2021MNRAS.500.5177L

Supernova remnants (SNRs) represent a powerful laboratory to study the cosmic ray acceleration processes at shocks, and their relation to the properties of the circumstellar medium. With the aim of studying the high-frequency radio emission and investigating the energy distribution of accelerated electrons and the magnetic field conditions, we performed single-dish observations of the large and complex Cygnus Loop SNR from 7.0-24.8 GHz with the Medicina and Sardinia Radio Telescopes, focusing on the northern filament (NGC 6992) and the southern shell. Both regions show a spectrum well fitted by a power-law function (S ∝ ν^–α), with spectral index α = 0.45 ± 0.05 for NGC 6992 and α = 0.49 ± 0.01 for the southern shell and without any indication of a spectral break. The spectra are significantly flatter than the whole Cygnus Loop spectrum (α = 0.54 ± 0.01), suggesting a departure from the plain shock acceleration mechanisms, which for NGC 6992 could be related to the ongoing transition towards a radiative shock. We model the integrated spectrum of the whole SNR considering the evolution of the maximum energy and magnetic field amplification. Through the radio spectral parameters, we infer a magnetic field at the shock of 10 µG. This value is compatible with purely adiabatic compression of the interstellar magnetic field, suggesting that the amplification process is currently inefficient.