Multiwavelength Observations of Short Time-Scale Variability in NGC 4151. IV. Analysis of Multiwavelength Continuum Variability
Edelson R.E., et al., 1996, ApJ, 470, 364

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 (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 ~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 Å to 6900 Å. These data do not probe extreme ultraviolet (1200 Å 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 <0.15 day between 1275 Å and the other ultraviolet bands, <0.3 day between 1275 Å and 1.5 keV, and < 1 day between 1275 Å and 5125 Å. 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 and medium energy X-ray lags indicate that the reprocessing region is smaller than ~0.15~lt-day in size. After correcting for strong (factor of <15) line of sight absorption, the medium energy X-ray luminosity variations appear adequate to 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/gamma-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 approx 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.