arXiv:0804.0910v2 [astro-ph] 22 Apr 2008 April 22, 2008 Long-term variability of the optical spectra of NGC 4151: I. Light curves and flux correlations. A.I. Shapovalova 1 , L. ˇ C. Popovi´ c 2,3 , S. Collin 4 , A.N. Burenkov 1 , V.H. Chavushyan 5 , N.G. Bochkarev 6 , E. Ben´ ıtez 7 , D. Dultzin-Hacyan 7 , A. Kovaˇ cevi´ c 8 , N. Borisov 1 , L. Carrasco 5 , J. Le ´ on-Tavares 5,9 , A. Mercado 10 , J.R. Valdes 5 , V.V. Vlasuyk 1 , and V.E. Zhdanova 1 1 Special Astrophysical Observatory of the Russian AS, Nizhnij Arkhyz, Karachaevo-Cherkesia 369167, Russia 2 Astronomical Observatory, Volgina 7, 11160 Belgrade 74, Serbia 3 Alexander von Humboldt Fellow, presently at Max Planck Institute for Radioastronomy, Bonn, Germany 4 LUTH, Observatoire de Paris, CNRS, Universit´ e Paris Diderot; 5 Place Jules Janssen, 92190 Meudon, France 5 Instituto Nacional de Astrof´ ısica, ´ Optica y Electr´ onica, Apartado Postal 51, CP 72000, Puebla, Pue. M´ exico 6 Sternberg Astronomical Institute, Moscow, Russia 7 Instituto de Astronom´ ıa, UNAM, Apartado Postal 70-264, CP 04510, M´ exico 8 Department of Astronomy, Faculty of Mathematics, University of Belgrade, Studentski trg 16, 11000 Belgrade, Serbia 9 Max-Planck Institute f¨ ur Radioastronomie, Auf dem H¨ ugel 69, 53121 Bonn, Germany 10 Universidad Polit´ ecnica de Baja California, Av. de la Industria # 291, CP 21010, Mexicali, B.C., M´ exico Received / Accepted ABSTRACT Aims. Results of a 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 Hα and Hβ) were obtained in the spectral range ∼ 4000 to 7500 Å, with a resolution between 5 and 15 Å, using the 6-m and the 1-m SAO’s telescopes (Russia), the GHAO’s 2.1-m telescope (Cananea, M´ exico), and the OAN-SPM’s 2.1-m telescope (San-Pedro, M´ exico). The observed fluxes of the Hα,Hβ,Hγ and HeIIλ4686 emission lines and of the continuum at the observed wavelength 5117Å, 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 Hα,Hγ and Heλ4686 fluxes were well correlated with the Hβ 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 Hα and Hβ varied differently. We considered three characteristic periods during which the Hβ and Hα 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 third period (2002-2006), the Hα and Hβ fluxes were well correlated to the continuum flux, meaning that the ionizing continuum was a good extrapolation of the optical continuum. We thus consider that the values of the time lags – line lagging continuum (0.81 +1.55 −0.81 days for Hα and 0.81 +2.19 −0.81 days for Hβ) for the third period give a more realistic estimation of the dimension of the BLR than during the other periods. Moreover, the time lags obtained by binning intervals of three years within the whole monitoring period indicate the permanent presence of a small component of the BLR (0.3-0.7 light days) Conclusions. We discuss the different responses of Hβ and Hα to the continuum during the monitoring period. Key words. galaxies: active - galaxies: individual: NGC 4151 1. Introduction The brightest Seyfert 1.5-type galaxy NGC 4151 has been stud- ied in detail at all wavelengths (e.g. Peterson 1988 and Ulrich 2000) The nucleus of this galaxy shows flux variability in a wide wavelength range, with time-scales from a few hours (in the hard X-ray, e.g. Yaqoob et al. 1993) to several months (in the infrared, e.g. Oknyanskij et al. 1999). In the optical range, the Active Galactic Nucleus (AGN) of this galaxy is also known to display flux variations of the con- tinuum and of the lines up to a factor ten or more (e.g. Peterson Send offprint requests to: A. I. Shapovalova, e-mail: ashap@sao.ru Tables 2-5 and 7-8 are given only in electronic form 1988, Clavel et al. 1990, Maoz et al. 1991, Shapovalova et al. 1996, Ulrich & Horne 1996, Sergeev et al. 2001, Lyuty 2005). These variations occur in time scales of several days (Maoz et al. 1991). NGC 4151 has been the subject of echo-mapping observa- tional campaigns. The main aim of AGN monitoring campaigns was to determine the size of the Broad Line Region (BLR) by measuring the time delay between the emission line fluxes, in response to the variations of the continuum flux (see Peterson 1993 for a review). It is interesting that different authors found different time lags: Antonucci & Cohen (1983) observed NGC 4151 at least once a month from 1980 May through 1981 July, and found that the BLR radius was less than 30 lt-days. These observations were also used by Peterson & Cota (1988) in com-