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Description
Results of a long-term spectral monitoring of the active galactic nucleus of NGC 4151 are presented (11 years, from 1996 to 2006). High quality spectra (S/N>50 in the continuum near H{alpha} and H{beta}) were obtained in the spectral range ~4000 to 7500{AA}, with a resolution between 5 and 15 {AA}, 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{AA}, were corrected for the position angle, the seeing and the aperture effects. 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 were well correlated 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 (1996-2001), the lines were not correlated with the continuum and saturated at high fluxes. In the second and third period (2002-2006), 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 a 1 to 50 light-days. We discuss the different responses of H{beta} and H{alpha} to the continuum during the monitoring period.
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