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 Hα and Hβ emission lines are given. 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 Å 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 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 Hα 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 Hβ 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±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 BLR gas distribution in the 5.5 years interval between the two campaigns. Subject headings: galaxies: individual (NGC 4151) — galaxies: active — galaxies: Seyfert This paper is dedicated to the memory of K.K. Chuvaev who passed away in the course of this work. School of Physics and Astronomy and the Wise Observatory, The Beverly and Raymond Sackler Faculty of Exact Sciences, Tel-Aviv University, Tel-Aviv 69978, Israel. Department of Astronomy, The Ohio State University, 174 West 18th Avenue, Columbus, OH 43210. Department of Astronomy, University of California at Berkeley, Berkeley, CA 94720. Mailing address: Lowell Observatory, 1400 West Mars Hill Road, Flagstaff, AZ 86001. Center for Astrophysics, University of Science and Technology, Hefei, Anhui, People’s Republic of China. Crimean Astrophysical Observatory, P/O Nauchny, 334413 Crimea, Ukraine. Deceased, 1994 November 15. Department of Physics and Astronomy, University of Iowa, Iowa City, IA 52242. Shanghai Observatory, Chinese Academy of Sciences, People’s Republic of China. Beijing Astronomical Observatory, Chinese Academy of Sciences, Beijing 100080, People’s Republic of China.