A double radio relic in the merging galaxy cluster ZwCl 0008.8+5215

Context. 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 (ICM) 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 giant metrewave radio telescope (GMRT) and Westerbork synthesis radio telescope (WSRT) observations of a new double relic in the galaxy cluster ZwCl 0008.8+5215. Aims. The aim of the observation is to understand the phenomenon of radio relics. Methods. We carried out radio continuum observations at 241 and 610 MHz with the GMRT, and 1.3-1.8 GHz observations with the WSRT in full polarization mode. Optical V, R, and I band images of the cluster were taken with the 2.5 m Isaac Newton Telescope (INT). An optical spectrum, to determine the redshift of the cluster, was taken with the William Herschel Telescope (WHT). Results. 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 +/- 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 similar to 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. Conclusions. 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.