drive the ultraviolet/optical variations. However, the medium energy X-ray NVA is 2-4 times that in the ultraviolet, and the single-epoch, absorption-corrected X-ray/γ-ray luminosity is only about 1/3 that of the ultraviolet/optical/infrared, suggesting that at most ∼1/3 of the total low-energy flux could be reprocessed high-energy emission.The strong wavelength dependence of the ultraviolet NVAs is consistent with an origin in an accretion disk, with the variable emission coming from the hotter inner regions and non-variable emission from the cooler outer regions. These data, when combined with the results of disk fits, indicate a boundary between these regions near a radius of order R ≈ 0.07 lt-day. No interband lag would be expected as reprocessing (and thus propagation between regions) need not occur, and the orbital time scale of ∼1 day is consistent with the observed variability time scale. However, such a model does not immediately explain the good correlation between ultraviolet and X-ray variations.

We present the results of an intensive ground-based spectrophotometric monitoring campaign of the Seyfert galaxy NGC 4151 for a period of over two months, with a typical temporal resolution of one day. Light curves for four optical continuum bands and the Hα and Hβ emission lines are given. During the monitoring period, the continuum at 6925 Å varied by ∼17% while the continuum at 4600 Å varied by ∼35%, with larger variations in the near UV. The wavelength dependence of the variation amplitude also extends into the far UV. The dependence in the 2700−7200 Å range can be explained by the different relative starlight contributions at different wavelengths, but the large variability at 1275 Å cannot be explained in this way. The continuum variability timescale is of order 13 days and is similar at all optical wavelength bands. No evidence for a time lag between the optical continuum and the UV continuum and emission lines was found. The Hα emission line flux varied by about 12% with a gradual rise throughout the campaign. Its cross correlation with the continuum light curve gives a lag of 0 − 2 days. The variations in the Hβ emission line flux are about 30% and lag the continuum by 0−3 days. This is in contrast to past results where a time lag of 9±2 days was found for both emission lines. This may be due to a different variability timescale of the ionizing continuum, or to a real change in the BLR gas distribution in the 5.5 years interval between the two campaigns.

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