AGN central masses and broad-line region sizes

From objects with multiple line measurements, we find that the highest precision measure of the virial product c{tau}{Delta}V^2^/G, where {tau} is the emission-line lag relative to continuum variations and {Delta}V is the emission-line width, is obtained by using the cross-correlation function centroid (as opposed to the cross-correlation function peak) for the time delay and the line dispersion (as opposed to FWHM) for the line width and by measuring the line width in the variable part of the spectrum.