  The galaxy cluster Abell 523 (A523) hosts an extended diffuse synchrotron
source historically classified as a radio halo. Its radio power at 1.4 GHz
makes it one of the most significant outliers in the scaling relations between
observables derived from multi-wavelength observations of galaxy clusters: it
has a morphology that is different and offset from the thermal gas, and it has
polarized emission at 1.4 GHz typically difficult to observe for this class of
sources. A magnetic field fluctuating on large spatial scales (~ 1 Mpc) can
explain these peculiarities but the formation mechanism for this source is not
yet completely clear. To investigate its formation mechanism, we present new
observations obtained with the LOw Frequency ARray at 120-168 MHz and the
Jansky Very Large Array at 1-2 GHz, which allow us to study the spectral index
distribution of this source. According to our data the source is observed to be
more extended at 144 MHz than previously inferred at 1.4 GHz, with a total size
of about 1.8 Mpc and a flux density S_144MHz = (1.52 +- 0.31) Jy. The spectral
index distribution of the source is patchy with an average spectral index alpha
~ 1.2 between 144 MHz and 1.410 GHz, while an integrated spectral index alpha ~
2.1 has been obtained between 1.410 GHz and 1.782 GHz. A previously unseen
patch of steep spectrum emission is clearly detected at 144 MHz in the south of
the cluster. Overall, our findings suggest that we are observing an overlapping
of different structures, powered by the turbulence associated with the primary
and a possible secondary merger.

Cosmology and Nongalactic Astrophysics