Evidence for a Merger-induced Shock Wave in ZwCl 0008.8+5215 with Chandra and Suzaku
We present the results from new deep Chandra (~410 ks) and Suzaku (~180 ks) observations of the merging galaxy cluster ZwCl 0008.8+5215 (z = 0.104). Previous radio observations revealed the presence of a double radio relic located diametrically west and east of the cluster center. Using our new Chandra data, we find evidence for the presence of a shock at the location of the western relic, RW, with a Mach number ${{ \mathcal M }}_{{S}_{X}}={1.48}_{-0.32}^{+0.50}$ from the density jump. We also measure ${{ \mathcal M }}_{{T}_{X}}={2.35}_{-0.55}^{+0.74}$ and ${{ \mathcal M }}_{{T}_{X}}={2.02}_{-0.47}^{+0.74}$ from the temperature jump, with Chandra and Suzaku, respectively. These values are consistent with the Mach number estimate from a previous study of the radio spectral index, under the assumption of diffusive shock acceleration (${{ \mathcal M }}_{\mathrm{RW}}={2.4}_{-0.2}^{+0.4}$). Interestingly, the western radio relic does not entirely trace the X-ray shock. A possible explanation is that the relic traces fossil plasma from nearby radio galaxies that is reaccelerated at the shock. For the eastern relic we do not detect an X-ray surface brightness discontinuity, despite the fact that radio observations suggest a shock with ${{ \mathcal M }}_{\mathrm{RE}}={2.2}_{-0.1}^{+0.2}$. The low surface brightness and reduced integration time for this region might have prevented the detection. The Chandra surface brightness profile suggests ${ \mathcal M }\lesssim 1.5$, while the Suzaku temperature measurements found ${{ \mathcal M }}_{{T}_{X}}={1.54}_{-0.47}^{+0.65}$. Lastly, we also detect a merger-induced cold front on the western side of the cluster, behind the shock that traces the western relic.