Complex Variability Pattern in NGC 4151. III. Variability of the Broad Emission Line Profiles over 11 Years
Abstract
We present results of an optical spectral monitoring campaign of the Seyfert galaxy NGC 4151 carried out with a CCD spectrograph at the 2.6 m Shajn Telescope of the Crimean Astrophysical Observatory in 1988-1998. 423 spectra of NGC 4151 were obtained in the Hα and Hβ spectral regions. After a very deep minimum in 1984-1989, the nucleus brightened gradually, reached its brightest recorded level in 1996, and then decreased gradually. The broad emission line profiles have shown strong changes in flux as well as in shape. The central red part of the Balmer lines strengthened systematically with respect to the total line flux, while the wings at +/-(2500-5000) km s-1 decreased. The He II λ4686 integrated flux and its profile shape show a significantly different variation to the Balmer lines. Root mean square profiles show that the strongest absolute changes in the Balmer lines occurred near the line center. Also, there are three local maxima at -3900, -1400, and +4300 km s-1. The velocity dependence of the line-continuum correlation and correlation matrices show that two regions of poor correlation (up to an anticorrelation), sometimes remarkably narrow, occurred, on average, at -4200 and +5000 km s-1. The line-continuum correlation for the total Hα flux since 1992 is much better than the correlation for the most of the individual profile segments. The observed evolution of the Balmer line profiles can be reasonably reproduced by a two-component model in which the changes in the profile shapes are due to changes in the relative strength of the two variable components with fixed profile shapes: double-peaked and single-peaked. We suppose that the ionizing continuum produced by the central engine is then redistributed between the two components. A double-peaked component may be emitted from the Keplerian disk ionized by scattered and reprocessed radiation, while a single-peaked component may be attributed to the gas system ionized by primary radiation. As to the total line fluxes, we suppose that the observed changes in the line-continuum correlation are driven by the changes in the spectral energy distribution (SED). Our results are also relevant to the problem of the two-dimensional reverberation mapping.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- June 2001
- DOI:
- 10.1086/321382
- Bibcode:
- 2001ApJ...554..245S
- Keywords:
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- Galaxies: Active;
- Galaxies: Individual: NGC Number: NGC 4151;
- Galaxies: Nuclei;
- Galaxies: Seyfert