High-rate active galaxy monitoring at the Wise Observatory. III - The broad-line region of NGC 4151

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Abstract

We present results of our AGN monitoring program for the Seyfert galaxy NGC 4151. The high sampling rate and flux calibration accuracy of our observations allowed us in the first two papers in this series to measure accurately the size of the broad line region (BLR) in two Seyfert galaxies, Mrk 279 and NGC 5548. In the present paper, these features of the observations, coupled with large emission-line variations in NGC 4151 during the monitoring period, give light curves whose quality allows, for the first time, a detailed study of the space and velocity distribution of the BLR gas in an AGN. Through cross-correlation, we find that both Hα and Hβ lag the continuum variations by 9 +/- 12 days. Through Fourier and maximum entropy deconvolutions of the continuum and emission-line light curves, we recover the approximate transfer function of the BLR in this object. We carry out a fitting procedure to the emission-line light curve to investigate the parameter space of the BLR. The latter two analyses suggest that the BLR can best be modeled as either a thick spherical shell or a thin disk viewed nearly edge-on. The ratio of outer to inner radii in these geometries is of order 10, with the inner radius of order several light-days. The line emission of the BLR decreases or is constant with distance from the center. Large regions of parameter space can be significantly excluded. In particular, flat geometries viewed nearly face-on, with line emission increasing with distance from the center, fail to reproduce the observations. The blue and the red wings of the Hα and Hβ line profile have an almost identical light curve with little time lag between them. Cross- correlation analysis and simulations significantly rule out pure unidirectional radial motion (either inflow or outflow) of the BLR gas. The consequences of the above findings to our current understanding of AGNs are briefly discussed.

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