On the escape of cosmic rays from radio galaxy cocoons

Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str.1, 85740 Garching, Germany Department of Physics, University of Toronto, 60 St. George Street, Toronto M5S1A7, Canada

Corresponding author: ensslin@mpa-garching.mpg.de

Received: 2 November 2000
Accepted: 12 November 2002

Abstract

The escape rate of cosmic ray (CR) particles from radio galaxy cocoons is a problem of high astrophysical relevance: e.g. if CR electrons are stored for long times in the dilute relativistic medium filling the radio cocoons (radio plasma in the following) they are protected against Coulomb losses and thus are able to produce a significant non-thermal Comptonisation signature on the CMB. On the other hand, CR protons and positrons which leak out of radio plasma can interact with the ambient medium, leading to characteristic gamma ray radiation by pion decay and pair annihilation. In order better understand such problems a model for the escape of CR particles from radio galaxy cocoons is presented here. It is assumed that the radio cocoon is poorly magnetically connected to the environment. An extreme case of this kind is an insulating boundary layer of magnetic fields, which can efficiently suppress particle escape. More likely, magnetic field lines are less organised and allow the transport of CR particles from the source interior to the surface region. For such a scenario two transport regimes are analysed: diffusion of particles along inter-phase magnetic flux tubes (leaving the cocoon) and cross field transport of particles in flux tubes touching the cocoon surface. The cross field diffusion is likely the dominate escape path, unless a significant fraction of the surface is magnetically connected to the environment. Major cluster merger should strongly enhance the particle escape by two complementary mechanisms. i) The merger shock waves shred radio cocoons into filamentary structures, allowing the CRs to easily reach the radio plasma boundary due to the changed morphology. ii) Also efficient particle losses can be expected for radio cocoons not compressed in shock waves. There, for a short period after the sudden injection of large scale turbulence, the (anomalous) cross field diffusion can be enhanced by several orders of magnitude. This lasts until the turbulent energy cascade has reached the microscopic scales, which determine the value of the microscopic diffusion coefficients.