1999AJ....117.2177M

We present observations of the nuclear region of the Seyfert galaxy NGC 4151, obtained with the 1.25 m telescope at the Crimean Astrophysical Observatory. On 96 nights during eight observational runs between 1989 February 11 and 1996 June 14, we performed over 1500 simultaneous measurements, using the 20" diameter aperture, in each spectral band of the Johnson UBVRI system. Estimated single-measurement accuracy is greater than 0.01 mag.

The light curve of NGC 4151 in each of the UBVRI bands shows a slow increase in intensity during almost the entire observation period, overlapped by rapid outbursts. The increase in the brightness of the nucleus began in 1990 February and continued through 1996 June, constituting an extraordinarily long-term brightening. Brightening amplitudes and gradients decreased from the U to the I band. Amplitude values were 2.0, 1.5, 0.9, 0.8, and 0.7 mag in UBVRI, respectively. During the 7.3 yr period of observation, variations in brightness gradients for the UBVRI bands were 0.30, 0.23, 0.15, 0.13, and 0.11 mag.yr^–1, respectively. Flares lasting 10-147 days are revealed on smooth UBVRI light curves. Flare amplitudes, A=F_max/F_min, decrease from the U to the I band. During flares, brightness gradients over tens of days in the nucleus exceed those of the observation period as a whole. The largest gradients are observed at the end of the observation period.

UBVRI energy distributions for flux excesses both during flares and during the observation period in general reveal a power-law form: F_ν~ν^α. We discuss our data in terms of a model in which an increase in the brightness in the nucleus of NGC 4151 is caused by the ejection in jet form of clouds emitting synchrotron radiation from the nucleus during its active period. Calculated spectral slopes for the flares are +0.18≥α_pl≥-1.83, for Δα_pl=2.01. The increase in spectral indices over time in the model we have adopted shows that the optical depth of the clouds emitting synchrotron radiation increased from the beginning to the end of the nuclear brightening.