Acceleration of primary and secondary particles in galaxy clusters by compressible MHD turbulence: from radio haloes to gamma-rays

Brunetti, G.; Lazarían, A.

doi:10.1111/j.1365-2966.2010.17457.x

Cited by 180 publications

(190 citation statements)

References 153 publications

(355 reference statements)

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“…Specifically, the emission of the relic could be projected on the halo emission causing a flattening of the spectrum at higher frequencies. We also note that recent modeling of turbulent re-acceleration of relativistic protons and their secondaries in the ICM predict a flattening of the synchrotron spectrum of radio halos at higher frequencies (Brunetti & Lazarian 2011). In these models the flattening marks the transition between the spectral component due to turbulent re-acceleration and the underlying spectrum generated by the continuous injection of secondary electrons; the spectral shape of A2256 would constrain the turbulent acceleration time-scale 0.5 Gyr.…”

Section: Radio Halomentioning

confidence: 99%

“…Relativistic electrons in the ICM can be re-accelerated in situ through interaction with turbulence generated in the ICM by cluster-cluster mergers (Brunetti et al 2001;Petrosian 2001), or secondary electrons can be continuously injected in the ICM by inelastic collisions between relativistic and thermal protons (Dennison 1980;Blasi & Colafrancesco 1999;Dolag & Enßlin 2000;Enßlin et al 2011). Models considering a combination of the two mechanisms, namely the re-acceleration of both relativistic protons and their secondaries by magnetohydrodynamical turbulence generated in the ICM have been recently considered (Brunetti & Blasi 2005;Dolag et al 2008;Brunetti & Lazarian 2011).…”

Section: Introductionmentioning

confidence: 99%

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First LOFAR observations at very low frequencies of cluster-scale non-thermal emission: the case of Abell 2256

Weeren¹,

Röttgering²,

Rafferty³

et al. 2012

A&A

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Abell 2256 is one of the best known examples of a galaxy cluster hosting large-scale diffuse radio emission that is unrelated to individual galaxies. It contains both a giant radio halo and a relic, as well as a number of head-tail sources and smaller diffuse steep-spectrum radio sources. The origin of radio halos and relics is still being debated, but over the last years it has become clear that the presence of these radio sources is closely related to galaxy cluster merger events. Here we present the results from the first LOFAR low band antenna (LBA) observations of Abell 2256 between 18 and 67 MHz. To our knowledge, the image presented in this paper at 63 MHz is the deepest ever obtained at frequencies below 100 MHz in general. Both the radio halo and the giant relic are detected in the image at 63 MHz, and the diffuse radio emission remains visible at frequencies as low as 20 MHz. The observations confirm the presence of a previously claimed ultra-steep spectrum source to the west of the cluster center with a spectral index of −2.3 ± 0.4 between 63 and 153 MHz. The steep spectrum suggests that this source is an old part of a head-tail radio source in the cluster. For the radio relic we find an integrated spectral index of −0.81 ± 0.03, after removing the flux contribution from the other sources. This is relatively flat which could indicate that the efficiency of particle acceleration at the shock substantially changed in the last ∼0.1 Gyr due to an increase of the shock Mach number. In an alternative scenario, particles are re-accelerated by some mechanism in the downstream region of the shock, resulting in the relatively flat integrated radio spectrum. In the radio halo region we find indications of low-frequency spectral steepening which may suggest that relativistic particles are accelerated in a rather inhomogeneous turbulent region.

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Section: Radio Halomentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

First LOFAR observations at very low frequencies of cluster-scale non-thermal emission: the case of Abell 2256

Weeren¹,

Röttgering²,

Rafferty³

et al. 2012

A&A

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“…Assuming these constraints, it has been suggested that during cluster mergers the re-acceleration of secondary electrons by compressible MHD turbulence can generate synchrotron radiation in good agreement with radio halos, while synchrotron emission ∼10 times fainter is generated in dynamically relaxed clusters when turbulence is dissipated (Brunetti & Lazarian 2011b). This theoretical conjecture is consistent with the observed radio bimodality in galaxy clusters (e.g., Brunetti et al 2009;Cassano et al 2010a), although it predicts a level of emission (from pure secondaries) in relaxed clusters that is close the upper limits derived for these clusters from present radio observations.…”

Section: Introductionmentioning

confidence: 99%

Radio halos in future surveys in the radio continuum

Cassano

Brunetti

Norris

et al. 2012

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Aims. Giant radio halos are Mpc-scale synchrotron sources detected in a significant fraction of massive and merging galaxy clusters. The statistical properties of radio halos can be used to discriminate among various models for the origin of non-thermal particles in galaxy clusters. Therefore, theoretical predictions are important as new radio telescopes are about to begin to survey the sky at low and high frequencies with unprecedented sensitivity. Methods. We carry out Monte Carlo simulations to model the formation and evolution of radio halos in a cosmological framework and extend previous calculations based on the hypothesis of turbulent-acceleration. We adopt a phenomenological approach by assuming that radio halos are either generated in turbulent merging clusters, or are purely hadronic sources generated in more relaxed clusters, "off-state" halos.Results. The models predict that the luminosity function of radio halos at high radio luminosities is dominated by the contribution of halos generated in turbulent clusters. The generation of these halos becomes less efficient in less massive systems causing a flattening of the luminosity function at lower radio luminosities, as also pointed out in previous studies. However, we find that potentially this can be more than compensated for by the intervening contribution of "off-state" halos that dominate at lower radio luminosities. We derive the expected number of halos to explore the potential of the EMU+WODAN surveys that will be carried out with ASKAP and Aperitif, respectively, in the near future. By restricting to clusters at redshifts ≤0.6, we show that the planned EMU+WODAN surveys at 1.4 GHz have the potential to detect up to about 200 new radio halos, increasing their number by one order of magnitude. A fraction of these sources will be "off-state" halos that should be found at flux level f 1.4 ≤ 10 mJy, presently accessible only to deep pointed observations. We also explore the synergy between surveys at different radio frequencies, the Tier 1 LOFAR survey at 150 MHz and the EMU+WODAN surveys at 1.4 GHz. We predict a larger number of radio halos in the LOFAR survey due to the high LOFAR sensitivity, but also due to the existence of halos with very steep spectrum that glow up preferentially at lower frequencies. These halos are only predicted in the framework of turbulent re-acceleration models and should not have counterparts in the EMU+WODAN surveys, thus the combination of the two surveys will test theoretical models.

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“…The origin of these components is still unclear and a subject of lively debate. Potentially these components contribute to the energy of the ICM and drive complex physical processes that may significantly alter our present (simplified) view of the ICM (Schekochihin et al 2005;Subramanian, Shukurov, & Haugen 2006;Brunetti & Lazarian 2011).…”

Section: Galaxy Clusters and Large-scale Structurementioning

confidence: 99%

“…Instead, there must be a pre-existing population of relativistic (or suprathermal) seed particles that are then reaccelerated by turbulent acceleration during mergers. One possibility is that the seeds are secondary electrons generated by collisions between CRs and thermal protons (Brunetti & Blasi 2005;Brunetti & Lazarian 2011). These hybrid models predict that Mpc-scale synchrotron emission from secondary electrons ('pure hadronic haloes') must also be present in relaxed clusters, with a radio luminosity 10-20 times smaller than that of merging clusters (Brunetti & Lazarian 2011).…”

Section: Radio Haloesmentioning

confidence: 99%