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 $\mathcal{M} < 2$ 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 $p$ be any prime, and let ${\mathcal B}(p)$ be the algebra of operations on the cohomology ring of any cocommutative $\mathbb{F}_p$-Hopf algebra. In this paper we show that when $p$ is odd (and unlike the $p=2$ case), ${\mathcal B}(p)$ cannot become an object in the Singer category of $\mathbb{F}_p$-algebras with coproducts, if we require that coproducts act on the generators of ${\mathcal B}(p)$ 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