Evidence for a Merger-induced Shock Wave in ZwCl 0008.8+5215 with Chandra and Suzaku

Abstract

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 }}_{SX}={1.48}_-0.32^+0.50 from the density jump. We also measure {{ \mathcal M }}_{TX}={2.35}_-0.55^+0.74 and {{ \mathcal M }}_{TX}={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 }}_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 }}_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 }≲ 1.5, while the Suzaku temperature measurements found {{ \mathcal M }}_{TX}={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.