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Astron. Astrophys. 324, 904-914 (1997)

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Complex variability pattern in NGC 4151

I. Sequences on the line-continuum diagram *

Yu.F. Malkov , V.I. Pronik and S.G. Sergeev

Crimean Astrophysical Observatory, P/O Nauchny, 334413 Crimea, Ukraine

Received 2 August 1996 / Accepted 28 January 1997

Abstract

We present the first results of an 8-year spectroscopic monitoring of the Seyfert 1.5 galaxy NGC 4151 carried out with a CCD spectrograph at the 2.6-m Shajn Telescope of the Crimean Astrophysical Observatory in 1988-1995. Total of 202 H [FORMULA] region spectra and 154 H [FORMULA] region spectra have formed the data set. All spectra were calibrated in flux using the narrow emission lines which were assumed to be constant over the duration of the monitoring program. A self-consistent seeing correction procedure not requiring the surface brightness distribution of the narrow-line region to be known is proposed. The seeing-corrected H [FORMULA], H [FORMULA], and He II   [FORMULA] 4686 integrated line fluxes and the optical continuum fluxes at 4480Å, 5120Å, 6230Å, and 6910Å are tabulated. During the monitoring program, the spectrum and luminosity of NGC 4151 have undergone dramatic changes. The broad-line fluxes grew by at least 4.6 times for the H [FORMULA], 6.5 times for the H [FORMULA], and 9 times for the He II   [FORMULA] 4686 lines whereas the non-thermal optical continuum at 5120Å increased by at least 4.8 times. The light curve of the He II   [FORMULA] 4686 line strongly differs from the H [FORMULA] and H [FORMULA] light curves. Moreover, there is a clearly pronounced, broad, variable emission near the observed wavelength 4540Å whose variations are not correlated with variations of the broad He II   [FORMULA] 4686 line.

The line-continuum diagrams for NGC 4151 reveal some features which contradict the simplest AGN paradigm where a single central source of the continuum emission has a steady relationship between the visible and ionizing continuum:

(1) each of the dependences of H [FORMULA], H [FORMULA], and He II   [FORMULA] 4686 emission-line fluxes on the optical continuum flux split into four sequences which cover different periods of observations and differ by most parameters of the line-continuum and line-line correlations, including the time lag between the continuum and line variations;

(2) some sequences do not pass through the zero-point of both broad-line and non-thermal optical continuum fluxes, and the relative position of these sequences on the line-continuum diagram is quite different for the hydrogen and He II lines;

(3) the transition time between two successive sequences ([FORMULA] 100 days or less) is much shorter than the dynamical time scale for the broad-line region ([FORMULA] several years), so the spatial redistribution of the line-emitting gas is obviously not the cause of the origin of sequences (and, hence, of the changes in the emission-line lag).

Key words: galaxies: active – galaxies: individual: NGC 4151 – galaxies: nuclei – galaxies: Seyfert – galaxies: emission lines

* Tables 1-4 and Fig. 3 are only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/Abstract.html

Send offprint requests to: Yu.F. Malkov

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© European Southern Observatory (ESO) 1997

Online publication: May 5, 1998

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