Multiwavelength Observations of Short-Timescale Variability in NGC 4151. IV. Analysis of Multiwavelength Continuum Variability

This paper combines data from the three preceding papers in order to analyze the multi-waveband variability and spectral energy distribution of the Seyfert~1 galaxy NGC~4151 during the December 1993 monitoring campaign. The source, which was near its peak historical brightness, showed strong, correlated variability at X-ray, ultraviolet, and optical wavelengths. The strongest variations were seen in medium energy ($sim$1.5~keV) X-rays, with a normalized variability amplitude (NVA) of 24%. Weaker (NVA = 6%) variations (uncorrelated with those at lower energies) were seen at soft $gamma$-ray energies of $sim$100~keV. No significant variability was seen in softer (0.1--1~keV) X-ray bands. In the ultraviolet/optical regime, the NVA decreased from 9% to 1% as the wavelength increased from 1275~AA to 6900~AA. These data do not probe extreme ultraviolet (1200~AA to 0.1~keV) or hard X-ray (2--50~keV) variability. The phase differences between variations in different bands were consistent with zero lag, with upper limits of $ls$0.15~day between 1275~AA and the other ultraviolet bands, $ls$0.3~day between 1275~AA and 1.5~keV, and $ls$1~day between 1275~AA and 5125~AA. These tight limits represent more than an order of magnitude improvement over those determined in previous multi-waveband AGN monitoring campaigns. The ultraviolet fluctuation power spectra showed no evidence for periodicity, but were instead well-fitted with a very steep, red power-law ($ a = -2.5 $). If photons emitted at a ``primary" waveband are absorbed by nearby material and ``reprocessed" to produce emission at a secondary waveband, causality arguments require that variations in the secondary band follow those in the primary band. The tight interband correlation and limits on the ultraviolet andComment: 35 pages, LaTeX (including aaspp4), including 7 PostScript figures; To appear in the ApJ (October 20, 1996) Vol. 47