Emission line formation in accretion discs

Abstract

We examine the formation of emission lines from accretion discs and develop simple expressions for computing the profiles of both optically thin and optically thick lines. The effect of the Keplerian velocity gradient on the transfer of line radiation in the disc is taken into account to first order in H/R. Line photons that are trapped in an optically thick emission layer can escape more easily in directions along which the Keplerian shear flow provides large Doppler gradients. This anisotropy in the local emission pattern alters the shape of the global emission line profile. We present synthetic emission line profiles for the optically thick and optically thin limits and covering a full range of inclinations. The resemblance between our high-inclination optically thick line profiles and the observed profiles of Baimer emission lines in eclipsing dwarf novae indicates that this local saturation effect is an important element of the line formation process in real discs.