7,997 research outputs found
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
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
Shocks and cold fronts in merging and massive galaxy clusters: new detections with Chandra
A number of merging galaxy clusters shows the presence of shocks and cold
fronts, i.e. sharp discontinuities in surface brightness and temperature. The
observation of these features requires an X-ray telescope with high spatial
resolution like Chandra, and allows to study important aspects concerning the
physics of the intra-cluster medium (ICM), such as its thermal conduction and
viscosity, as well as to provide information on the physical conditions leading
to the acceleration of cosmic rays and magnetic field amplification in the
cluster environment. In this work we search for new discontinuities in 15
merging and massive clusters observed with Chandra by using different imaging
and spectral techniques of X-ray observations. Our analysis led to the
discovery of 22 edges: six shocks, eight cold fronts and eight with uncertain
origin. All the six shocks detected have derived from density
and temperature jumps. This work contributed to increase the number of
discontinuities detected in clusters and shows the potential of combining
diverse approaches aimed to identify edges in the ICM. A radio follow-up of the
shocks discovered in this paper will be useful to study the connection between
weak shocks and radio relics.Comment: Matched to the MNRAS published version, minor grammar and typo fixe
Particle reacceleration by compressible turbulence in galaxy clusters: effects of reduced mean free path
Direct evidence for in situ particle acceleration mechanisms in the
inter-galactic-medium (IGM) is provided by the diffuse Mpc--scale synchrotron
emissions observed from galaxy clusters. It has been proposed that MHD
turbulence, generated during cluster-cluster mergers, may be a source of
particle reacceleration in the IGM. Calculations of turbulent acceleration must
account self-consistently for the complex non--linear coupling between
turbulent waves and particles. This has been calculated in some detail under
the assumption that turbulence interacts in a collisionless way with the IGM.
In this paper we explore a different picture of acceleration by compressible
turbulence in galaxy clusters, where the interaction between turbulence and the
IGM is mediated by plasma instabilities and maintained collisional at scales
much smaller than the Coulomb mean free path. In this regime most of the energy
of fast modes is channeled into the reacceleration of relativistic particles
and the acceleration process approaches a universal behaviour being
self-regulated by the back-reaction of the accelerated particles on turbulence
itself. Assuming that relativistic protons contribute to several percent (or
less) of the cluster energy, consistent with the FERMI observations of nearby
clusters, we find that compressible turbulence at the level of a few percent of
the thermal energy can reaccelerate relativistic electrons at GeV energies,
that are necessary to explain the observed diffuse radio emission in the form
of giant radio halos.Comment: 8 pages, 3 figures. Accepted in MNRAS (October 28, 2010
A Note on the Algebra of Operations for Hopf Cohomology at Odd Primes
Let be any prime, and let be the algebra of operations
on the cohomology ring of any cocommutative -Hopf algebra. In
this paper we show that when is odd (and unlike the case), cannot become an object in the Singer category of
-algebras with coproducts, if we require that coproducts act on
the generators of coherently with their nature of cohomology
operation
The contribution of AGN to the X-ray background: the effect of iron features
The contribution of the iron emission line, commonly detected in the X-ray
spectra of Seyfert (Sey) galaxies, to the cosmic X-ray background (XRB)
spectrum is evaluated in the framework of the XRB synthesis models based on AGN
unification schemes. To derive the mean line properties, we have carried out a
search in the literature covering a sample of about 70 AGN. When adopting line
parameters in agreement with the observations, it turns out that the maximum
contribution of the iron line to the XRB is less than 7% at a few keV. This is
still below the present uncertainties in the XRB spectrum measurements.Comment: 21 LaTeX pages with 5 Postscript figures. Accepted for publication in
New Astronom
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