Crossing the veil of the brightest radio relic in the sky MACS J0717.6+3745

TL;DR

This study presents high-frequency, full-polarisation VLA observations at X-band (10 GHz) of the MACS J0717.5$+$3745 radio relic and combines them with lower-frequency data to perform Stokes QU-fitting across a broad 2–12 GHz range. The analysis reveals a notable change in magneto-ionic structure at high frequency, with results favoring an internal Faraday depolarisation scenario in several regions, indicating a more ordered magnetic-field configuration and significant RM dispersion consistent with early-stage merger shocks. These findings imply that high-frequency polarimetry probes different, more compact synchrotron-emitting regions and potentially fossil AGN electron populations, offering deeper insights into particle acceleration mechanisms at relic shocks. The work underscores the importance of broad-band, high-sensitivity spectropolarimetry for tracing intrinsic magneto-ionic properties and motivates future wide-band observations with next-generation facilities to fully characterize relic magnetic fields and acceleration processes.

Abstract

We present high-frequency, full-polarisation Jansky Very Large Array (VLA) radio data at X-band of the radio relic: MACS J0717.5+3745. Radio relics trace shock waves in the intracluster medium (ICM) 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 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 polarisation analysis presented here shows, for the first time, a change in the magneto-ionic structure compared to the low-frequency data available in the literature. These high-frequency, polarised data could be interpreted also with an internal depolarisation behaviour and this new finding may be used to investigate possible particle acceleration mechanism. If that is true, the change in the behaviour of the polarised signal could be tracing physical properties of a population of non-thermal particles that are undergoing to a re-acceleration of particles in the relic by 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-bands data will clarify this. Finally, we conclude that high-frequency, high-sensitive, 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.

Figures (8)

• Figure 1: a) The radio relic's total intensity image at 10 GHz. The location of the NAT is indicated by an arrow, while the red dashed lines indicate the regions used for the spectropolarimetric analysis. b) The polarisation intensity map; black vectors indicating the magnetic field direction, white contours represent the polarisation intensity at [3, 6, 12] $\times$$\sigma^{Pol}_{\text{rms}}$ levels. The size of the synthesized beam $\theta$=2$\farcs$967$\times$2$\farcs$244 is indicated in the bottom left corner of each map.
• Figure 2: Spectral index maps computed between S- and C-bands a) and C- and X-bands b). Image c) shows the spectral curvature map. No strong negative curvature is detected along radio relic. The black contours are the total intensity values at [3, 6, 9, 12, 18, 24, 48, 86] $\times$$\sigma^{I}_{\text{rms}}$ levels.
• Figure 3: Result of the Stokes QU-fitting assuming one external Faraday screen for the 6 regions. All points correspond to the S-, C-, and X-bands. For each plot, the upper panel shows the fractional polarisation ($p$) and the lower panel shows the polarisation angle ($\chi$), both versus $\lambda^{2}$ for the six regions: NR, R1, R2, R3, R4 and NAT. Blue dashed lines is the fit to one external depolarisation model (equation e4.7) with its corresponding 3$\sigma$ error.
• Figure 4: Corner plots of the Stokes QU-fitting modelling (one external Faraday screen) for the NR and R1 regions.
• Figure 5: Corner plots of the Stokes QU-fitting modelling (one external Faraday screen) for the R2 and the NAT regions.