  We study the structure of the galaxy cluster Abell 523 (A523) at z=0.104
using new spectroscopic data for 132 galaxies acquired at the Telescopio
Nazionale Galileo, new photometric data from the Isaac Newton Telescope, and
X-ray and radio data from the Chandra and Very Large Array archives. We
estimate the velocity dispersion of the galaxy population, sigmaV=949 km/s, and
the X-ray temperature of the hot intracluster medium, kT=5.3 keV. We infer that
A523 is a massive system: M200 about 7-9 x10E14 Msun. The analysis of the
optical data confirms the presence of two subclusters, 0.75 Mpc apart, tracing
the SSW-NNE direction and dominated by the two brightest cluster galaxies (BCG1
and BCG2). The X-ray surface brightness is strongly elongated towards the NNE
direction, and its peak is clearly offset from both the BCGs. We confirm the
presence of a 1.3 Mpc large radio halo, elongated in the ESE-WNW direction and
perpendicular to the optical/X-ray elongation. We detect a significant
radio/X-ray offset and radio polarization, two features which might be the
result of a magnetic field energy spread on large spatial scales. A523 is found
consistent with most scaling relations followed by clusters hosting radio
haloes, but quite peculiar in the Pradio-LX relation: it is underluminous in
the X-rays or overluminous in radio. A523 can be described as a binary head--on
merger caught after a collision along the SSW-NNE direction. However, minor
optical and radio features suggest a more complex cluster structure, with A523
forming at the crossing of two filaments along the SSW-NNE and ESE-WNW
directions.
