Rapid continuum variability in NGC 4151

Intensive monitoring of NGC 4151 showed strong, correlated variability at X-ray, ultraviolet, and optical wavelengths. The strongest variations were seen in similar to 1.5 keV X-rays, with weaker variations at similar to 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 Angstrom to 6900 Angstrom. The phase differences between variations in different bands were consistent with zero lag, with upper limits of less than or similar to 0.15 day between 1275 Angstrom and the other ultraviolet bands, less than or similar to 0.3 day between 1275 Angstrom and 1.5 keV, and 1 day between 1275 Angstrom, and 5125 Angstrom, 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 similar to 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. (C)1998 COSPAR. Published by Elsevier Science Ltd.