UBVRI light curves of the Seyfert galaxy NGC 7469 during 1990-1998: Microvariability

Observations of the nuclear region of the Seyfert galaxy NGC 7469 obtained at Crimean Astrophysical Observatory with the 1.25 m telescope are presente d. During 64 nights on nine observational runs between 1990 September 24 an d 1998 October 22 in each spectral band of the Johnson UBVRI system, about 1500 measurements have been performed simultaneously through the round aper ture 20 " in diameter using differential photometry techniques. The estimat ed accuracy of each measurement is about 0.01 mag. During the observing per iod 1990-1996 the mean luminosity of the nucleus was almost constant; only overlapping brightness fluctuations:were observed. The mean luminosity leve l has been raised in 1996 October. The peak amplitude (maximum flux/minimum flux) F-max/F-min = 2.09 on the light curves was observed in the U band, w hile the minimum:amplitude F-max/F-min = 1.32 was in the I band for the ent ire observation period. Using structure function (SF) analysis, the following conclusions have been made: (1) Long-term variability is caused by the same processes in the opt ical, because the slope b of the SF is approximately equal for all wave ban ds, except for the I band the slope is appreciably distinguished from the o thers. This would be an indication of the presence of an independent IR ene rgy source in NGC 7469. (2) Considering the same time interval (from 6 minu tes to 2 hr) for intranight variability on SFs at different wave bands, one can conclude that flicker noise causes variations observed on the light cu rve at the UV region (U and B bands), while at the near-IR region the light curve is formed by mixed shot noise and flicker noise-the greater the wave length, the more the contribution of shot noise processes. (3) On intranigh t light curves of the NGC 7469 there exist rapid hares with durations simil ar to 25 minutes at U band, similar to 55 minutes at B, V bands, and simila r to 2 hr at R, I bands-a typical timescale of intranight variability incre asing with the increasing wavelength. In order to examine the intranight variations of the nucleus of NGC 7469, s tandard deviations (SDs) of the nightly averaged flux, F, and a measure of intranight variability, SD/F, were calculated for each night of observation s. The ratios P = N-i/N-tot of number of nights with SD/F greater than or e qual to 1%, 2%, 3%, and so forth (N-i) to all 64 observational nights (N-to t) were expressed as a percentage of probability to detect variability at a given level; they were plotted versus the appropriate parameter SD/F. It i s interesting that a probability to detect intranight variability at a leve l given by a parameter SD/F may be fitted well by a "probability curve" for a given galaxy. The homogeneity of data obtained with the same telescope u sing the same technique has made statistical comparisons possible among dif ferent galaxies and different wave bands. Therefore, analogous curves for t he Seyfert galaxies NGC 1275 and NGC 4151 were plotted and were averaged by bands. Probability curves for the two galaxies of the same type (Seyfert 1 ; NGC 4151 and NGC 7469) coincided well. The probability curve for the gala xy NGC 1275 of BL Lacertae type shows more nights with variations of amplit ude less than 7%. The probability to observe intranight variations with an amplitude of more than 7% is approximately equal for both types of objects. An excess of nights with intranight variability parameter SD/F in the rang e of similar to 4%-8% is a common characteristic of the two Seyfert 1 galax ies (NGC 4151 and NGC 7469) and of the BL Lac object (NGC 1275) studied in this paper. This excess may be connected with a specific type of intranight galaxy variability. All probability curves are fitted best by the second-o rder exponential decay. As a result, one can conclude that intranight varia bility is really transient in character and has manifested itself with diff erent probabilities for different galaxies. The efficiency of the energy so urce for every galaxy to produce intranight variability on a given level (d uty cycle of the central energy engine) was estimated using these probabili ty curves. For a given threshold of detection estimated as an error of a gi ven technique (for instance, 1% for observations presented in this paper), one can conclude that duty cycle of NGC 1275 is about 100%. Vice versa, NGC 4151 has a more quiet energy source: only similar to 60% of nights show in tranight variability on a level greater than 1%. The efficiency of the ener gy source producing the near-IR radiation of NGC 7469 is more than 0.9, whi le at the optical bands it is equal to similar to 0.7 +/- 0.1. It may be an indication of the presence of an independent IR energy source, too.