Quantifying the impact of variable BLR diffuse continuum contributions on measured continuum interband delays
Abstract
We investigate the contribution of reprocessed continuum emission (1000-10 000 Å) originating in broad-line region (BLR) gas, the diffuse continuum (DC), to the wavelength-dependent continuum delays measured in AGN disc reverberation mapping experiments. Assuming a spherical BLR geometry, we adopt a Local Optimally emitting Cloud (LOC) model for the BLR that approximately reproduces the broad emission-line strengths of the strongest UV lines (Ly α and C IV) in NGC 5548. Within this LOC framework, we explore how assumptions about the gas hydrogen density and column density distributions influence flux and delay spectra of the DC. We find that: (i) models which match well measured emission-line luminosities and time delays also produce a significant DC component, (ii) increased n_H and/or N_H, particularly at smaller BLR radii, result in larger DC luminosities and reduced DC delays, (iii) in a given continuum band the relative importance of the DC component to the measured interband delays is proportional (though not 1:1) to its fractional contribution to the total light in that band, (iv) the measured DC delays and DC variability amplitude depend also on the variability amplitude and characteristic variability time-scale of the driving continuum, (v) the DC radial surface emissivity distributions F(r) approximate power laws in radius with indices close to -2 (≈1:1 response to variations in the driving continuum flux), thus their physics is relatively simple and less sensitive to the unknown geometry and uncertainties in radiative transfer. Finally, we provide a simple recipe for estimating the DC contribution in disc reverberation mapping experiments.
- Publication:
-
Monthly Notices of the Royal Astronomical Society
- Pub Date:
- November 2019
- DOI:
- 10.1093/mnras/stz2330
- arXiv:
- arXiv:1908.07757
- Bibcode:
- 2019MNRAS.489.5284K
- Keywords:
-
- methods: numerical;
- galaxies: active;
- quasars: emission lines;
- galaxies: Seyfert;
- Astrophysics - Astrophysics of Galaxies
- E-Print:
- 20 pages, 17 figures. Accepted for publication in MNRAS