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 ov er 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 69 25 Angstrom varied by similar to 17% while the continuum at 4600 Angst rom varied by similar to 35%, with larger variations in the near-UV. T he wavelength dependence of the variation amplitude also extends into the far-UV. The dependence in the 2700-7200 Angstrom range can be expl ained by the different relative starlight contributions at different w avelengths, but the large variability at 1275 Angstrom cannot be expla ined in this way. The continuum variability timescale is of order 13 d ays and is similar in all optical wavelength bands. No evidence of a t ime lag between the optical continuum and the UV continuum and emissio n lines was found. The H alpha emission-line flux varied by similar to 12%, with a gradual rise throughout the campaign. Its cross-correlati on with the continuum light curve yields a lag of 0-2 days. The variat ions in the H beta emission-line flux are similar to 30% and lag the c ontinuum by 0-3 days. This is in contrast to past results in which a t ime lag of 9+/-2 days was found for both emission lines, This may be d ue to a different variability timescale of the ionizing continuum or t o a real change in the broad-line region gas distribution in the 5.5 y r interval between the two campaigns.