Gamma-ray halos around supernova remnants

Supernova remnants (SNRs) are known to accelerate particles to relativistic energies which is indirectly confirmed by the detection of nonthermal emission from these objects across the whole electromagnetic spectrum from radio to very-high-energy gamma-rays. The particularities of the acceleration mechanism and particle transfer are still debated. Here, we analyze how particle escape modifies the observable spectra and morphology of SNRs and discuss the prospects of detection of characteristic imprints in observed emission. We use our time-dependent acceleration code RATPaC to study the formation of extended gamma-ray halos around supernova remnants and the morphological implications that arise when the high-energy particles start to escape from the remnant. We find a strong difference in the morphology of the gamma-ray emission from supernova remnants at later stages, dependent on the emission process. At early times, both leptonic and hadronic morphologies are shell-like. However, as soon as the maximum energy of freshly accelerated particles starts to decrease, the leptonic morphology starts to become center-filled, whereas the hadronic morphology keeps its shell-like shape. Both emission-spectra show a spectral softening caused by the escape of the highest-energetic particles. Escaping high-energy electrons start to form an emission halo around the remnant at this time. There are good prospects for detecting this spectrally hard emission with the future Cerenkov Telescope Array, as there are for detecting variations in the gamma-ray spectral index across the interior of the remnant. Due to the projection effects there is no significant variation of the spectral index expected to be detected with the current generation of gamma-ray observatories.