Multiwavelength observations of short-timescale variability in NGC 4151 .2. 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 H alpha and H beta emission lines are tabulated. During the monitoring period, the continuum at 6925 Angstrom varied by similar to 17% while the continuum at 4600 Angstrom varied by similar to 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 Angstrom range can be explained by the different relative starlight contributions at different wavelengths, but the large variability at 1275 Angstrom 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 H alpha emission-line flux varied by similar to 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 H beta emission-line flux are similar to 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.