Spectral activity of the Seyfert galaxy Markarian 6 in 1970–1991

Abstract

We present results of the reduction of the archive spectra of the nucleus of Mrk 6 obtained in 1970-1991 using the image tube spectrograph at the 2.6-m Shajn telescope of the Crimean Astrophysical Observatory. We analyze 363 spectra for 181 dates obtained in the H spectral region. The spectra in the H region (194 spectra for 179 dates) were used to get light curves. The light curves in the H and H lines and also in the adjacent continuum show appreciable variability. Broad-line profile parameters (such as centroid, width, blue-to-red ratio of fluxes) seem to be independent of or dependent very weakly on the continuum flux. However, there is a tendency for the blue-to-red ratio of fluxes to increase with increasing continuum flux, and also a weak tendency for the profile width (profile dispersion) to decrease with increasing continuum flux. An anti- correlation between H shapes at the blue and red wings, positive correlation between blue shape and line/continuum flux, and anti-correlation between red shape and line/continuum flux show that the blue segments respond slightly better to the continuum changes than the red segment and total H line. We assume that the observed broad profile consists of at least two components with fixed profiles. We found that one of the components is nearly symmetric and has a single peak, while the other component has a blue bump and extended red wing. In terms of such a two-broad-components profile model, the evolution of the observed H profiles can be roughly reproduced by changes in the relative strength of the two variable components. However, the evolution of the broad H profile in Mrk 6 is complicated and not clear at all. The evolution is driven, most likely, by a set of various factor, including a multi-component BLR model with redistribution of ionizing radiation between dierent parts of the BLR, the matter redistribution, reverberation eects, and continuum-dependent changes.