Optical variability of the nuclei of the Seyfert galaxies NGC 3516 and NGC 5548 on time scales from 20 years to 20 minutes

We have monitored the UBV brightness of the nuclei of the Seyfert galaxies NGC 3516 and NGC 5548 since 1969. In the present paper we give new UBV observations (since 1976) of these nuclei and analyze all of the available UBV observations. We also analyze our observations of each object over four nights in one band with 7 min time resolution. The variability of the nuclei of these galaxies is manifested over different time intervals and is most pronounced in NGC 5548. In both nuclei, against a background of slowly varying brightness with a characteristic time ~9 years for NGC 3516 and ~20 years for NGC 5548, fluctuations are observed with amplitude 0.2^m^-0.4^m^ in the U band and a characteristic time ~3 days. The maximum amplitude of variation in the U band reaches 1.6^m^ in NGC 3516 and 2.4^m^ in NGC 5548. Fades by ~1.5^m^l (U) in 1990 and 1992 are a noteworthy feature of the light curve of NGC 5548. The amplitude of microvariability in NGC 3516 is of the same order as that in NGC 4151 and NGC 7469, i.e., ~10% in 15-20 min, while that in NGC 5548 reaches 0.5^m^ in 12 min in the U band and 0.16^m^ in 30 min in the V band. This is the highest rate of variability in the visible ever observed in Seyfert nuclei. The color indices of the variable source in the nucleus of NGC 3516 do not vary, on the average, as its brightness varies, and it has an ultraviolet emission excess in comparison to synchrotron emission. The main variable source in the nucleus of NGC 5548 also has an ultraviolet excess in comparison to synchrotron emission, but in contrast to NGC 3516, its color indices decrease with increasing brightness. Moreover, 10% of the time, emission even bluer than in the main source is observed in the nucleus of NGC 5548, and 70% of those cases occur in two deep fades. The color characteristics of that emission correspond to a gaseous continuum and probably have little relationship to the main photoionization mechanism.