Multiwavelength observations of short time-scale variability in NGC 4151. II. Optical observations

We present the results of an intensive ground-based spectrophotometric monitoring campaign of the Seyfert galaxy NGC~4151 for a period of over two months, with a typical temporal resolution of one day. Light curves for four optical continuum bands and the H$\alpha$ and H$\beta$ emission lines are given. During the monitoring period, the continuum at 6925~\AA\ varied by $\sim$17% while the continuum at 4600~\AA\ varied by $\sim$35%, with larger variations in the near UV. The wavelength dependence of the variation amplitude also extends into the far UV. The dependence in the 2700$-$7200~\AA\ range can be explained by the different relative starlight contributions at different wavelengths, but the large variability at 1275~\AA\ cannot be explained in this way. The continuum variability timescale is of order 13 days and is similar at all optical wavelength bands. No evidence for a time lag between the optical continuum and the UV continuum and emission lines was found. The H$\alpha$ emission line flux varied by about 12% with a gradual rise throughout the campaign. Its cross correlation with the continuum light curve gives a lag of $0-2$ days. The variations in the H$\beta$ emission line flux are about 30% and lag the continuum by 0$-$3 days. This is in contrast to past results where a time lag of 9$\pm$2 days was found for both emission lines. This may be due to a different variability timescale of the {\em{ionizing}} continuum, or to a real change in the BLR gas distribution in the 5.5 years interval between the two campaigns.