Spiral waves in accretion discs - observations

I review the observational evidence for spiral structure in the accretion discs of cataclysmic variables (CVs). Doppler tomography is ideally suited to resolve and map such co-rotating patterns and allows a straightforward comparison with theory. The dwarf nova IP Pegasi presents the best studied case, carrying two spiral arms in a wide range of emission lines throughout its outbursts. Both arms appear at the locations where tidally driven spiral waves are expected, with the arm closest to the gas stream weaker in the lines compared to the arm closest to the companion. Eclipse data indicates sub-Keplerian velocities in the outer disc. The dramatic disc structure changes in dwarf novae on timescales of days to weeks, provide unique opportunities for our understanding of angular momentum transport and the role of density waves on the structure of accretion discs. I present an extension to the Doppler tomography technique that relaxes one of the basic assumptions of tomography, and is able to map modulated emission sources. This extension allows us to fit anisotropic emission from, for example, spiral shocks, the irradiated companion star and disc-stream interaction sites. 1 Accretion discs and angular momentum The energetic phenomena associated with a wide range of accreting systems rely on the efficient conversion of potential energy into radiation and heat. In close binaries, the deep potential well of the compact object leads to mass transfer and accretion once the companion star evolves and Roche lobe overflow commences. The efficiency of accretion is proportional to the compactness, M/R, of the accreting compact star with massM and radius R. As matter spills over near the first Lagrangian point, it sets off on a ballistic trajectory towards the accretor. Its potential energy is converted into kinetic energy, but its net angular momentum, due to orbital motion of the mass donor, prevents a straightforward path to the accretor. The natural orbit for such matter is a circular Keplerian orbit corresponding to its specific angular momentum. Instead of dumping material directly onto the compact star, the primary Roche lobe is slowly filled with a near Keplerian disc. Angular momentum needs to be dispersed within this accretion disc in order to allow gas to spiral inwards towards the compact star [7]. It is the detailed process of angular momentum transport that determines the structure of this accretion disc and therefore the rate at which gas, supplied from the mass donor, is actually accreted by the compact object. Although so fundamental to the process of accretion through discs, our understanding of angular momentum dispersal is very limited. We can roughly divide the possible physical