7,164 research outputs found
The challenge of turbulent acceleration of relativistic particles in the intra-cluster medium
Acceleration of cosmic-ray electrons (CRe) in the intra-cluster-medium (ICM)
is probed by radio observations that detect diffuse, Mpc-scale, synchrotron
sources in a fraction of galaxy clusters. Giant radio halos are the most
spectacular manifestations of non-thermal activity in the ICM and are currently
explained assuming that turbulence driven during massive cluster-cluster
mergers reaccelerates CRe at several GeV. This scenario implies a hierarchy of
complex mechanisms in the ICM that drain energy from large-scales into
electromagnetic fluctuations in the plasma and collisionless mechanisms of
particle acceleration at much smaller scales. In this paper we focus on the
physics of acceleration by compressible turbulence. The spectrum and damping
mechanisms of the electromagnetic fluctuations, and the mean-free-path (mfp) of
CRe are the most relevant ingredients that determine the efficiency of
acceleration. These ingredients in the ICM are however poorly known and we show
that calculations of turbulent acceleration are also sensitive to these
uncertainties. On the other hand this fact implies that the non-thermal
properties of galaxy clusters probe the complex microphysics and the weakly
collisional nature of the ICM.Comment: 13 pages, 3 figures. Invited paper for the 42th EPS conference on
plasma physics (2015), accepted for publication in Plasma Physics and
Controlled Fusion. This is an author-created, un-copyedited version of an
article accepted for publication in PPCF. IOP Publishing Ltd is not
responsible for any errors or omissions in this version of the manuscript or
any version derived from i
Anisotropic inverse Compton scattering from the trans-relativistic to the ultra-relativistic regime and application to the radio galaxies
The problem of the anisotropic Inverse Compton scattering between a
monochromatic photon beam and relativistic electrons is revisited and formally
solved without approximations. Solutions are given for the single scattering
with an electron beam and with a population of electrons isotropically
distributed, under the assumption that the energy distribution of the
relativistic particles follows a simple power law as it is the case in many
astrophysical applications. Both the Thomson approximation and the
Klein-Nishina regime are considered for the scattering of an unpolarized photon
beam. The equations are obtained without the ultra-relativistic approximation
and are compared with the ultra-relativistic solutions given in the literature.
The main characteristics of the power distribution and spectra of the scattered
radiation are discussed for relevant examples. In the Thomson case for an
isotropic electron population simple formulae holding down to
mildly-relativistic energies are given. As an application the formulae of the
anisotropic inverse Compton scattering are used to predict the properties of
the X and -ray spectra from the radio lobes of strong FR II radio
galaxies due to the interaction of the relativistic electrons with the incoming
photons from the nucleus. The dependence of the emitted power on the
relativistic electron energy distribution and on its evolution with time is
discussed.Comment: 35 pages, 17 .ps figures, LaTex, to appear in Astroparticle Physic
Anisotropic inverse Compton emission in the radio galaxy 3C 265
We present the results from a Chandra observation of the powerful radio
galaxy 3C 265. We detect X-ray emission from the nucleus, the radio hotspots
and lobes. In particular, the lobe X-ray emission is well explained as
anisotropic inverse Compton scattering of the nuclear photons by the
relativistic electrons in the radio lobes; the comparison between radio
synchrotron and IC emission yields a magnetic field strength a factor about 2
lower than that calculated under minimum energy conditions. The X-ray spectrum
of the nucleus is consistent with that of a powerful, strongly absorbed quasar
and the X-ray emission of the south-eastern hotspot can be successfully
reproduced by a combination of synchro-self Compton and inverse Compton
emission assuming a magnetic field slightly lower than equipartition.Comment: 5 pages, 6 figures, to be published as a Letter on Monthly Notices of
the Royal Astronomical Societ
Cluster Mergers, Radio Halos and Hard X-ray Tails: A Statistical Magneto-Turbulent Model
There is now firm evidence that the ICM consists of a mixture of hot plasma,
magnetic fields and relativistic particles. The most important evidences for
non-thermal phenomena in galaxy clusters comes from the diffuse Mpc-scale
synchrotron radio emission (radio halos) observed in a growing number of
massive clusters (Feretti 2003) and from hard X-ray (HXR) excess emission
(detected in a few cases) which can be explained in terms of IC scattering of
relativistic electrons off the cosmic microwave background photons
(Fusco-Femiano et al. 2003). There are now growing evidences that giant radio
halos may be naturally accounted for by synchrotron emission from relativistic
electrons reaccelerated by some kind of turbulence generated in the cluster
volume during merger events (Brunetti 2003). With the aim to investigate the
connection between thermal and non-thermal properties of the ICM, we have
developed a statistical magneto-turbulent model which describes the evolution
of the thermal and non-thermal emission from clusters. We calculate the energy
and spectrum of the magnetosonic waves generated during cluster mergers, the
acceleration and evolution of relativistic electrons and thus the resulting
synchrotron and inverse Compton spectra. Here we give a brief description of
the main results, while a more detailed discussion will be presented in a
forthcoming paper. Einstein-De Sitter cosmology, km
, , is assumed.Comment: 3 pages, 2 figures. To appear in the proceedings of IAU Colloquium
195 - "Outskirts of galaxy clusters: intense life in the suburbs", Torino,
Italy, March 12-16, 200
Stochastic reacceleration of relativistic electrons by turbulent reconnection: a mechanism for cluster-scale radio emission ?
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 intra-cluster-medium (ICM). In view
of large-scale turbulence in the ICM we adopt a reconnection scheme that is
based on turbulent reconnection and 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 halos 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.Comment: 13 pages, 2 figures, MNRAS in pres
A giant radio halo in the massive and merging cluster Abell 1351
We report on the detection of diffuse radio emission in the X-ray luminous
and massive galaxy cluster A1351 (z=0.322) using archival Very Large Array data
at 1.4 GHz. Given its central location, morphology, and Mpc-scale extent, we
classify the diffuse source as a giant radio halo. X-ray and weak lensing
studies show A1351 to be a system undergoing a major merger. The halo is
associated with the most massive substructure. The presence of this source is
explained assuming that merger-driven turbulence may re-accelerate high-energy
particles in the intracluster medium and generate diffuse radio emission on the
cluster scale. The position of A1351 in the logP - logL plane
is consistent with that of all other radio-halo clusters known to date,
supporting a causal connection between the unrelaxed dynamical state of massive
() clusters and the presence of giant radio halos.Comment: 4 pages, 3 figures, proof corrections include
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