Magnetic field evolution in giant radio relics using the example of CIZA J2242.8+5301

Donnert, J.; Stroe, Andra; Brunetti, G.; Hoang, D. N.; Roettgering, H.

doi:10.1093/mnras/stw1792

Cited by 37 publications

(70 citation statements)

References 65 publications

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“…Top right: spectrum of the Toothbrush radio relic. The flux densities at frequencies lower than 4.85 GHz are taken from Table 3 magnetic field strength (Donnert et al 2016). We found spectral indices of 0.90 ± 0.04 and 1.00 ± 0.04 for the Sausage and the Toothbrush, respectively, indicating that models that describe the origin of relics have to include effects beyond the assumptions made above.…”

Section: Integrated Flux Densities and Spectral Indicesmentioning

confidence: 92%

“…A break in the spectrum would indicate a more complicated origin of the relativistic electrons, for example re-acceleration of aged seed electrons (e.g., Markevitch et al 2005), or a significant increase of the downstream magnetic field strength within the cooling time of cosmic-ray electrons (CREs; Donnert et al 2016). The steep spectra of radio relics hampers the detection of polarized emission at high frequencies.…”

Section: Introductionmentioning

confidence: 99%

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Relics in galaxy clusters at high radio frequencies

Kierdorf

Beck

Hoeft

et al. 2017

A&A

110

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Aims. We investigated the magnetic properties of radio relics located at the peripheries of galaxy clusters at high radio frequencies, where the emission is expected to be free of Faraday depolarization. The degree of polarization is a measure of the magnetic field compression and, hence, the Mach number. Polarization observations can also be used to confirm relic candidates. Methods. We observed three radio relics in galaxy clusters and one radio relic candidate at 4.85 and 8.35 GHz in total emission and linearly polarized emission with the Effelsberg 100-m telescope. In addition, we observed one radio relic candidate in X-rays with the Chandra telescope. We derived maps of polarization angle, polarization degree, and Faraday rotation measures. Results. The radio spectra of the integrated emission below 8.35 GHz can be well fitted by single power laws for all four relics. The flat spectra (spectral indices of 0.9 and 1.0) for the so-called Sausage relic in cluster CIZA J2242+53 and the so-called Toothbrush relic in cluster 1RXS 06+42 indicate that models describing the origin of relics have to include effects beyond the assumptions of diffuse shock acceleration. The spectra of the radio relics in ZwCl 0008+52 and in Abell 1612 are steep, as expected from weak shocks (Mach number ≈2.4). Polarization observations of radio relics offer a method of measuring the strength and geometry of the shock front. We find polarization degrees of more than 50% in the two prominent Mpc-sized radio relics, the Sausage and the Toothbrush, which are among the highest percentages of linear polarization detected in any extragalactic radio source to date. This is remarkable because the large beam size of the Effelsberg single-dish telescope corresponds to linear extensions of about 300 kpc at 8.35 GHz at the distances of the relics. The high degree of polarization indicates that the magnetic field vectors are almost perfectly aligned along the relic structure, as expected for shock fronts that are observed edge-on. The polarization degrees correspond to Mach numbers of >2.2. Polarized emission is also detected in the radio relics in ZwCl 0008+52 and, for the first time, in Abell 1612. The smaller sizes and lower degrees of polarizations of the latter relics indicate a weaker shock and/or an inclination between the relic and the sky plane. Abell 1612 shows a complex X-ray surface brightness distribution, indicating a recent major merger and supporting the classification of the radio emission as a radio relic. In our cluster sample, no wavelength-dependent Faraday depolarization is detected between 4.85 GHz and 8.35 GHz, except for one component of the Toothbrush relic. Faraday depolarization between 1.38 GHz and 8.35 GHz varies with distance from the center of the host cluster 1RXS 06+42, which can be explained by a decrease in electron density and/or in strength of a turbulent (or tangled) magnetic field. Faraday rotation measures show large-scale gradients along the relics, which cannot be explained by variations in the Milk...

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Section: Integrated Flux Densities and Spectral Indicesmentioning

confidence: 92%

Section: Introductionmentioning

confidence: 99%

Relics in galaxy clusters at high radio frequencies

Kierdorf

Beck

Hoeft

et al. 2017

A&A

110

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show abstract

“…Note, then, that since the downstream flow speed in the shock rest frame increases behind a spherical shock, the advection length in a given timescale is somewhat longer than that estimated for a planar shock (Donnert et al 2016). The factor Q depends on the postshock magnetic field strength, B 2 , as Being only about half the observed width of the B1 region of the Toothbrush relic at this frequency, it seems too small to allow the width to be set by radiative cooling alone following acceleration at the shock surface.…”

Section: Introductionmentioning

confidence: 87%

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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“…Interplay between the turbulence and shocks may be important in other respects, as well. For instance, turbulent amplification of magnetic fields by shocks and associated second-order Fermi acceleration leading to radio relic emission has been explored in several recent studies (Iapichino & Brüggen 2012;Donnert et al 2016;Ji et al 2016;Fujita et al 2015;Fujita, Akamatsu, & Kimura 2016;Donnert et al 2016). The relative contributions from solenoidal and compressive turbulent components will depend on the manner in which the turbulence is generated (Federrath et al 2010;Porter et al 2015) and its intensity (Vazquez-Semadeni 1994).…”

Section: Introductionmentioning

confidence: 99%

Turbulence and vorticity in Galaxy clusters generated by structure formation

Vazza

Jones

Brüggen

et al. 2016

Mon. Not. R. Astron. Soc.

Self Cite

124

150

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Turbulence is a key ingredient for the evolution of the intracluster medium, whose properties can be predicted with high resolution numerical simulations. We present initial results on the generation of solenoidal and compressive turbulence in the intracluster medium during the formation of a small-size cluster using highly resolved, non-radiative cosmological simulations, with a refined monitoring in time. In this first of a series of papers, we closely look at one simulated cluster whose formation was distinguished by a merger around z ∼ 0.3. We separate laminar gas motions, turbulence and shocks with dedicated filtering strategies and distinguish the solenoidal and compressive components of the gas flows using Hodge-Helmholtz decomposition. Solenoidal turbulence dominates the dissipation of turbulent motions (∼ 95%) in the central cluster volume at all epochs. The dissipation via compressive modes is found to be more important (∼ 30% of the total) only at large radii ( 0.5 r vir ) and close to merger events. We show that enstrophy (vorticity squared) is good proxy of solenoidal turbulence. All terms ruling the evolution of enstrophy (i.e. baroclinic, compressive, stretching and advective terms) are found to be significant, but in amounts that vary with time and location. Two important trends for the growth of enstrophy in our simulation are identified: first, enstrophy is continuously accreted into the cluster from the outside, and most of that accreted enstrophy is generated near the outer accretion shocks by baroclinic and compressive processes. Second, in the cluster interior vortex stretching is dominant, although the other terms also contribute substantially.

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Magnetic field evolution in giant radio relics using the example of CIZA J2242.8+5301

Cited by 37 publications

References 65 publications

Relics in galaxy clusters at high radio frequencies

Relics in galaxy clusters at high radio frequencies

Shock Acceleration Model for the Toothbrush Radio Relic

Turbulence and vorticity in Galaxy clusters generated by structure formation

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