Crossing the veil of the brightest radio relic in the sky: MACS J0717.5+3745

ABSTRACT We present high-frequency, full-polarization Jansky Very Large Array (VLA) radio data for the X band of the radio relic MACS J0717.5+3745. Radio relics trace shock waves in the intracluster medium (ICM) that are produced during mergers. Understanding the physical characteristics of relics is important for determining their nature – whether, for example, they are thermal ICM electrons that are accelerated, or whether they are fossil electrons re-accelerated by a merger event. Radio spectropolarimetric analysis, such as through the Stokes QU-fitting, provides a diagnostic of the nature and structure of the magnetized plasma internal or external to the source, with important implications for theoretical models. The high-frequency polarization analysis presented here shows, for the first time, a change in the magneto-ionic structure compared with that for the low-frequency data available in the literature. These high-frequency, polarized data could also be interpreted with an internal depolarization behaviour, and this new finding may be used to investigate possible particle acceleration mechanisms. If that is true, the change in the behaviour of the polarized signal could be tracing the physical properties of a population of non-thermal particles that are undergoing a re-acceleration in the relic caused by the large-scale internal shocks of active galactic nuclei jet fossil particles ejected from the central Narrow Angle Tail radio galaxy. New upcoming broad-band VLA X- and Ku-band data will clarify this. Finally, we conclude that high-frequency, high-sensitivity, spectropolarimetric radio data should be explored further, as they can effectively trace shock fronts and thereby provide insights into the intrinsic magneto-ionic properties of radio components.