Stochastic reacceleration of relativistic electrons by turbulent reconnection: a mechanism for cluster-scale radio emission?

Brunetti, G.; Lazarían, A.

doi:10.1093/mnras/stw496

Cited by 121 publications

(106 citation statements)

References 98 publications

(128 reference statements)

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“…On the other hand, fast-mode compressive turbulence remains isotropic down to dissipation scales, so it has become favored in treatments of stochastic reacceleration of electrons producing radio halos during cluster mergers (Brunetti & Lazarian 2007, 2011. We emphasize, at the same time, that solenoidal turbulence, likely to be energetically dominant on large scales, could still play a reacceleration role through turbulent magnetic reconnection (e.g., Brunetti & Lazarian 2016) or generation of small-scale slow-mode MHD waves that might interact resonantly with CRe (e.g., Lynn et al 2014). In our study, we do not depend on TA to produce a flat electron spectrum at the shock, but rather we explore its potential role as an effective means of slowing energy loss downstream of the shock.…”

Section: Introductionmentioning

confidence: 92%

“…Since we do not need to resolve the diffusive shock precursor or follow the DSA process in detail, this scheme allows us to use a much coarser grid, reducing dramatically the required computation time. 5 We mention for completeness a proposed alternate scenario in which solenoidal turbulence leads to fast magnetic reconnection and produces a hybrid first-second order reacceleration process (Brunetti & Lazarian 2016).…”

Section: Dsa Solutions At the Shockmentioning

confidence: 99%

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Shock Acceleration Model for the Toothbrush Radio Relic

Kang¹,

Ryu²,

Jones³

2017

ApJ

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Although many of the observed properties of giant radio relics detected in the outskirts of galaxy clusters can be explained by relativistic electrons accelerated at merger-driven shocks, significant puzzles remain. In the case of the so-called Toothbrush relic, the shock Mach number estimated from X-ray observations (is substantially weaker than that inferred from the radio spectral index (. Toward understanding such a discrepancy, we here consider the following diffusive shock acceleration (DSA) models: (1) weak-shock models with  M 2 s and a preexisting population of cosmic-ray electrons (CRe) with a flat energy spectrum, and (2) strong-shock models with » M 3 s and either shock-generated suprathermal electrons or preexisting fossil CRe. We calculate the synchrotron emission from the accelerated CRe, following the time evolution of the electron DSA, and the subsequent radiative cooling and postshock turbulent acceleration (TA). We find that both models could reproduce reasonably well the observed integrated radio spectrum of the Toothbrush relic, but the observed broad transverse profile requires the stochastic acceleration by downstream turbulence, which we label "turbulent acceleration" or TA to distinguish it from DSA. Moreover, to account for the almost uniform radio spectral index profile along the length of the relic, the weak-shock models require a preshock region over 400kpc with a uniform population of preexisting CRe with a high cutoff energy (40 GeV). Due to the short cooling time, it is challenging to explain the origin of such energetic electrons. Therefore, we suggest the strong-shock models with low-energy seed CRe (150 MeV) are preferred for the radio observations of this relic.

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Section: Introductionmentioning

confidence: 92%

Section: Dsa Solutions At the Shockmentioning

confidence: 99%

Shock Acceleration Model for the Toothbrush Radio Relic

Kang¹,

Ryu²,

Jones³

2017

ApJ

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“…In other words, the process of acceleration is a random walk process, but with substantial increments in energy. In Brunetti and Lazarian (2016) this process was applied to the acceleration of cosmic rays in galaxy clusters and was termed 1.5 Fermi acceleration. Later this 1.5 Fermi acceleration was also employed to account for the acceleration of electrons in Gamma-ray bursts (Xu and Zhang, 2017;Xu, Yang, and Zhang, 2018) and pulsar wind nebulae (Xu et al, 2019b), which successfully explains the features of their synchrotron spectra.…”

Section: Stochastic Acceleration In Mhd Turbulencementioning

confidence: 99%

3D turbulent reconnection: Theory, tests, and astrophysical implications

Lazarían¹,

et al. 2020

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Magnetic reconnection, topological change in magnetic fields, is a fundamental process in magnetized plasmas. It is associated with energy release in regions of magnetic field annihilation, but this is only one facet of this process. Astrophysical fluid flows normally have very large Reynolds numbers and are expected to be turbulent, in agreement with observations. In strong turbulence magnetic field lines constantly reconnect everywhere and on all scales, thus making magnetic reconnection an intrinsic part of the turbulent cascade. We note in particular that this is inconsistent with the usual practice of regarding magnetic field lines as persistent dynamical elements. A number of theoretical, numerical, and observational studies starting with the Lazarian & Vishniac 1999 paper proposed that 3D turbulence makes magnetic reconnection fast and that magnetic reconnection and turbulence are intrinsically connected. In particular, we discuss the dramatic violation of the textbook concept of magnetic flux-freezing in the presence of turbulence. We demonstrate that in the presence of turbulence the plasma effects are subdominant to turbulence as far as the magnetic reconnection is concerned. The latter fact justifies an MHD-like treatment of magnetic reconnection on all scales much larger than the relevant plasma scales. We discuss numerical and observational evidence supporting the turbulent reconnection model. In particular, we demonstrate that the tearing reconnection is suppressed in 3D and, unlike the 2D settings, 3D reconnection induces turbulence that makes magnetic reconnection independent of resistivity. We show that turbulent reconnection dramatically affects key astrophysical processes, e.g. star formation, turbulent dynamo, acceleration of cosmic rays. We provide criticism of the concept of "reconnection-mediated turbulence" and explain why turbulent reconnection is very different from enhanced turbulent resistivity and hyper-resistivity, and why the latter have fatal conceptual flaws.

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“…In this paper we have assumed that relativistic particles are re-accelerated and decelerated in a systematic way in reconnecting and magnetic-dynamo regions, respectively, and on longer time-scales undergo a stochastic second order Fermi process diffusing across these sites in super-Alfvénic MHD turbulence [37]. This mechanisms, proposed for the ICM, was applied also to gamma-ray bursts and Pulsars Wind Nebulae [38,39].…”

Section: Comparison With Ttd Accelerationmentioning

confidence: 99%

“…4, 5, 36]. More recently, Brunetti and Lazarian [37] proposed a mechanism that operates in large-scale super-Alfvénic solenoidal turbulence in the ICM, where particles are reaccelerated stochastically diffusing across regions of magnetic reconnection and dynamo [see 38,39, for application to gamma-ray bursts and Pulsar wind nebulae]. On much smaller scales, situations involving first order and second order Fermi-like acceleration are also observed in simulations of reconnection regions [e.g., [40][41][42][43].…”

mentioning

confidence: 99%

Second-order Fermi Reacceleration Mechanisms and Large-Scale Synchrotron Radio Emission in Intracluster Bridges

Brunetti

Vazza

2020

Phys. Rev. Lett.

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Radio observations at low frequencies with the Low Frequency Array (LOFAR) start discovering gigantic radio bridges connecting pairs of massive galaxy clusters. These observations probe unexplored mechanisms of in situ particle acceleration that operate on volumes of several Mpc 3 . Numerical simulations suggest that such bridges are dynamically complex and that weak shocks and super-Alfvénic turbulence can be driven across the entire volume of these regions. In this Letter we explore, for the first time, the role of second order Fermi mechanisms for the reacceleration of relativistic electrons interacting with turbulence in these peculiar regions. We assume the turbulent energy flux measured in simulations and adopt a scenario in which relativistic particles scatter with magnetic field lines diffusing in super-Alfvénic turbulence and magnetic fields are amplified by the same turbulence. We show that steep spectrum and volume filling synchrotron emission can be generated in the entire intra-cluster bridge region thus providing a natural explanation for radio bridges. Consequently, radio observations have the potential to probe the dissipation of energy on scales larger than galaxy clusters and second order Fermi mechanisms operating in physical regimes that are still poorly explored. This has a potential impact on several branches of astrophysics and cosmology.

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Stochastic reacceleration of relativistic electrons by turbulent reconnection: a mechanism for cluster-scale radio emission?

Cited by 121 publications

References 98 publications

Shock Acceleration Model for the Toothbrush Radio Relic

Shock Acceleration Model for the Toothbrush Radio Relic

3D turbulent reconnection: Theory, tests, and astrophysical implications

Second-order Fermi Reacceleration Mechanisms and Large-Scale Synchrotron Radio Emission in Intracluster Bridges

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