Evidence for a merger induced shock wave in ZwCl\,0008.8+5215 with {\it Chandra} and {\it Suzaku}

Abstract

We present the results from new deep {\it Chandra} ($\sim410$~ks) and {\it Suzaku} ($\sim180$ 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 {\it Chandra} data, we find evidence for the presence of a shock at the location of the western relic, RW, with a Mach number $\mathcal{M}_{S_X}=1.48^{+0.50}_{-0.32}$ from the density jump. We also measure $\mathcal{M}_{T_X}=2.35^{+0.74}_{-0.55}$ and $\mathcal{M}_{T_X}=2.02^{+0.74}_{-0.47}$ from the temperature jump, with {\it Chandra} and {\it 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 ($\mathcal{M}_{\rm RW}=2.4^{+0.4}_{-0.2}$). 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 which is re-accelerated 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 $\mathcal{M}_{\rm RE}=2.2^{+0.2}_{-0.1}$. The low surface brightness and reduced integration time for this region might have prevented the detection. {\it Chandra} surface brightness profile suggests $\mathcal{M}\lesssim1.5$, while {\it Suzaku} temperature measurements found $\mathcal{M}_{T_X}=1.54^{+0.65}_{-0.47}$. Finally, we also detect a merger induced cold front on the western side of the cluster, behind the shock that traces the western relic.