Annu. Rev. Astron. Astrophys. 1997. 35: 445-502 Copyright © 1997 by Annual Reviews. All rights reserved

4. EMISSION LINE VARIABILITY: RATIONALE AND METHODS

4.1. Rationale for Emission Line Variability Study

The exciting prospect of determining the mass of the central black hole is the main motivation for monitoring emission line variations in AGN. The mass is estimated in the following way. Variations in the emission line strengths of AGN are observed to echo the continuum variations with a time delay, which can be interpreted as the light travel time between the central source and the surrounding high velocity gas clouds (the BLR). Combining the radial distance to the line-emitting gas with its velocity (assuming virialized motions) allows determination of the mass of the central black hole.

It is clearly important to assess whether the gas is gravitationally bound, as well as to search for kinematic evidence of accretion and/or ordered gas motions such as infall or a rotating disk. Reverberation (or echo) mapping is a technique for inferring the structure and velocity field of the BLR from the time delays between continuum and line variations. The basic assumptions are that the BLR is ionized by a central continuum point source, the light travel time between continuum and gas clouds is much longer than the ionization or recombination times, and the line intensity is linearly correlated with the incident continuum flux (e.g. Peterson 1993).