Compressible turbulence in galaxy clusters: physics and stochastic particle re-acceleration

By Gianfranco Brunetti, A. Lazarian. 2007

306 citations recorded by the source corpus.

We attempt to explain the non-thermal emission arising from galaxy clusters as a result of the re-acceleration of electrons by compressible turbulence induced by cluster mergers. On the basis of the available observational facts we put forward a simplified model of turbulence in clusters of galaxies focusing our attention on the compressible motions. In our model intracluster medium (ICM) is represented by a high-beta plasma in which turbulent motions are driven at large scales. The corresponding injection velocities are higher than the Alfven velocity. As a result, the turbulence is approximately isotropic up to the scale at which the turbulent velocity gets comparable with the Alfven velocity. These motions are most important for the energetic particle acceleration, but at the same time they are subjected to most of the plasma damping. Under the hypothesis that turbulence in the ICM is highly super-Alfvenic the magnetic field is passively advected and the field lines are bended on scales smaller than that of the classical, unmagnetized, ion-ion mean free path. This affects ion diffusion and the strength of the effective viscosity. Under these conditions the bulk of turbulence in hot (5-10 keV temperature) galaxy clusters is likely to be dissipated at collisionless scales via resonant coupling with thermal and fast particles. We use collisionless physics to derive the amplitude of the different components of the energy of the compressible modes, and review and extend the treatment of plasma damping in the ICM. We calculate the acceleration of both protons and electrons taking into account both transit time damping acceleration and non-resonant acceleration by large-scale compressions. We find that relativistic electrons can be re-accelerated in the ICM up to energies of several GeV provided that the rms velocity of the compressible turbulent-eddies is (V L /c s ) 2 ≈ 0.15-0.3; c s is the sound speed in the ICM. We find that under typical conditions ≈2-5 per cent of the energy flux of the cascading of compressible motions injected at large scales goes into the acceleration of fast particles and that this may explain the observed non-thermal emission from merging galaxy clusters.

Subfields: Astronomy and Astrophysics, Nuclear and High Energy Physics

View Paper: https://www.semanticscholar.org/paper/c81f9f0fbc8884f5795629a2136ef1d46b343254

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Citations in the corpus

An improved SPH scheme for cosmological simulations (2015, background): “As most baryons on cosmological scales are in weakly collisional plasmas, numerical models should aim to minimize viscosity where possible (see e.g. Brunetti & Lazarian 2007).”
Massive and refined - II. The statistical properties of turbulent motions in massive galaxy clusters with high spatial resolution (2010, background): “…that the e ﬀ ective kinematic viscosity, ν , must be smaller than ν ∼ 10 29 c m 2 s − 1 6 . to At smaller scales there are physical reasons believe that the turbulence in the ICM is ﬁnally dissipated in a collision-less regime, e.g. by accelerating relativistic particles (Brunetti & Lazarian 2007).”
Acceleration of primary and secondary particles in galaxy clusters by compressible MHD turbulence: from radio haloes to gamma-rays (2010, methodology): “In this paper we have assumed the picture of MHD turbulence in galaxy clusters as described in Brunetti & Lazarian (2007) to calculate the acceleration of primary protons and secondary particles by compressible MHD turbulence.”
Cosmic-Ray Propagation: Nonlinear Diffusion Parallel and Perpendicular to Mean Magnetic Field (2007, methodology): “For instance, Brunetti & Lazarian (2007), treated acceleration of CRs for plasma in clusters of galaxies appealing to the fast modes, which is the approach to CRs similar to that in Paper I.”
DIFFUSIVE SHOCK ACCELERATION SIMULATIONS OF RADIO RELICS (2012, background): “In addition, in ICMs, nonthermal particles can be produced via turbulent re-acceleration (e.g., Brunetti & Lazarian 2007, 2011).”
RESIDUAL GAS MOTIONS IN THE INTRACLUSTER MEDIUM AND BIAS IN HYDROSTATIC MEASUREMENTS OF MASS PROFILES OF CLUSTERS (2009, background): “Random gas motions can also maintain and amplify cluster magnetic fields via dynamo processes (Roettiger et al. 1999; Subramanian et al. 2006) and contribute to the acceleration of cosmic rays in the ICM (e.g., Brunetti & Lazarian 2007).”
ASTROPHYSICAL GYROKINETICS: KINETIC AND FLUID TURBULENT CASCADES IN MAGNETIZED WEAKLY COLLISIONAL PLASMAS (2007, background): “…magnetic field anplasma effects in cluster; for further discussions and disagreements, see Enßlin & Vogt (2006); Subramanian et al. (2006); Brunetti & Lazarian (2007). long filaments (Clarke & Enßlin 2006), suggest that the reversal scale is much larger than the ion gyroscale: thus, the…”
Stochastic Acceleration by Turbulence (2012, background): “…2005), to giants radio 13This can be ascertained by plugging in for the diffusion coefficients in Equation (8) the expressions in Equation (16). galaxy lobes (Lacombe 1977; Achterberg 1979; Eilek 1979), and to intra cluster medium of clusters of galaxies (Petrosian 2001, Brunetti & Lazarian 2007).”
On the Momentum Diffusion of Radiating Ultrarelativistic Electrons in a Turbulent Magnetic Field (2008, background): “…sources of high-energy radiation and particles, such as accretion disks (e.g., Liu et al. 2004, 2006), clusters of galaxies (e.g., Petrosian 2001; Brunetti & Lazarian 2007), gamma-ray bursts (e.g., Stern & Poutanen 2004), solar flares (e.g., Petrosian & Donaghy 1999; Petrosian & Liu 2004),…”
Turbulent Motions and Shocks Waves in Galaxy Clusters simulated with AMR (2009, background): “Remarkably, the interplay between Cosmic Rays (CR) and turbulent magnetic fields may drive still poorly explored plasma processes that may potentially affect our simplified view of the IGM (Subramanian et al. 2006, Schekochihin et al.2007, Brunetti & Lazarian 2007, Guo & Oh 2008).”
Cosmic ray transport in galaxy clusters: implications for radio halos, gamma-ray signatures, and cool core heating (2010, methodology): “…al. 2004a); – significantly curved radio spectra are also very natural, since the interplay of acceleration and cooling produces spectral bumps and cut-offs (Feretti et al. 2004a; Brunetti & Lazarian 2007, 2011); – the Lν−LX relation holds also for reasonable magnetic fields (Cassano et al. 2007).”
Simulating cosmic rays in clusters of galaxies – II. A unified scheme for radio haloes and relics with predictions of the γ-ray emission (2007, background): “…within the ICM (Sarazin2002), until they experience continuous in-situ acceleration either via interactions with magneto-hydrodynamic waves, or through turbulent spectra (Jaffe 1977; Schlickeiser et al. 1987; Brunetti et al. 2001; Ohno et al. 2002; Brunetti et al. 2004; Brunetti & Lazarian 2007).”
Feedback heating by cosmic rays in clusters of galaxies (2007, background): “The MHD turbulence driven by cluster mergers may reaccelerate cosmic rays, which has been studied by many authors to explain radio halos and hard X-ray tails in some galaxy clusters (e.g. Petrosian 2001; Brunetti et al. 2004; Brunetti & Lazarian 2007).”
SEARCH FOR COSMIC-RAY-INDUCED GAMMA-RAY EMISSION IN GALAXY CLUSTERS (2013, background): “…through the second order Fermi process to sufficiently high energies to explain the observed radio emission (Schlickeiser et al. 1987; Brunetti et al. 2001; Petrosian 2001; Brunetti et al. 2004; Brunetti & Blasi 2005; Brunetti & Lazarian 2007, 2011; Brunetti et al. 2009; Donnert et al. 2013).”
Diffuse Radio Emission from Galaxy Clusters (2019, unknown): “In addition, the Fermi-II acceleration by postshock turbulence via transit-time-damping resonance has been invoked to explain the broad downstream steepening of the radio spectrum behind the observed radio shocks (Brunetti & Lazarian 2007; Kang et al. 2017; Kang 2017).”