Cosmic Ray Acceleration and Nonthermal Radiation at Accretion Shocks in the Outer Regions of Galaxy Clusters

Abstract

Cosmology models predict that external accretion shocks form in the outer region of galaxy clusters due to supersonic gas infall from filaments and voids in the cosmic web. They are characterized by high sonic and Alfvénic Mach numbers, $M_s\sim 10-10^2$ and $M_A\sim 10^2-10^3$, and propagate into weakly magnetized plasmas of $\beta \equiv P_g/P_B\gtrsim 10^2$. Although strong accretion shocks are expected to be efficient accelerators of cosmic rays (CRs), nonthermal signatures of shock-accelerated CRs around clusters have not been confirmed, and detailed acceleration physics at such shocks has yet to be understood. In this study, we first establish through two-dimensional particle-in-cell simulations that at strong high-$\beta$ shocks electrons can be pre-energized via stochastic Fermi acceleration owing to the ion-Weibel instability in the shock transition region, possibly followed by injection into diffusive shock acceleration. Hence, we propose that the models derived from conventional thermal leakage injection may be employed for the acceleration of electrons and ions at accretion shocks as well. Applying these analytic models to numerical shock zones identified in structure formation simulations, we estimate nonthermal radiation, such as synchrotron and inverse-Compton (IC) emission due to CR electrons, and ${\pi }^0$-decay $\gamma$-rays due to CR protons, around simulated clusters. Our models with the injection parameter, $Q\approx 3.5-3.8$, predict synthetic synchrotron maps, which seem consistent with recent radio observations of the Coma cluster. However, the detection of nonthermal IC X-rays and $\gamma$-rays from accretion shocks would be quite challenging. We suggest that the proposed analytic models may be adopted as generic recipes for CR production at cosmological shocks.

arXiv comment21 pages, 12 figures

References in corpus (67)

• Seven-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Cosmological Interpretation
• Energy spectra of gamma-rays, electrons and neutrinos produced at proton-proton interactions in the very high energy regime
• Particle acceleration in relativistic collisionless shocks: Fermi process at last?
• Simulations of Ion Acceleration at Non-relativistic Shocks. I. Acceleration Efficiency
• Diffuse Radio Emission from Galaxy Clusters
• Cosmological Shock Waves and Their Role in the Large Scale Structure of the Universe
• Turbulence and Magnetic Fields in the Large Scale Structure of the Universe
• The Maximum Energy of Accelerated Particles in Relativistic Collisionless Shocks
• Particle Acceleration on Megaparsec Scales in a Merging Galaxy Cluster
• Observation of magnetic field generation via the Weibel instability in interpenetrating plasma flows
• Properties of Cosmic Shock Waves in Large Scale Structure Formation
• The microphysics of collisionless shock waves
• Detecting shock waves in cosmological smoothed particle hydrodynamics simulations
• Simultaneous Acceleration of Protons and Electrons at Nonrelativistic Quasiparallel Collisionless Shocks
• Gamma-Ray Background from Structure Formation in the Intergalactic Medium
• Simulations of Ion Acceleration at Non-relativistic Shocks. II. Magnetic Field Amplification
• Shock Waves in Eulerian Cosmological Simulations: Main Properties and Acceleration of Cosmic Rays
• Non-relativistic Collisionless Shocks in Unmagnetized Electron-Ion Plasmas
• Cosmological Shocks in Adaptive Mesh Refinement Simulations and the Acceleration of Cosmic Rays
• Non-Thermal Electron Acceleration in Low Mach Number Collisionless Shocks. I. Particle Energy Spectra and Acceleration Mechanism
• Gamma-Rays from Intergalactic Shocks
• Weibel-instability-mediated collisionless shocks in laboratory with ultraintense lasers
• Electron Injection by Whistler Waves in Non-relativistic Shocks
• Cosmological Shock Waves in the Large Scale Structure of the Universe: Non-gravitational Effects
• Synthetic Spectra from PIC Simulations of Relativistic Collisionless Shocks
• Search for gamma-ray emission from the Coma Cluster with six years of Fermi-LAT data
• Simulating the gamma-ray emission from galaxy clusters: a universal cosmic ray spectrum and spatial distribution
• Numerical Studies of Cosmic Ray Injection and Acceleration
• Non-Thermal Electron Acceleration in Low Mach Number Collisionless Shocks. II. Firehose-Mediated Fermi Acceleration and its Dependence on Pre-Shock Conditions
• Cluster Accretion Shocks as Possible Acceleration Sites for Ultra High Energy Protons below the Greisen Cutoff
• Properties of Merger Shocks in Merging Galaxy Clusters
• Diffuse radio emission from clusters in the MareNostrum Universe simulation
• Numerical Modeling of Gamma Radiation from Galaxy Clusters
• Electron Shock Surfing Acceleration in Multidimensions: Two-dimensional Particle-In-Cell Simulation of Collisionless Perpendicular Shock
• Diffusive Shock Acceleration at Cosmological Shock Waves
• Forming Clusters of Galaxies as the Origin of Unidentified GeV Gamma-Ray Sources
• Electron Surfing and Drift Accelerations in a Weibel-dominated High-Mach-number Shock
• Constraining the efficiency of cosmic ray acceleration by cluster shocks
• Non-linear diffusive shock acceleration with free escape boundary
• Shock finding on a moving-mesh: I. Shock statistics in non-radiative cosmological simulations
• A double radio relic in the merging galaxy cluster ZwCl 0008.8+5215
• Nonrelativistic collisionless shocks in weakly magnetized electron--ion plasmas: two-dimensional particle-in-cell simulation of perpendicular shock
• Shock Waves and Cosmic Ray Acceleration in the Outskirts of Galaxy Clusters
• Theory of Stochastic Shock Drift Acceleration for Electrons in the Shock Transition Region
• Electron Accelerations at High Mach Number Shocks: Two-Dimensional Particle-In-Cell Simulations in Various Parameter Regimes
• Electron Preacceleration in Weak Quasi-perpendicular Shocks in High-beta Intracluster Medium
• Magnetic field amplification by the Weibel instability at planetary and astrophysical high-Mach-number shocks
• Proton Acceleration in Weak Quasi-parallel Intracluster Shocks: Injection and Early Acceleration
• Diffusive Shock Acceleration in Test-Particle Regime
• Electron Pre-Acceleration at Nonrelativistic High-Mach-Number Perpendicular Shocks
• Kinetic simulations of nonrelativistic perpendicular shocks of young supernova remnants. III. Magnetic reconnection
• The Efficiency of Magnetic Field Amplification at Shocks by Turbulence
• A NuSTAR observation of the center of the Coma Cluster
• A statistical detection of gamma-ray emission from galaxy clusters: implications for the gamma-ray background and structure formation
• A Diffusive Shock Acceleration Model for Protons in Weak Quasi-parallel Intracluster Shocks
• Kinetic simulations of nonrelativistic perpendicular shocks of young supernova remnants. I. Electron shock-surfing acceleration
• Effects of Multi-scale Plasma Waves on Electron Preacceleration at Weak Quasi-perpendicular Intracluster Shocks
• A joint XMM-NuSTAR observation of the galaxy cluster Abell 523: constraints on Inverse Compton emission
• Kinetic simulations of nonrelativistic perpendicular shocks of young supernova remnants. II. Influence of shock-surfing acceleration on downstream electron spectra
• Preliminary evidence for a virial shock around the Coma galaxy cluster
• Electron Acceleration at Rippled Low-Mach-number Shocks in High-beta Collisionless Cosmic Plasmas
• Limiting the shock acceleration of cosmic-ray protons in the ICM
• Non-linear diffusive shock acceleration: A recipe for injection of electrons
• Hard X-ray emission from accretion shocks around galaxy clusters
• Microinstabilities in the Transition Region of Weak Quasi-Perpendicular Intracluster Shocks
• Electron Firehose Instabilities in High-$\beta$ ICM shocks
• Possible non-thermal origin of the hard X-ray emission in the merging galaxy cluster SPT-CL J2031-4037

Cited by in corpus (4)

• A formation mechanism for "Wrong Way" Radio Relics
• Simulating the LOcal Web (SLOW) -- III: Synchrotron emission from the local cosmic web
• Criteria for ion acceleration in laboratory magnetized quasi-perpendicular collisionless shocks: when are 2D simulations enough?
• A Hierarchical Shock Model of Ultra-High-Energy Cosmic Rays