Multiwavelength Observations of Short-Timescale Variability in NGC 4151. II. Optical Observations
Abstract
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 A varied by ~17% while the continuum at 4600 A 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 A 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 Hα 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.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- October 1996
- DOI:
- 10.1086/177870
- arXiv:
- arXiv:astro-ph/9605080
- Bibcode:
- 1996ApJ...470..336K
- Keywords:
-
- GALAXIES: INDIVIDUAL NGC NUMBER: NGC 4151;
- GALAXIES: ACTIVE;
- GALAXIES: SEYFERT;
- Astrophysics
- E-Print:
- 16 pages, LaTeX (including aas2pp4 and epsf), including 10 PostScript figures