Long-term variability of the optical spectra of NGC 4151^*

I. Light curves and flux correlations

¹ Special Astrophysical Observatory of the Russian AS, Nizhnij Arkhyz, Karachaevo-Cherkesia 369167, Russia e-mail: ashap@sao.ru
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, Université Paris Diderot, 5 Place Jules Janssen, 92190 Meudon, France
⁵ Instituto Nacional de Astrofísica, Óptica y Electrónica, Apartado Postal 51, CP 72000, Puebla, Pue. México, México
⁶ Sternberg Astronomical Institute, Moscow, Russia
⁷ Instituto de Astronomía, UNAM, Apartado Postal 70-264, CP 04510, México
⁸ Department of Astronomy, Faculty of Mathematics, University of Belgrade, Studentski trg 16, 11000 Belgrade, Serbia
⁹ Max-Planck Institute für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
¹⁰ Universidad Politécnica de Baja California, Av. de la Industria # 291, CP 21010, Mexicali, B.C., México

Received: 20 November 2007
Accepted: 7 April 2008

Abstract

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 ( 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, México), and the OAN-SPM's 2.1-m telescope (San-Pedro, México). The observed fluxes of the Hα, Hβ, Hγ, and HeIIλ4686 emission lines and of the continuum at the observed wavelength 5117 Å  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 correlated well 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 (1996–1999), 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 Hα and Hβ 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 Hβ and Hα to the continuum during the monitoring period.