Abstract

Two Seyfert 1 galaxies of comparable intrinsic luminosity, NGC 5548 and Markarian 279, have varied by a factor 2 in the optical band within a period of 5 years (1985–1990) during which both have been frequently observed. The large amplitude of the long-term variations reveals evidence that the profile of the broad emission lines depends on the luminosity of the line itself, in the sense that the asymmetry of the lines is stronger when the objects are brighter. This indicates that the structure of the broad line region in these two sources is stable on time scales of 5 years, and that their transfer functions are illumination-dependent in at least part of the radial velocity space. The effect, however, is absent in other, lower luminosity Seyfert 1 galaxies.

Introduction

The technique of reverberation mapping described by Blandford & McKee (1982) has been applied successfully in recent years to derive the transfer function (TF) of the broad line region (BLR) of well-monitored Seyfert 1 galaxies (e.g. Krolik et al. 1991). Until now the common assumption has been that the shape of the TF does not depend on the illumination of the BLR: this implies that if the continuum of a source were constant, the resulting broad lines would have the same profile irrespective of their intensity. The data presented in this paper, however, indicate that the broad lines of two Seyfert 1 galaxies, Markarian (Mkn) 279 and NGC 5548, display different characteristic profiles at different intensities, and therefore that the respective TFs have illumination-dependent shapes in at least part of the radial velocity range.