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

We present our UBVRI observations of the nucleus of the Seyfert Galaxy NGC 5548 from 1990 until 2000 at four astronomical observatories: the Crimean S tation of the Sternberg Astronomical Institute, the Special Astrophysical O bservatory of the Russian Academy of Sciences, the Crimean Astrophysical Ob servatory (Ukraine), and the Mount Maidanak Observatory of the Ulugbek Astr onomical Institute (Uzbekistan). All data have been reduced to a single sys tem and are given for an A = 14.3 " aperture. Large light variations have b een observed in the galactic nucleus over the last ten years: the total var iability amplitude through the A = 14.3 " aperture was 1.(m)8, 1.(m)4, 0.(m )94, 0.(m)65, and 0.(m)35 in U, B, V, R-J, and I-J, respectively. The struc ture function of the variable component shows a power-law dependence on the time shift with the index b approximate to 0.7. Treating the variability a s a superposition of random outbursts with various durations and amplitudes is consistent with the observations. The maximum outburst duration inferre d from the structure function is about 500 days. The best agreement between the observed and computed color-magnitude relations is achieved when the g alaxy'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 it s brightness. This implies that the outburst color indices are constant to a first approximation. However, there is evidence that the energy distribut ion of some outbursts differs from the average one. Some outbursts may exhi bit enhanced ultraviolet and optical U radiation. (C) 2001 MAIK "Nauka/Inte rperiodica".