Simulating Disky Broad Line Region Reverberation

Abstract

Variability studies of the broad emission lines of Active Galactic Nuclei (AGNs) and quasars show stochastic radial velocity variations (i.e., fluctuations in the centroid of the line), 'jitter', on timescales of weeks to months. This jitter may be intrinsic as the broad-line emitting region (BLR) reverberates from the AGN continuum. There are also coordinated variations in the width of the broad emission lines and the luminosity of the central source ('breathing' or 'anti-breathing') which remain unexplained. These can be used as a tool for testing models of the BLR. We have constructed a pipeline to simulate a disk-like BLR geometry that reverberates in response to various chosen continuum light curves and produce synthetic emission line profiles. These profiles can then be characterized by measured shape parameters (centroid velocity shift, velocity dispersion, and Pearson skewness coefficient) and compared to observed time series of those same parameters. We have found that through our pipeline, we can recreate the velocity jitter at similar variations found in observations. The computational tools presented in this paper will also be applicable to case studies of quasars observed under the Sloan Digital Sky Survey V (SDSS-V) Black Hole Mapper reverberation mapping program. This paper is the first in a series of papers -- in this paper, we present the model and pipeline, and in future papers, we will present applications.

Figures (15)

• Figure 1: Example of an emissivity map with a single power-law emissivity. The power-law goes as $r^{-3}$. The inner and outer radii of the BLR are given in units of the gravitational radius, as defined in Section \ref{['subsec:linecalc']}, and are 1750 and 14000 respectively. The brightness is relative to the flux at the inner radius.
• Figure 2: Example of an emissivity map with a single-arm spiral. The same geometrical parameters from Figure \ref{['fig:powerlaw']} are applied here. Parameters of the spiral arm are: the azimuthal angle $\phi_0 = 30^{\circ}$, the pitch angle $p = 20^{\circ}$, the width $\delta = 30^{\circ}$, and the contrast $A=10.0$. The shading represents the excess brightness relative to the underlying BLR emissivity.
• Figure 3: Example of the escape probability map of line photons emitted at a specific location on the disk. The same geometrical parameters from Figure \ref{['fig:powerlaw']} are applied here. The radiative transfer parameters from equation (\ref{['tau']}) are $\tau_0 = 10.0$ and $\eta = 0.5$, as constrained by the work of chajet13. The observer is to the right of the disk, with an inclination angle of 30$^{\circ}$ between the direction of the observer and the axis of the disk.
• Figure 4: The basic process of the pipeline, with arrows to indicate the progression. Boxes in blue are represent statistical or empirical processes, boxes in red represent physical processes, and boxes in white represent outputs that can be utilized and examined.
• Figure 5: Examples of light curves generated using the Damped Random Walk model, as described in macleod10. The input parameters were a rest-frame wavelength of 900 Å and an Eddington ratio of 0.1. We varied the mass of the central black hole in the range of $10^7-10^9 M_{\odot}$ and normalized the light curves by their maximum value.