Nuclear activity of NGC 5548 from UBVRI observations during 1990–2000

We present our UBV RI observations of the nucleus of the Seyfert Galaxy NGC 5548 from 1990 until 2000 at four astronomical observatories: the Crimean Station of the Sternberg Astronomical Institute, the Special Astrophysical Observatory of the Russian Academy of Sciences, the Crimean Astrophysical Observatory (Ukraine), and the Mount Maidanak Observatory of the Ulugbek Astronomical Institute (Uzbekistan). All data have been reduced to a single system and are given for an A=14.3″ aperture. Large light variations have been observed in the galactic nucleus over the last ten years: the total variability amplitude through the A=14.3″ aperture was \(1\mathop .\limits^m 8\), \(1\mathop .\limits^m 4\), \(0\mathop .\limits^m 94\), \(0\mathop .\limits^m 65\), and \(0\mathop .\limits^m 35\) in U, B, V, R_J, and I_J, respectively. The structure function of the variable component shows a power-law dependence on the time shift with the index b≈0.7. Treating the variability as a superposition of random outbursts with various durations and amplitudes is consistent with the observations. The maximum outburst duration inferred from the structure function is about 500 days. The best agreement between the observed and computed color—magnitude relations is achieved when the galaxy’s brightness and color indices through the A=14.3″ aperture are V=14.17, U-B=0.23, B-V=0.95, V-R_J=1.06, V-I_J=1.68, while the colors of the variable component are, on average, U-B=−1.10, B-V=0.15, V-R_J=0.50, V-I_C=0.75, and they vary only slightly with its brightness. This implies that the outburst color indices are constant to a first approximation. However, there is evidence that the energy distribution of some outbursts differs from the average one. Some outbursts may exhibit enhanced ultraviolet and optical U radiation.