Complex variability pattern in NGC 4151. III. Variability of the broad emission line profiles over 11 years

We present results of an optical spectral monitoring campaign of the Seyfer t galaxy NGC 4151 carried out with a CCD spectrograph at the 2.6 m Shajn Te lescope of the Crimean Astrophysical Observatory in 1988-1998. 423 spectra of NGC 4151 were obtained in the H alpha and H beta spectral regions. After a very deep minimum in 1984-1989, the nucleus brightened gradually, reache d its brightest recorded level in 1996, and then decreased gradually. The b road emission line profiles have shown strong changes in flux as well as in shape. The central red part of the Balmer lines strengthened systematicall y with respect to the total line flux, while the wings at +/-(2500-5000) km s(-1) decreased. The He II lambda 4686 integrated flux and its profile sha pe show a significantly different variation to the Balmer lines. Root mean square profiles show that the strongest absolute changes in the Balmer line s occurred near the line center. Also, there are three local maxima at -390 0, -1400, and +4300 km s(-1). The velocity dependence of the line-continuum correlation and correlation matrices show that two regions of poor correla tion (up to an anticorrelation), sometimes remarkably narrow, occurred, on average, at -4200 and + 5000 km s(-1). The line-continuum correlation for t he total H alpha flux since 1992 is much better than the correlation for th e most of the individual profile segments. The observed evolution of the Ba lmer 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 b y the central engine is then redistributed between the two components. A do uble-peaked component may be emitted from the Keplerian disk ionized by sca ttered and reprocessed radiation, while a single-peaked component may be at tributed to the gas system ionized by primary radiation. As to the total li ne fluxes, we suppose that the observed changes in the line-continuum corre lation are driven by the changes in the spectral energy distribution (SED). Our results are also relevant to the problem of the two-dimensional reverb eration mapping.