According to structure formation simulations, weak shocks with typical Mach number, Ms < 3, are expected to form in merging galaxy clusters. The presence of such shocks has been indicated by X-ray and radio observations of many merging clusters. In particular, diffuse radio sources known as radio relics could be explained by synchrotron-emitting electrons accelerated via diffusive shock acceleration (Fermi I) at quasi-perpendicular shocks. Here we also consider possible roles of stochastic acceleration (Fermi II) by compressive MHD turbulence downstream of the shock. Then we explore a puzzling discrepancy that for some radio relics, the shock Mach number inferred from the radio spectral index is substantially larger than that estimated from X-ray observations. This problem could be understood, if shock surfaces associated with radio relics consist of multiple shocks with different strengths. In that case, X-ray observations tend to pick up the part of shocks with lower Mach numbers and higher kinetic energy flux, while radio emissions come preferentially from the part of shocks with higher Mach numbers and higher cosmic ray (CR) production. We also show that the Fermi I reacceleration model with preexisting fossil electrons supplemented by Fermi II acceleration due to postshock turbulence could reproduce observed profiles of radio flux densities and integrated radio spectra of two giant radio relics. This study demonstrates the CR electrons can be accelerated at collisionless shocks in galaxy clusters just like supernova remnant shock in the interstellar medium and interplanetary shocks in the solar wind.
|Original language||English (US)|
|Journal||Proceedings of Science|
|State||Published - 2017|
|Event||35th International Cosmic Ray Conference, ICRC 2017 - Bexco, Busan, Korea, Republic of|
Duration: Jul 10 2017 → Jul 20 2017
Bibliographical noteFunding Information:
HK was supported by Basic Science Research Program through the National Research Foundation of Korea funded by the Ministry of Education (2014R1A1A2057940). DR was supported by the National Research Foundation of Korea through grant 2016R1A5A1013277. TWJ was supported by the US National Science Foundation through grant AST1211595. The authors thank A. Stroe and R. J. van Weeren for providing the radio flux data for the Sausage relic in  and the Toothbrush relic in , respectively.
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