Supernova remnants in the new radio astronomy era
Supernova remnants (SNRs) are what is left after stellar explosions, when the stellar ejecta, the explosion shock and the circumstellar medium interact. Despite being among the first objects studied in radio astronomy, observational difficulties have so far prevented a definitive characterisation, which would help answer open questions related to these sources. It is debated which is the contribution of SNRs to Galactic cosmic rays, or how the interaction with the surrounding environments influences the particle energetics. The SKA precursors are providing valuable and unexpected discoveries on SNRs, thanks to their unique capabilities to probe spatial scales from a few arcseconds to a few degrees with a sensitivity of tens of microjansky. Accurate integrated flux density measurements and arcsecond-scale spectral-index maps are now possible for tens of SNRs, substantially expanding the small subset of remnants traditionally studied in great detail. SKA will markedly enhance current observations by providing: higher sensitivity, enabling the detection of fainter SNRs also in polarisation, revealing diffuse structures and the underlying magnetic field configuration; higher angular resolution, allowing detailed mapping of compact remnants and reducing depolarisation in fine structures, tracing filaments and shocks fronts; wider frequency coverage to probe unexplored spectral windows, where spectral turnovers and breaks or cut-off may occur, establishing a direct connection to X-ray and {\gamma}-ray emission that constrains the electron population, and enabling accurate modelling of the non-thermal emission across the electromagnetic spectrum; improved image fidelity for more reliable cross-matching with other wavelengths, leading to a better understanding of the SNR-interstellar medium interplay.