Curved Radio Spectra of Weak Cluster Shocks

Abstract

In order to understand certain observed features of arc-like giant radio relics such as the rareness, uniform surface brightness, and curved integrated spectra, we explore a diffusive shock acceleration (DSA) model for radio relics in which a spherical shock impinges on a magnetized cloud containing fossil relativistic electrons. Toward this end, we perform DSA simulations of spherical shocks with the parameters relevant for the Sausage radio relic in cluster CIZA J2242.8+5301, and calculate the ensuing radio synchrotron emission from re-accelerated electrons. Three types of fossil electron populations are considered: a delta-function like population with the shock injection momentum, a power-law distribution, and a power-law with an exponential cutoff. The surface brightness profile of radio-emitting postshock region and the volume-integrated radio spectrum are calculated and compared with observations. We find that the observed width of the Sausage relic can be explained reasonably well by shocks with speed $u_s \sim 3\times 10^3 \kms$ and sonic Mach number $M_s\sim 3$. These shocks produce curved radio spectra that steepen gradually over $(0.1-10){\nu }_{\mathrm{br}}$ with break frequency ${\nu }_{\mathrm{br}}\sim 1$ GHz, if the duration of electron acceleration is $\sim 60-80$ Myr. However, the abrupt increase of spectral index above $\sim 1.5$ GHz observed in the Sausage relic seems to indicate that additional physical processes, other than radiative losses, operate for electrons with ${\gamma }_e\gtrsim 10^4$.

accepted for publication in ApJ

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• Suzaku observations of the merging galaxy cluster Abell2255: The northeast radio relic
• Can cluster merger shocks reproduce the luminosity and shape distribution of radio relics?
• Exploring the spectral properties of radio relics I: Integrated spectral index and Mach number
• Revived Fossil Plasma Sources in Galaxy Clusters
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• X-ray study of the double radio relic Abell 3376 with Suzaku
• Discovering the most elusive radio relic in the sky: Diffuse Shock Acceleration caught in the act?
• Magnetic Field Evolution in Giant Radio Relics using the example of CIZA J2242.8+5301
• Turbulence Dynamo in Galaxy Clusters
• Shock Acceleration Model for the Toothbrush Radio Relic
• Using Collisions of AGN Outflows with ICM Shocks as Dynamical Probes
• Ultra-steep spectrum emission in the merging galaxy cluster Abell 1914
• Discovery of a radio relic in the low mass, merging galaxy cluster PLCK G200.9-28.2
• Shocks and Tides Quantified in the "Sausage" Cluster, CIZA J2242.8+5301, using N-body/hydro-dynamical Simulations
• Spectro-polarimetric observations of the CIZA J2242.8+5301 northern radio relic: no evidence of high-frequency steepening
• Simulations of the Galaxy Cluster CIZA J2242.8+5301 I: Thermal Model and Shock Properties
• Multi-wavelength Analysis of the Merging Galaxy Cluster A115
• Turbulent Cosmic-Ray Reacceleration and the Curved Radio Spectrum of the Radio Relic in the Sausage Cluster
• Discovery of a double radio relic in ZwCl1447.2+2619: A rare testbed for shock acceleration models with a peculiar surface brightness ratio
• The correlation between radio power and Mach number for radio relics in galaxy clusters
• Shocked Narrow-Angle Tail Radio Galaxies: Simulations and Emissions
• Electron Preacceleration at Weak Quasi-perpendicular Intracluster Shocks: Effects of Preexisting Nonthermal Electrons
• Discovery of large-scale diffuse radio emission in low-mass galaxy cluster Abell 1931
• First detection at 5.5 and 9 GHz of the radio relics in bullet cluster with ATCA
• Re-acceleration model for the `Sausage' Radio Relic
• The peculiar cluster MACS J0417.5-1154 in the C and X-bands