X-ray and optical variability in NGC 4051 and the nature of narrow-line Seyfert 1 galaxies

We report on the results of a three-year program of coordinated X-ray and optical monitoring of the narrow-line Seyfert 1 galaxy NGC 4051. The rapid continuum variations observed in the X-ray spectra are not detected in the optical, although the time-averaged X-ray and optical continuum fluxes are well correlated. Variations in the flux of the broad H beta line are found to lag behind the optical continuum variations by 6 days (with an uncertainty of 2-3 days), and combining this with the line width yields a virial mass estimate of similar to1.1 x 10(6) M-., at the very low end of the distribution of active galactic nucleus masses measured by line reverberation. Strong variability of He II lambda 4686 is also detected, and the response time measured is similar to that of H beta but with a much larger uncertainty. The He II lambda 4686 line is almost 5 times broader than H beta, and it is strongly blueward asymmetric, as are the high-ionization UV lines recorded in archival spectra of NGC 4051. The data are consistent with the Balmer lines arising in a low-to-moderate-inclination disklike configuration and the high-ionization lines arising in an outflowing wind, of which we observe preferentially the near side. Previous observations of the narrow-line region morphology of this source suggest that the system is inclined by similar to 50 degrees, and if this is applicable to the broad H beta -emitting region, a central mass of similar to1.4 x 10(6) M-. can be inferred. During the third year of monitoring, both the X-ray continuum and the He II lambda 4686 line went into extremely low states, although the optical continuum and the H beta broad line were both still present and variable. We suggest that the inner part of the accretion disk may have gone into an advection-dominated state, yielding little radiation from the hotter inner disk.