In this paper, we investigate a situation where relativistic particles are reaccelerated diffusing across regions of reconnection and magnetic dynamo in super-Alfvenic, incompressible large-scale ...turbulence. We present an exploratory study of this mechanism in the intracluster medium (ICM). In view of large-scale turbulence in the ICM, we adopt a reconnection scheme that is based on turbulent reconnection and magnetohydrodynamics (MHD) turbulence. In this case, particles are accelerated and decelerated in a systematic way in reconnecting and magnetic-dynamo regions, respectively, and on longer time-scales undergo a stochastic process diffusing across these sites (similar to second-order Fermi). Our study extends on larger scales numerical studies that focused on the acceleration in and around turbulent reconnecting regions. We suggest that this mechanism may play a role in the reacceleration of relativistic electrons in galaxy clusters providing a new physical scenario to explain the origin of cluster-scale diffuse radio emission. Indeed differently from current turbulent reacceleration models proposed for example for radio haloes, this mechanism is based on the effect of large-scale incompressible and super-Alfvenic turbulence. In this new model, turbulence governs the interaction between relativistic particles and magnetic field lines that diffuse, reconnect and are stretched in the turbulent ICM.
Context.
Radio relics in galaxy clusters are giant diffuse synchrotron sources powered in cluster outskirts by merger shocks. Although the relic–shock connection has been consolidated in recent years ...by a number of observations, the details of the mechanisms leading to the formation of relativistic particles in this environment are still not well understood.
Aims.
The diffusive shock acceleration (DSA) theory is a commonly adopted scenario to explain the origin of cosmic rays at astrophysical shocks, including those in radio relics in galaxy clusters. However, in a few specific cases it has been shown that the energy dissipated by cluster shocks is not enough to reproduce the luminosity of the relics via DSA of thermal particles. Studies based on samples of radio relics are required to further address this limitation of the mechanism.
Methods.
In this paper, we focus on ten well-studied radio relics with underlying shocks observed in the X-rays and calculate the electron acceleration efficiency of these shocks that is necessary to reproduce the observed radio luminosity of the relics.
Results.
We find that in general the standard DSA cannot explain the origin of the relics if electrons are accelerated from the thermal pool with an efficiency significantly smaller than 10%. Our results show that other mechanisms, such as shock re-acceleration of supra-thermal seed electrons or a modification of standard DSA, are required to explain the formation of radio relics.
Abstract
The Fermi-LAT (Large Area Telescope) collaboration recently published deep upper limits to the gamma-ray emission of the Coma cluster, a cluster hosting the prototype of giant radio haloes. ...In this paper, we extend previous studies and use a formalism that combines particle reacceleration by turbulence and the generation of secondary particles in the intracluster medium to constrain relativistic protons and their role for the origin of the radio halo. We conclude that a pure hadronic origin of the halo is clearly disfavoured as it would require excessively large magnetic fields. However, secondary particles can still generate the observed radio emission if they are reaccelerated. For the first time the deep gamma-ray limits allow us to derive meaningful constraints if the halo is generated during phases of reacceleration of relativistic protons and their secondaries by cluster-scale turbulence. In this paper, we explore a relevant range of parameter space of reacceleration models of secondaries. Within this parameter space, a fraction of model configurations is already ruled out by current gamma-ray limits, including the cases that assume weak magnetic fields in the cluster core, B ≤ 2–3 μG. Interestingly, we also find that the flux predicted by a large fraction of model configurations assuming magnetic fields consistent with Faraday rotation measures (RMs) is not far from the limits. This suggests that a detection of gamma-rays from the cluster might be possible in the near future, provided that the electrons generating the radio halo are secondaries reaccelerated and the magnetic field in the cluster is consistent with that inferred from RM.
Radio observations discovered large-scale non-thermal sources in the central Mpc regions of dynamically disturbed galaxy clusters (radio haloes). The morphological and spectral properties of these ...sources suggest that the emitting electrons are accelerated by spatially distributed and gentle mechanisms, providing some indirect evidence for turbulent acceleration in the intergalactic medium (IGM).
Only deep upper limits to the energy associated with relativistic protons in the IGM have been recently obtained through gamma and radio observations. Yet these protons should be (theoretically) the main non-thermal particle component in the IGM implying the unavoidable production, at some level, of secondary particles that may have a deep impact on the gamma-ray and radio properties of galaxy clusters.
Following Brunetti & Lazarian, in this paper we consider the advances in the theory of magnetohydrodynamics (MHD) turbulence to develop a comprehensive picture of turbulence in the IGM and extend our previous calculations of particle acceleration by compressible MHD turbulence by considering self-consistently the re-acceleration of both primary and secondary particles. Under these conditions we expect that radio to gamma-ray emission is generated from galaxy clusters with a complex spectrum that depends on the dynamics of the thermal gas and dark matter. The non-thermal emission results in very good agreement with radio observations and with present constraints from hard X-ray and gamma-ray observations. In our model giant radio haloes are generated in merging (turbulent) clusters only. However, in case secondaries dominate the electron component in the IGM, we expect that the level of the Mpc-scale synchrotron emission in more relaxed clusters is already close to that of the radio upper limits derived by present observations of clusters without radio haloes. Important constraints on cluster physics from future observations with present and future telescopes are also discussed.
Radio galaxies play an important role in the seeding of cosmic rays and magnetic fields in galaxy clusters. Here, we simulate the evolution of relativistic electrons injected into the intracluster ...medium by radio galaxies. Using passive tracer particles added to magnetohydrodynamical adaptive-mesh simulations, we calculated the evolution of the spectrum of relativistic electrons, taking into account energy losses and re-acceleration mechanisms associated with the dynamics of the intracluster medium. Re-acceleration can occur at shocks via diffusive shock acceleration, and in turbulent flows via second-order Fermi re-acceleration. This study confirms that relativistic electrons from radio galaxies can efficiently fill the intracluster medium over scales of several hundreds of Myr and that they create a stable reservoir of fossil electrons that remains available for further re-acceleration by shock waves and turbulent gas motions. Our results also show that late evolution of radio lobes and remnant radio galaxies is significantly affected by the dynamics of the surrounding intracluster medium. Here, the diffusive re-acceleration couples the evolution of relativistic particles to the gas perturbations. In the near future, deep radio observations, especially at low frequencies, will be able to probe such mechanisms in galaxy clusters.
A shock at the radio relic position in Abell 115 Botteon, A; Gastaldello, F; Brunetti, G ...
Monthly notices of the Royal Astronomical Society. Letters,
07/2016, Letnik:
460, Številka:
1
Journal Article
Recenzirano
Odprti dostop
We analysed a deep Chandra observation (334 ks) of the galaxy cluster Abell 115 and detected a shock cospatial with the radio relic. The X-ray surface brightness profile across the shock region ...presents a discontinuity, corresponding to a density compression factor
$\mathcal {C}=2.0\pm 0.1$
, leading to a Mach number
$\mathcal {M}=1.7\pm 0.1$
(
$\mathcal {M}=1.4{\rm -}2$
including systematics). Temperatures measured in the upstream and downstream regions are consistent with what expected for such a shock:
$T_{\rm u}=4.3^{+1.0}_{-0.6}\:\rm {keV}$
and
$T_{\rm d}=7.9^{+1.4}_{-1.1}\:\rm {keV}$
, respectively, implying a Mach number
$\mathcal {M}=1.8^{+0.5}_{-0.4}$
. So far, only few other shocks discovered in galaxy clusters are consistently detected from both density and temperature jumps. The spatial coincidence between this discontinuity and the radio relic edge strongly supports the view that shocks play a crucial role in powering these synchrotron sources. We suggest that the relic is originated by shock re-acceleration of relativistic electrons rather than acceleration from the thermal pool. The position and curvature of the shock and the associated relic are consistent with an off-axis merger with unequal mass ratio where the shock is expected to bend around the core of the less massive cluster.
Abstract
Faraday rotation and synchrotron emission from extragalactic radio sources give evidence for the presence of magnetic fields extending over ∼ Mpc scales. However, the origin of these fields ...remains elusive. With new high-resolution grid simulations, we studied the growth of magnetic fields in a massive galaxy cluster that in several aspects is similar to the Coma cluster. We investigated models in which magnetic fields originate from primordial seed fields with comoving strengths of 0.1 nG at redshift z = 30. The simulations show evidence of significant magnetic field amplification. At the best spatial resolution (3.95 kpc), we are able to resolve the scale where magnetic tension balances the bending of magnetic lines by turbulence. This allows us to observe the final growth stage of the small-scale dynamo. To our knowledge, this is the first time that this is seen in cosmological simulations of the intracluster medium. Our mock observations of Faraday rotation provide a good match to observations of the Coma cluster. However, the distribution of magnetic fields shows strong departures from a simple Maxwellian distribution, suggesting that the three-dimensional structure of magnetic fields in real clusters may be significantly different than what is usually assumed when inferring magnetic field values from rotation measure observations.
A number of relaxed, cool-core galaxy clusters exhibit diffuse, steep-spectrum radio sources in their central regions, known as radio mini-halos. It has been proposed that the relativistic electrons ...responsible for the emission have been reaccelerated by turbulence generated by the sloshing of the cool core gas. We present a high-resolution MHD simulation of gas sloshing in a galaxy cluster coupled with subgrid simulations of relativistic electron acceleration to test this hypothesis. Our simulation shows that the sloshing motions generate turbulence on the order of deltaupsilon ~ 50-200 km s super(-1) on spatial scales of ~50-100 kpc and below in the cool core region within the envelope of the sloshing cold fronts, whereas outside the cold fronts, there is negligible turbulence. This turbulence is potentially strong enough to reaccelerate relativistic electron seeds (with initial gamma ~ 100-500) to gamma ~ 10 super(4) via damping of magnetosonic waves and non-resonant compression. The seed electrons could remain in the cluster from, e.g., past active galactic nucleus activity. In combination with the magnetic field amplification in the core, these electrons then produce diffuse radio synchrotron emission that is coincident with the region bounded by the sloshing cold fronts, as indeed observed in X-rays and the radio. The result holds for different initial spatial distributions of pre-existing relativistic electrons. The power and the steep spectral index (alpha approximately 1-2) of the resulting radio emission are consistent with observations of mini-halos, though the theoretical uncertainties of the acceleration mechanisms are high. We also produce simulated maps of inverse-Compton hard X-ray emission from the same population of relativistic electrons.
We present the first high-resolution magnetohydrodynamic simulation of cosmic-ray electron re-acceleration by turbulence in cluster mergers. We use an idealized model for cluster mergers, combined ...with a numerical model for the injection, cooling and re-acceleration of cosmic-ray electrons, to investigate the evolution of cluster-scale radio emission in these objects. In line with theoretical expectations, we for the first time show in a simulation that re-acceleration of cosmic-ray electrons has the potential to reproduce key observables of radio haloes. In particular, we show that clusters evolve being radio loud or radio quiet, depending on their evolutionary stage during the merger. We thus recover the observed transient nature of radio haloes. In the simulation, the diffuse emission traces the complex interplay between the spatial distribution of turbulence injected by the halo infall and the spatial distribution of the seed electrons to re-accelerate. During the formation and evolution of the halo, the synchrotron emission spectra show the observed variety: from power laws with spectral index of 1-1.3 to curved and ultra-steep spectra with index >1.5.
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