Cosmic ray diffusion close to supernova remnants: Isotropic vs anisotropic model

According to the most popular scenario, cosmic rays are accelerated at supernova remnant shocks and released gradually in the interstellar medium during the lifetime of the remnant. However, the details of the particle escape process from the shock are little understood. A way to investigate this is by searching for the gamma-ray emission that escaping cosmic rays are expected to produce in the surrounding of the supernova remnant as a result of interactions with the ambient gas. Here we consider two opposite scenarios in which the cosmic ray diffusion coefficient is either spatially isotropic or strongly anisotropic, with particles propagating along magnetic field lines only. The differences in the spectrum and morphology of the resulting gamma-ray emission are discussed. This study is motivated by the detection of gamma rays from the vicinity of two supernova remnants: W28 and W44. Proving that such emission is linked to particles that escaped the remnant might serve as a proof of the fact that supernova remnants indeed are the sources of cosmic rays. (authors)