Long-term variability of the optical spectra of NGC 4151 -: I.: Light curves and flux correlations

Aims. Results of long-term spectral monitoring of the active galactic nucleus of NGC 4151 are presented (11 years, from 1996 to 2006). Methods. High quality spectra (S/N > 50 in the continuum near H alpha and H beta) were obtained in the spectral range similar to 4000 to 7500 angstrom, with a resolution between 5 and 15 angstrom, using the 6-m and the 1-m SAO's telescopes (Russia), the GHAO's 2.1-m telescope (Cananea, Mexico), and the OAN-SPM's 2.1-m telescope (San-Pedro, Mexico). The observed fluxes of the H alpha, H beta, H gamma, and HeII lambda 4686 emission lines and of the continuum at the observed wavelength 5117 angstrom were corrected for the position angle, the seeing, and the aperture effects. Results. We found that the continuum and line fluxes varied strongly (up to a factor 6) during the monitoring period. The emission was maximum in 1996-1998, and there were two minima in 2001 and in 2005. As a consequence, the spectral type of the nucleus changed from a Sy1.5 in the maximum activity state to a Sy1.8 in the minimum state. The H alpha, H gamma, and He lambda 4686 fluxes correlated well with the H beta flux. The line profiles were strongly variable, showing changes of the blue and red asymmetry. The flux ratios of the blue/red wings and of the blue (or red) wing/core of H alpha and H beta varied differently. We considered three characteristic periods during which the H beta and H alpha profiles were similar: 1996-1999, 2000-2001, and 2002-2006. The line-to-continuum flux ratios were different; in particular during the first period (1996 -1999), the lines were not correlated with the continuum and saturated at high fluxes. In the second and third periods (2000 -2006), where the continuum flux was small, the H alpha and H beta fluxes were well correlated to the continuum flux, meaning that the ionizing continuum was a good extrapolation of the optical continuum. The CCFs are often asymmetrical and the time lags between the lines and the continuum are badly defined, indicating the presence of a complex BLR, with dimensions from 1 to 50 light-days. Conclusions. We discuss the different responses of H beta and H alpha to the continuum during the monitoring period.