Reverberation mapping
= Reverberation mapping
{title2=$M=f_{\rm geom}c\tau\Delta v^2/G$}
Time-dependent inference of an emitting region's response to a variable illuminating continuum. For an active nucleus, a broad-line delay estimates a light-travel scale and a line width estimates a velocity scale. A virial <mass> estimate then requires a geometrical and kinematic factor $f_{\rm geom}$; the lag alone neither fixes the geometry nor establishes purely gravitational motion.