On the observability of spiral structures in cataclysmic variable accretion discs

We use the grid of hydrodynamic accretion disc calculations of Stehle to construct orbital phase-dependent emission-line profiles of thin discs carrying spiral density waves. The observational signatures of spiral waves are explored to establish the feasibility of detecting spiral waves in cataclysmic variable discs using prominent emission lines in the visible range of the spectrum. For high Mach number accretion discs (Mvcs≃ 15 — 30), we find that the spiral shock arms are so tightly wound that they leave few obvious finger prints in the emission lines. Only a minor variation of the double peak separation in the line profile at a level of ∼8 per cent is produced. For accretion discs in outburst (M≃ 5 — 20) however, the lines are dominated by the emission from an m=2 spiral pattern in the disc. We show that reliable Doppler tomograms of spiral shock patterns can be reconstructed provided that a signal-to-noise ratio of at least 15, a wavelength resolution of ∼80 km s−1 and a time resolution of ∼50 spectra per binary orbit are achieved. We confirm that the observed spiral pattern in the disc of IP Pegasi can be reproduced by tidal density waves in the accretion disc and demands the presence of a large, hot disc, at least in the early outburst stages.