Cluster Merger Shock Constraints on Particle Acceleration and Nonthermal Pressure in the Intracluster Medium

X-ray observations of galaxy cluster merger shocks can be used to constrain nonthermal processes in the intracluster medium (ICM). The presence of nonthermal pressure components in the ICM, as well as the shock acceleration of particles and their escape, all affect shock jump conditions in distinct ways. Therefore, these processes can be constrained usingX-ray surface brightness and temperaturemaps of merger shock fronts. Herewe use these observations to place constraints on particle acceleration efficiency in intermediate Mach number (M 2 3) shocks and explore the potential to constrain the contribution of nonthermal components (e.g., cosmic rays, magnetic field, and turbulence) to ICM pressure in cluster outskirts.Wemodel the hydrodynamic jump conditions in merger shocks discovered in the galaxy clusters A520 (M 2) and 1E 0657 56 (M 3) using a multifluid model comprising a thermal plasma, a nonthermal plasma, and a magnetic field. Based on the published X-ray spectroscopic data alone, we find that the fractional contribution of cosmic rays accelerated in these shocks is P10% of the shock downstream pressure. Current observations do not constrain the fractional contribution of nonthermal components to the pressure of the undisturbed shock upstream. Future X-ray observations, however, have the potential to either detect particle acceleration in these shocks through its effect on the shock dynamics, or place a lower limit on the nonthermal pressure contributions in the undisturbed ICM.Webriefly discuss implications formodels of particle acceleration in collisionless shocks and the estimates of galaxy cluster masses derived from X-ray and Sunyaev-Zel’dovich effect observations. Subject headinggs: cosmic rays — galaxies: clusters: general — galaxies: clusters: individual (A520, 1E 0657 06) — intergalactic medium — shock waves — turbulence — X-rays: galaxies: clusters Online material: color figures