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Multiwavelength observations of short time-scale variability in NGC 4151. II. Optical observations | | S. Kaspi et al
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15 May 1996 | | Abstract: | 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 H$alpha$ and H$eta$ emission lines are given. During the monitoring period, the continuum at 6925~AA varied by $sim$17\% while the continuum at 4600~AA varied by $sim$35\%, with larger variations in the near UV. The wavelength dependence of the variation amplitude also extends into the far UV. The dependence in the 2700$-$7200~AA range can be explained by the different relative starlight contributions at different wavelengths, but the large variability at 1275~AA cannot be explained in this way. The continuum variability timescale is of order 13 days and is similar at all optical wavelength bands. No evidence for a time lag between the optical continuum and the UV continuum and emission lines was found. The H$alpha$ 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 H$eta$ emission line flux are about 30\% and lag the continuum by 0$-$3 days. This is in contrast to past results where a time lag of 9$pm$2 days was found for both emission lines. This may be due to a different variability timescale of the {em{ionizing}} continuum, or to a real change in the BLR gas distribution in the 5.5 years interval between the two campaigns. | | Source: | arXiv, astro-ph/9605080 | | Services: | Forum | Review | PDF | Favorites | |
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