al 2012;Lawther et al 2018). The position of a parcel of gas in this plane is only a crude measure of its ionization state, since it encodes no information about the shape of the ionizing spectral energy distribution (SED) and says little about the absolute emissivity of the parcel.…”

“…1 and means that a streamline will always intersect regions of the φ H -n H plane that correspond to different ionization state. This advantageous behaviour is specific to axisymmetric geometries; spherically symmetric models such as 'pressure-law' models (Rees et al 1989;Netzer et al 1992;Goad et al 1993;Lawther et al 2018) or a simple spherical wind instead produce single power-law slopes.…”

“…We refer to this model and its derivatives as locally optimally-emitting cloud (LOC) models. LOC models have been applied relatively successfully to observations of AGN, both in general and for individual objects such as NGC 5548 (Korista & Goad 2000, 2004Hamann et al 2002;Nagao, Marconi & Maiolino 2006;Baldwin et al 2004;Lawther et al 2018).…”