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 been 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 performed simultaneously in each of the UBVRI bands (3600 Angstrom lambda(eff) less than or equal to 8300 Angstrom) with the 1.25 m telescope at the Crimean Astrophysical Observatory. Flux densities (F) with their standard deviations (SD) and the measure of intranight variability (SD/F) were calculated for each night of observations. On 40% of the nights, intranight activity in the nucleus was seen at a level of variability SD/F greater than or equal 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-level 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 that during 1989-1994 a process operated in the nucleus of the galaxy NGC 1275 that caused variations of the optical flux on a timescale of several hours. We obtain preliminary evidence that the causes of intranight and internight variations seem to be different. All of the data obtained are discussed from the point of view of current models of active galactic nuclei.