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

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Abstract

This paper combines data from the three preceding papers in order to analyze the multi-wave-band variability and spectral energy distribution of the Seyfert I 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 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 to 6900 A. These data do not probe extreme ultraviolet (1200 A 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 A and the other ultraviolet bands, <~0.3 day between 1275 A and 1.5 keV, and <~1 day between 1275 and 512 A. These tight limits represent more than an order of magnitude improvement over those determined in previous multi wave band 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" wave band are absorbed by nearby material and "reprocessed" to produce emission at a secondary wave band, 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 (a 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 one third of that of the ultraviolet optical/infrared, suggesting that at most about a third 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 nonvariable 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 timescale of 1 day is consistent with the observed variability timescale. However, such a model does not immediately explain the good correlation between ultraviolet and X-ray variations.

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