Images of accretion discs - I. The eclipse mapping method

Abstract

A method of mapping the surface brightness distributions of accretion discs in eclipsing cataclysmic binaries is described and tested with synthetic eclipse data. Accurate synthetic light curves are computed by numerical simulation of the accretion disc eclipse, and images of the disc are reconstructed by maximum entropy methods. The conventional definition of entropy leads to a distorted image of the disc, from which only the locations of compact bright spots are reliably determined. A modified form of entropy, sensitive to the azimuthal structure of the image but not to its radial profile, suppresses azimuthal structure but correctly recovers the radial structure of the accretion disc. This eclipse mapping method permits powerful tests of accretion disc theory by deriving the spatial structure of discs from observational data with a minimum of model-dependent assumptions. Mass transfer rates for eclipsing cataclysmic variables can be measured by this method with an uncertainty of about a factor of two.