Echo Mappping of the Broad Line Region of AGNs -- A Critical Appraisal

The results of recent AGN monitoring campaigns confirm the ``big picture'' of the echo paradigm, but the details of the emission-line light curves cannot be accurately reproduced with only the simplest assumptions, some of which must be invalid. I discuss possible solutions. I present some preliminary optical light curves from Wise Observatory for NGC 4151 during the December 1993 multi-satellite campaign. The optical data show a continuity with the complex behavior observed in the IUE data, and may explain the peculiarities in emission-line response seen in this and other AGNs. I review some recent results on quasar emission line variability from the Steward-Wise PG quasar monitoring program, which allow extension of the observed AGN BLR Radius--Luminosity relation to higher luminosities than previously feasible. Agreement with the expected $R\propto L^{1/2}$ relation is suggested. Finally, I criticize the trend to attribute significance to the details of transfer functions recovered by inversion techniques. I show, as an example, that the model emission-line light curves produced by convolving the 5-year continuum light curve of NGC 5548 with transfer functions peaked at zero or non-zero lag have differences much smaller that the uncertainties in the H$\beta$ light curve. Transfer functions of both kinds can reproduce the data equally well. I emphasize the need to use modeling, rather than inversion methods, in order to delineate the regions of BLR parameter space allowed and ruled out by the data.