A double radio relic in the merging galaxy cluster ZwCl 0008.8+5215
  Some merging galaxy clusters host diffuse elongated radio sources, also
called radio relics. It is proposed that these radio relics trace shock waves
in the intracluster medium created during a cluster merger event. Within the
shock waves particles are accelerated to relativistic energies, and in the
presence of a magnetic field synchrotron radiation will be emitted. Here we
present GMRT and WSRT observations of a new double relic in the galaxy cluster
ZwCl 0008.8+5215. Optical V, R, and I band images of the cluster were taken
with the INT. An optical spectrum, to determine the redshift of the cluster,
was taken with the WHT. Our observations show the presence of a double radio
relic in the galaxy cluster ZwCl 0008.8+5215, for which we find a spectroscopic
redshift of z = 0.1032 \pm 0.0018 from an optical spectrum of one of the cD
galaxies. The spectral index of the two relics steepens inwards to the cluster
center. For part of the relics, we measure a polarization fraction in the range
~ 5-25%. A ROSAT X-ray image displays an elongated ICM and the large-scale
distribution of galaxies reveals two cluster cores, all pointing towards a
binary cluster merger event. The radio relics are located symmetrically with
respect to the X-ray center of the cluster, along the proposed merger axis. The
relics have a linear extent of 1.4 Mpc and 290 kpc. This factor of five
difference in linear size is unlike that of previously known double relic
systems, for which the sizes do not differ by more than a factor of two. We
conclude that the double relics in ZwCl 0008.8+5215 are best explained by two
outward moving shock waves in which particles are (re)accelerated trough the
diffusive shock acceleration (DSA) mechanism. [abridged]
