Turbulent magnetic fields in the merging galaxy cluster MACS
  J0717.5+3745: polarization analysis
  We present wideband polarimetric observations, obtained with the Karl G.
Jansky Very Large Array (VLA), of the merging galaxy cluster MACS J0717.5+3745,
which hosts one of the most complex known radio relic and halo systems. We use
both Rotation Measure Synthesis and QU-fitting, and find a reasonable agreement
of the results obtained with these methods, in particular, when the Faraday
distribution is simple and the depolarization is mild. The relic is highly
polarized over its entire length reaching a fractional polarization ${>}30\%$
in some regions. We also observe a strong wavelength-dependent depolarization
for some regions of the relic. The northern part of the relic shows a complex
Faraday distribution suggesting that this region is located in or behind the
intracluster medium (ICM). Conversely, the southern part of the relic shows a
Rotation Measure very close to the Galactic foreground, with a rather low
Faraday dispersion, indicating very little magnetoionic material intervening
the line-of-sight. From spatially resolved polarization analysis, we find that
the scatter of Faraday depths correlates with the depolarization, indicating
that the tangled magnetic field in the ICM causes the depolarization. At the
position of a well known narrow-angle-tailed galaxy (NAT), we find evidence of
two components clearly separated in Faraday space. The high Faraday dispersion
component seems to be associated with the NAT, suggesting the NAT is embedded
in the ICM while the southern part of the relic lies in front of it. The
magnetic field orientation follows the relic structure indicating a
well-ordered magnetic field. We also detect polarized emission in the halo
region; however the absence of significant Faraday rotation and a low value of
Faraday dispersion suggests the polarized emission, previously considered as
the part of the halo, has a shock(s) origin.
