1991; Peterson 1993). The cross correlation lag $\tau$ can be taken to be the light-travel time across the BLR, so the BLR size is given by $r=c\tau$. By combining this with the emission-line width $\vFWHM$, the mass of the central source can be inferred to be $$M = \frac{f \vFWHM^2 c\tau}{G},$$ where $f$ is a factor of order unity that depends on the still unknown geometry and kinematics of the BLR. There is an implicit assumption that the gravitational force of the central object dominates the kinematics of the BLR; this is formally unproven, but at least in the case of the well-studied Seyfert 1 galaxy NGC 5548, the reverberation-mapping data are consistent with the required $\vFWHM \propto r^{-1/2}$ relationship (Peterson & Wandel 1999). Virial mass estimates based on reverberation-mapping data are now available for nearly 40 AGNs (Wandel, Peterson, & Malkan 1999; Kaspi et al. 2000).

Beginning in early 1996, we undertook a program of contemporaneous X-ray and optical spectroscopic monitoring of the galaxy NGC 4051, the only NLS1 galaxy in Seyfert’s (1943) original list of high surface-brightness galaxies with strong emission lines. The X-ray variability characteristics of NGC 4051 are typical of the NLS1 class (Lawrence et al. 