Deep low-frequency radio observations of Abell 2256 I: The filamentary
  radio relic
  We present deep and high fidelity images of the merging galaxy cluster Abell
2256 at low frequencies, using the upgraded Giant Metrewave Radio Telescope
(uGMRT) and LOw-Frequency ARray (LOFAR). This cluster hosts one of the most
prominent known relics, with a remarkably spectacular network of filamentary
substructures. The new uGMRT (300-850 MHz) and LOFAR (120-169 MHz)
observations, combined with the archival Karl G. Jansky Very Large Array (VLA;
1-4 GHz) data, allowed us to carry out the first spatially resolved spectral
analysis of the exceptional relic emission down to 6 arcsec resolution over a
broad range of frequencies. Our new sensitive radio images confirm the presence
of complex filaments of magnetized relativistic plasma also at low frequencies.
We find that the integrated spectrum of the relic is consistent with a single
power law, without any sign of spectral steepening, at least below 3 GHz.
Unlike previous claims, the relic shows an integrated spectral index of
$-1.07\pm0.02$ between 144 MHz and 3 GHz, which is consistent with the
(quasi)stationary shock approximation. The spatially resolved spectral analysis
suggests that the relic surface very likely traces the complex shock front,
with a broad distribution of Mach numbers propagating through a turbulent and
dynamically active intracluster medium. Our results show that the northern part
of the relic is seen edge-on and the southern part close to face-on. We suggest
that the complex filaments are regions where higher Mach numbers dominate the
(re-)acceleration of electrons that are responsible for the observed radio
emission.
