SIMBAD references

Observations of the nuclear region of the Seyfert galaxy NGC 7469 obtained at Crimean Astrophysical Observatory with the 1.25 m telescope are presented. During 64 nights on nine observational runs between 1990 September 24 and 1998 October 22 in each spectral band of the Johnson UBVRI system, about 1500 measurements have been performed simultaneously through the round aperture 20" in diameter using differential photometry techniques. The estimated accuracy of each measurement is about 0.01 mag. During the observing period 1990-1996 the mean luminosity of the nucleus was almost constant; only overlapping brightness fluctuations were observed. The mean luminosity level 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, while the minimum amplitude F_max/F_min=1.32 was in the I band for the entire 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 optical, because the slope b of the SF is approximately equal for all wave bands, except for the I band the slope is appreciably distinguished from the others. This would be an indication of the presence of an independent IR energy source in NGC 7469. (2) Considering the same time interval (from 6 minutes to 2 hr) for intranight variability on SFs at different wave bands, one can conclude that flicker noise causes variations observed on the light curve 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 wavelength, the more the contribution of shot noise processes. (3) On intranight light curves of the NGC 7469 there exist rapid flares with durations ∼25 minutes at U band, ∼55 minutes at B, V bands, and ∼2 hr at R, I bands–a typical timescale of intranight variability increasing with the increasing wavelength.

In order to examine the intranight variations of the nucleus of NGC 7469, standard deviations (SDs) of the nightly averaged flux, F, and a measure of intranight variability, SD/F, were calculated for each night of observations. The ratios P=N_i/N_tot of number of nights with SD/F≥1%, 2%, 3%, and so forth (N_i) to all 64 observational nights (N_tot) were expressed as a percentage of probability to detect variability at a given level; they were plotted versus the appropriate parameter SD/F. It is interesting that a probability to detect intranight variability at a level 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 using the same technique has made statistical comparisons possible among different galaxies and different wave bands. Therefore, analogous curves for the 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 galaxy NGC 1275 of BL Lacertae type shows more nights with variations of amplitude 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 range of ∼4%-8% is a common characteristic of the two Seyfert 1 galaxies (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-order exponential decay. As a result, one can conclude that intranight variability is really transient in character and has manifested itself with different probabilities for different galaxies. The efficiency of the energy source for every galaxy to produce intranight variability on a given level (duty cycle of the central energy engine) was estimated using these probability curves. For a given threshold of detection estimated as an error of a given 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 ∼60% of nights show intranight variability on a level greater than 1%. The efficiency of the energy source producing the near-IR radiation of NGC 7469 is more than 0.9, while at the optical bands it is equal to ∼0.7±0.1. It may be an indication of the presence of an independent IR energy source, too.