Rapid continuum variability in NGC 4151

Abstract

Intensive monitoring of NGC 4151 showed strong, correlated variability at X-ray, ultraviolet, and optical wavelengths. The strongest variations were seen in ~1.5 keV X-rays, with weaker variations at ~100 keV and no significant variability in the 0.1-1 keV bands. In the ultraviolet/optical regime, the fractional variability amplitude decreased from 9% to 1% as the wavelength increased from 1275 A˚ to 6900 A˚. 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 A˚ and 5125 A˚, an order of magnitude improvement over limits 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 red power-law. The tight limits on the lags indicate that size of the putative reprocessing region is smaller than ~0.15 lt-day in size. After correcting for absorption, the X-ray luminosity variations appear adequate to drive the ultraviolet/optical variations, but energy budget and other arguments indicate that only a fraction of the total low-energy flux could be reprocessed high-energy emission. The data are also consistent with the variable emission coming from the hotter inner regions of an accretion disk and non-variable emission from the cooler outer regions.