Multiwavelength observations of short-timescale variability in NGC 4151. II. Optical observations

S. Kaspi*, D. Maoz, H. Netzer, B. M. Peterson, T. Alexander, A. J. Barth, R. Bertram, F. Z. Cheng, K. K. Chuvaev, R. A. Edelson, A. V. Filippenko, S. Hemar, L. C. Ho, O. Kovo, T. Matheson, R. W. Pogge, B. C. Qian, S. M. Smith, R. M. Wagner, H. WuS. J. Xue, Z. L. Zou

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review


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 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 A 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 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 Ha 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.

Original languageEnglish
Pages (from-to)336-348
Number of pages13
JournalAstrophysical Journal
Issue number1
StatePublished - 1996


FundersFunder number
Directorate for Mathematical and Physical Sciences8957063


    • Galaxies: Seyfert
    • Galaxies: active
    • Galaxies: individual (NGC 4151)


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