Radio and X-ray Shocks in Clusters of galaxies

TL;DR

This study uses cosmological simulations and shock acceleration models to create mock radio and X-ray maps of galaxy clusters, analyzing shock properties and comparing them with observations to understand radio relics and shock behaviors.

Contribution

It introduces a method to generate synthetic radio and X-ray maps from simulations, revealing how shock properties vary with projection and observational method, and discusses discrepancies with real observations.

Findings

Radio and X-ray shocks often involve multiple shock surfaces along line of sights.

Radio observations tend to detect stronger shocks with flatter spectra.

Synthetic relics show some differences from observed relics, suggesting possible alternative acceleration mechanisms.

Abstract

Radio relics detected in the outskirts of galaxy clusters are thought to trace radio-emitting relativistic electrons accelerated at cosmological shocks. In this study, using the cosmological hydrodynamic simulation data for the large-scale structure formation and adopting a diffusive shock acceleration (DSA) model for the production of cosmic-ray (CR) electrons, we construct mock radio and X-ray maps of simulated galaxy clusters that are projected in the sky plane. Various properties of shocks and radio relics, including the shock Mach number, radio spectral index and luminosity are extracted from the synthetic maps and compared with observations. A substantial fraction of radio and X-ray shocks identified in these maps involve multiple shock surfaces along line of sights (LoSs), and the morphology of shock distributions in the maps depends on the projection direction. Among multiple shocks in a given LoS, radio observations tend to pick up stronger shocks with flatter radio spectra, while X-ray observations preferentially select weaker shocks with larger kinetic energy flux. As a result, the shock Mach numbers and locations derived from radio and X-ray observations could differ from each other in some cases. We also find that the distributions of the spectral index and radio power of the synthetic radio relics are somewhat inconsistent with those of observed real relics; a bit more radio relics have been observed closer to the cluster core and with steeper spectral indices. We suggest the inconsistency could be explained, if very weak shocks with $M_s \la 2$ accelerate CR electrons more efficiently, compared to the DSA model adopted here.