UBVRI observations of the nucleus of NGC 1275 from 1989 to 1994: Microvariability

Intranight variability of the NGC 1275 nucleus in the optical range has bee n studied using five series of photoelectric observations obtained over 37 nights from 1989 December 22 to 1994 December 29. The UBVRI monitoring on a single night continued for up to 4.5 hr. In total, 820 measurements were p erformed simultaneously in each of the UBVRI bands (3600 Angstrom lambda(ef f) less than or equal to 8300 Angstrom) with the 1.25 m telescope at the Cr imean Astrophysical Observatory. Flux densities (F) with their standard dev iations (SD) and the measure of intranight variability (SD/F) were calculat ed for each night of observations. On 40% of the nights, intranight activit y in the nucleus was seen at a level of variability SD/F greater than or eq ual to 3% in all spectral bands. Independent activity in the nucleus in the I band (the spectral region around 8300 Angstrom) is suspected on about 10 % of nights. The peak amplitude of the variability, F-max/F-min similar to 40%, was observed only once during the 37 nights. There was suspected low-l evel intranight variability in each of the UBVRI spectral bands for nights with SD/F less than or equal to 3%. The data we obtained lead us to argue t hat during 1989-1994 a process operated in the nucleus of the galaxy NGC 12 75 that caused variations of the optical flux on a timescale of several hou rs. We obtain preliminary evidence that the causes of intranight and intern ight variations seem to be different. All of the data obtained are discusse d from the point of view of current models of active galactic nuclei.