2002A&A...387...40M

An ongoing program on photometric and spectral monitoring of some bright Seyfert galaxies has been carried out at the Crimean Astrophysical Observatory since 1989.
Result. of photometric observations of NGC 5548 obtained with the 1.25m telescope are reported in this paper; it focuses on the analysis of intranight variations. During 44 observational nights in 1993-1999 in each spectral band of the Johnson UBVRI system, 672 measurements have been performed simultaneously through the round aperture (diameter 15'') using differential photometry techniques. The estimated accuracy of each measurement is about 0.01 mag. The peak amplitude R_ max_=Fmax_/Fmin_=2.99 for the whole light curve was observed in the U band, while the minimum amplitude F_max/F_min=1.37 occurred in the I band during the full observation period. UBVRI observations and good sampled data of international monitoring campaigns of NGC 5548, were used to calculate Structure Functions. A comparison is made of the characteristics of the long and short time scale variations of NGC 5548 with those of NGC 4151, NGC 7469 and NGC 1275. In order to examine the intranight variations of the nucleus of NGC 5548, standard deviations (SD) of the nightly averaged flux F, and a measure of intranight variability - SD/F were calculated for each night. Using this parameter, a probability characteristics is introduced, and duty cycles (the fraction of time when the galaxy is variable), characterizing the efficiency of the central energy source, were evaluated. It is concluded that intranight variability is really transient in character and manifests itself with different probabilities for different galaxies.