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} ($\sim 410$~ks) and {\it Suzaku} ($\sim 180$ 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 {\it Chandra} data, we find evidence for the presence of a shock at the location of the western relic, RW, with a Mach number ${\mathcal{M}}_{S_X}=1.48_{-0.32}^{+0.50}$ 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 {\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 (${\mathcal{M}}_{\mathrm{RW}}=2.4_{-0.2}^{+0.4}$). 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 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 ${\mathcal{M}}_{\mathrm{RE}}=2.2_{-0.1}^{+0.2}$. The low surface brightness and reduced integration time for this region might have prevented the detection. {\it Chandra} surface brightness profile suggests $\mathcal{M}\lesssim 1.5$, while {\it Suzaku} temperature measurements found ${\mathcal{M}}_{T_X}=1.54_{-0.47}^{+0.65}$. Finally, we also detect a merger induced cold front on the western side of the cluster, behind the shock that traces the western relic.

21 pages, 15 figures, 4 tables; accepted for publication in ApJ