Spiral waves in accretion discs - observations

TL;DR

This paper reviews observational evidence of spiral structures in accretion discs of cataclysmic variables, highlighting Doppler tomography's role in mapping these features and discussing their implications for understanding disc dynamics.

Contribution

It presents new extensions to Doppler tomography that enable mapping of anisotropic and modulated emission sources in accretion discs.

Findings

Spiral arms observed in IP Pegasi's accretion disc during outbursts.

Spiral arms align with theoretical predictions of tidally driven waves.

Sub-Keplerian velocities detected in the outer disc.

Abstract

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.