  We studied the prominent bow shock in the merging galaxy cluster A520 using a
deep Chandra X-ray observation and archival VLA radio data. This shock is a
useful diagnostic tool, owing to its clear geometry and relatively high Mach
number. At the "nose" of the shock, we measure a Mach number of
$M=2.4_{-0.2}^{+0.4}$. The shock becomes oblique away from the merger axis,
with the Mach number falling to $\simeq$1.6 around 30$^{\circ}$ from the nose.
The electron temperature immediately behind the shock nose is consistent with
that from the Rankine-Hugoniot adiabat, and is higher (at a 95% confidence)
than expected for adiabatic compression of electrons followed by Coulomb
electron-proton equilibration, indicating the presence of equilibration
mechanisms faster than Coulomb collisions. This is similar to an earlier
finding for the Bullet cluster. We also combined four archival VLA datasets to
obtain a better image of the cluster's giant radio halo at 1.4 GHz. An abrupt
edge of the radio halo traces the shock front, and no emission is detected in
the pre-shock region. If the radio edge were due only to adiabatic compression
of relativistic electrons in pre-shock plasma, we would expect a pre-shock
radio emission detectable in this radio dataset; however, an interferometric
artifact dominates the uncertainty, so we cannot rule this model out. Other
interesting features of the radio halo include a peak at the remnant of the
cool core, suggesting that the core used to have a radio minihalo, and a peak
marking a possible region of high turbulence.
