MULTIWAVELENGTH OBSERVATIONS OF SHORT-TIMESCALE VARIABILITY IN NGC-4151 .4. ANALYSIS OF MULTIWAVELENGTH CONTINUUM VARIABILITY

This paper combines data from the three preceding papers in order to a nalyze the multi-wave-band variability and spectral energy distributio n of the Seyfert 1 galaxy NGC 4151 during the 1993 December monitoring campaign. The source, which was near its peak historical brightness, showed strong, correlated variability at X-ray, ultraviolet, and optic al wavelengths. The strongest variations were seen in medium-energy (s imilar to 1.5 keV) X-rays, with a normalized variability amplitude (NV A) of 24%. Weaker (NVA = 6%) variations (uncorrelated with those at lo wer energies) were seen at soft gamma-ray energies of similar to 100 k eV. No significant variability was seen in softer (0.1-1 keV) X-ray ba nds. In the ultraviolet/optical regime, the NVA decreased from 9% to 1 % as the wavelength increased from 1275 to 6900 Angstrom These data do not probe extreme ultraviolet (1200 Angstrom to 0.1 keV) or hard X-ra y (2-50 keV) variability. The phase differences between variations in different bands were consistent with zero lag, with upper limits of le ss than or similar to 0.15 day between 1275 Angstrom and the other ult raviolet bands, less than or similar to 0,3 day between 1275 Angstrom and 1.5 keV, and less than or similar to 1 day between 1275 and 5125 A ngstrom These tight limits represent more than an order of magnitude i mprovement over those determined in previous multi-wave-band AGN monit oring campaigns. The ultraviolet fluctuation power spectra showed no e vidence for periodicity, but were instead well fitted with a very stee p, red power law (a less than or equal to -2.5). If photons emitted at a ''primary'' wave band are absorbed by nearby material and ''reproce ssed'' to produce emission at a secondary wave band, causality argumen ts require that variations in the secondary band follow those in the p rimary band. The tight interband correlation and limits on the ultravi olet and medium-energy X-ray lags indicate that the reprocessing regio n is smaller than similar to 0.15 it-day in size. After correcting for strong (a factor of greater than or similar to 15) line-of-sight abso rption, the medium-energy X-ray luminosity variations appear adequate to drive the ultraviolet/optical variations. However, the medium-energ y X-ray NVA is 2-4 times that in the ultraviolet, and the single-epoch , absorption-corrected X-ray/gamma-ray luminosity is only about one-th ird of that of the ultraviolet/optical/infrared, suggesting that at mo st about a third of the total low-energy flux could be reprocessed hig h-energy emission. The strong wavelength dependence of the ultraviolet NVAs is consistent with an origin in an accretion disk, with the vari able emission coming from the hotter inner regions and nonvariable emi ssion from the cooler outer regions. These data, when combined with th e results of disk fits, indicate a boundary between these regions near a radius of order R approximate to 0.07 1t-day. No interband lag woul d be expected, as reprocessing (and thus propagation between regions) need not occur, and the orbital timescale of similar to 1 day is consi stent with the observed variability timescale. However, such a model d oes not immediately explain the good correlation between ultraviolet a nd X-ray variations.