The use of the colour-colour plot in the analysis of radio synchrotron spectra

Peter C. Tribble

Institute of Astronomy, Madingley Road, Cambridge CB3 0HA


Katz-Stone et al. have recently introduced the concept of the colour-colour plot for analyzing radio synchrotron spectra. I present theoretical colour-colour plots for realistic synchrotron spectra that allow for the effects of a random magnetic field, inverse Compton losses, and diffusion of relativistic electrons. I consider the effect of varying the magnetic field distribution and the set of observing frequencies. The model colour-colour plots presented here agree with the data for Cygnus A much better than standard models. The models indicate that the field strength in Cygnus A exceeds 8.7microGauss, and that electrons are diffusing slowly between regions of different field strength with a mean free path to scattering of a few parsecs or less.

Key words: magnetic fields -- radio continuum: galaxies -- radiation mechanisms: cyclotron and synchrotron Contents:

Model colour-colour diagrams
Sheared field models
Other field distributions
Realistic diffusion models
Two component models
Application to Cygnus A
Peter Tribble,