  The recent discovery of a large number of galaxy clusters using the
Sunyaev-Zel'dovich (SZ) effect has opened a new era on the study of the most
massive clusters in the Universe. Multi-wavelength analyses are required to
understand the properties of these new sets of clusters, which are a sensitive
probe of cosmology. We aim at a multi-wavelength characterization of PLCK
G004.5$-$19.5, one of the most massive X-ray validated SZ effect-selected
galaxy clusters discovered by the Planck satellite. We have observed PLCK
G004.5$-$19.5 with GMOS on Gemini South for optical imaging and spectroscopy,
and performed a strong lensing analysis. We also searched for associated radio
emission in published catalogs. An analysis of the optical images confirms that
this is a massive cluster, with a dominant central galaxy (the BCG) and an
accompanying red sequence of galaxies, plus a $14''$-long strong lensing arc.
Longslit pectroscopy of 6 cluster members shows that the cluster is at
$z=0.516\pm0.002$. We also targeted the strongly lensed arc, and found $z_{\rm
arc}=1.601$. We use LensTool to carry out a strong lensing analysis, from which
we measure a median Einstein radius $\theta_E(z_s=1.6)\simeq30''$ and estimate
an enclosed mass $M_E=2.45_{-0.47}^{+0.45}\times10^{14}\,M_\odot$. By
extrapolating an NFW profile we find a total mass
$M_{500}^{SL}=4.0_{-1.0}^{+2.1}\times10^{14}M_\odot$. Including a constraint on
the mass from previous X-ray observations yields a slightly higher mass,
$M_{500}^{SL+X}=6.7_{-1.3}^{+2.6}\times10^{14}M_\odot$, marginally consistent
with the value from strong lensing alone. High-resolution radio images from
TGSS at 150~MHz reveal that PLCK G004.5$-$19.5 hosts a powerful radio relic on
scales $\lesssim500$ kpc. Emission at the same location is also detected in low
resolution images at 843~MHz and 1.4~GHz. This is one of the higher redshift
radio relics known to date.
