Electron Preacceleration at Weak Quasi-perpendicular Intracluster Shocks: Effects of Preexisting Nonthermal Electrons

Abstract

Radio relics in the outskirts of galaxy clusters imply the diffusive shock acceleration (DSA) of electrons at merger-driven shocks with Mach number $M_s\lesssim 3-4$ in the intracluster medium (ICM). Recent studies have suggested that electron preacceleration and injection, prerequisite steps for DSA, could occur at supercritical shocks with $M_s\gtrsim 2.3$ in the ICM, thanks to the generation of multiscale waves by microinstabilities such as the Alfvén ion cyclotron (AIC) instability, the electron firehose instability (EFI), and the whistler instability (WI). On the other hand, some relics are observed to have subcritical shocks with $M_s\lesssim 2.3$, leaving DSA at such weak shocks as an outstanding problem. Reacceleration of preexisting nonthermal electrons has been contemplated as one of possible solutions for that puzzle. To explore this idea, we perform Particle-in-Cell (PIC) simulations for weak quasi-perpendicular shocks in high-$\beta$ ($\beta =P_{\mathrm{gas}}/P_B$) plasmas with power-law suprathermal electrons in addition to Maxwellian thermal electrons. We find that suprathermal electrons enhance the excitation of electron-scale waves via the EFI and WI. However, they do not affect the ion reflection and the ensuing generation of ion-scale waves via the AIC instability. The presence of ion-scale waves is the key for the preacceleration of electrons up to the injection momentum, thus the shock criticality condition for electron injection to DSA is preserved. Based on the results, we conclude that preexisting nonthermal electrons in the preshock region alone would not resolve the issue of electron preacceleration at subcritical ICM shocks.

12 pages, 7 figures, to appear in ApJ

References in corpus (25)

• Shocks and cold fronts in galaxy clusters
• Diffuse Radio Emission from Galaxy Clusters
• Turbulence and Magnetic Fields in the Large Scale Structure of the Universe
• Particle Acceleration on Megaparsec Scales in a Merging Galaxy Cluster
• Detecting shock waves in cosmological smoothed particle hydrodynamics simulations
• Simultaneous Acceleration of Protons and Electrons at Nonrelativistic Quasiparallel Collisionless Shocks
• The Case for Electron Re-Acceleration at Galaxy Cluster Shocks
• Simulations of relativistic collisionless shocks: shock structure and particle acceleration
• Shock Waves in Eulerian Cosmological Simulations: Main Properties and Acceleration of Cosmic Rays
• Simulations and Theory of Ion Injection at Non-relativistic Collisionless Shocks
• 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
• Non-Thermal Electron Acceleration in Low Mach Number Collisionless Shocks. II. Firehose-Mediated Fermi Acceleration and its Dependence on Pre-Shock Conditions
• Shock finding on a moving-mesh: I. Shock statistics in non-radiative cosmological simulations
• 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
• Relics in galaxy clusters at high radio frequencies
• Electron Preacceleration in Weak Quasi-perpendicular Shocks in High-beta Intracluster Medium
• Exploring the spectral properties of radio relics I: Integrated spectral index and Mach number
• Revived Fossil Plasma Sources in Galaxy Clusters
• The Acceleration of Electrons at Collisionless Shocks Moving Through a Turbulent Magnetic Field
• First observations of irregular surface of interplanetary shocks at ion scales by Cluster
• Electron Heating in Low Mach Number Perpendicular shocks. I. Heating Mechanism
• Limiting the shock acceleration of cosmic-ray protons in the ICM
• Electron Acceleration at Rippled Low-Mach-number Shocks in High-beta Collisionless Cosmic Plasmas

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