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 gala xies NGC 3516 and NGC 5548 since 1969. In the present paper we give ne w 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 approximately 7 min time reso lution. The variability of the nuclei of these galaxies is manifested over different time intervals and is most pronounced in NGC 5548. In b oth nuclei, against a background of slowly varying brightness with a c haracteristic time approximately 9 years for NGC 3516 and approximatel y 20 years for NGC 5548, fluctuations are observed with amplitude 0.2m -0.4m in the U band and a characteristic time approximately 3 days. Th e maximum amplitude of variation in the U band reaches 1.6m in NGC 351 6 and 2.4m in NGC 5548. Fades by approximately 1.5m (U) in 1990 and 19 92 are a noteworthy feature of the light curve of NGC 5548. The amplit ude of microvariability in NGC 3516 is of the same order as that in NG C 4151 and NGC 7469, i.e., approximately 10% in 15-20 min, while that in NGC 5548 reaches 0.5m in 12 min in the U band and 0.16m in 30 min i n the V band. This is the highest rate of variability in the visible e ver observed in Seyfert nuclei. The color indices of the variable sour ce in the nucleus of NGC 3516 do not vary, on the average, as its brig htness varies, and it has an ultraviolet emission excess in comparison to synchrotron emission. The main variable source in the nucleus of N GC 5548 also has an ultraviolet excess in comparison to synchrotron em ission, but in contrast to NGC 3516, its color indices decrease with i ncreasing brightness. Moreover, 10% of the time, emission even bluer t han 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 litt le relationship to the main photoionization mechanism.