A 3.5 Mpc-long radio relic in the galaxy cluster ClG 0217+70
  Mega-parsec scale radio sources in the form of halos and relics are often
detected in dynamically disturbed galaxy clusters. Although they are associated
with merger-induced turbulence and shocks, respectively, their formation is not
fully understood. We aim to identify the mechanisms responsible for particle
acceleration and magnetic field amplification in the halo and relics of the
galaxy cluster ClG 0217+70. We observed ClG 0217+70 with LOFAR at 141 MHz and
VLA at 1.5 GHz, and combine these observations with VLA 1.4 GHz archival data
to study the morphological and spectral properties of the diffuse sources. We
add Chandra archival data to examine the thermal and non-thermal properties of
the halo. Our LOFAR and VLA data confirm the presence of a giant radio halo in
the cluster centre and multiple relics in the outskirts. The radio and X-ray
emission from the halo are correlated, implying a tight relation between the
thermal and non-thermal components. The diffuse radio structure in the south
east with a projected size of 3.5 Mpc is the most extended radio relic detected
to date. The spectral index across the relic width steepens towards the cluster
centre, suggesting the electron ageing in the post-shock regions. The shock
Mach numbers for the relics derived from the spectral index map range between
2.0 and 3.2. However, the integrated spectral indices lead to increasingly high
Mach numbers for the relics farther from the cluster centre. This discrepancy
could be because the relation between injection and integrated spectra does not
hold for distant shocks, suggesting that the cooling time for the
radio-emitting electrons is longer than the crossing time of the shocks. The
variations in the surface brightness of the relics and the low Mach numbers
imply that the radio-emitting electrons are re-accelerated from fossil plasma
that could originate in active galactic nuclei.
