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

We present the results of an intensive ground-based spectrophotometric monitoring campaign on the Seyfert galaxy NGC 4151 for a period of over 2 months, with a typical temporal resolution of 1 day. Light curves for four optical continuum bands and the Hα and Hβ emission lines are tabulated. During the monitoring period, the continuum at 6925 Å varied by ∼17% while the continuum at 4600 Å varied by ∼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 A range can be explained by the different relative starlight contributions at different wavelengths, but the large variability at 1275 Å cannot be explained in this way. The continuum variability timescale is of order 13 days and is similar in all optical wavelength bands. No evidence of a time lag between the optical continuum and the UV continuum and emission lines was found. The Ha emission-line flux varied by ∼12%, with a gradual rise throughout the campaign. Its cross-correlation with the continuum light curve yields a lag of 0-2 days. The variations in the Hβ emission-line flux are ∼30% and lag the continuum by 0-3 days. This is in contrast to past results in which a time lag of 9 ± 2 days was found for both emission lines. This may be due to a different variability timescale of the ionizing continuum or to a real change in the broad-line region gas distribution in the 5.5 yr interval between the two campaigns.