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

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

Statistics of Velocity from Spectral Data: Modified Velocity Centroids

We address the problem of studying interstellar turbulence using spectral line data. We find a criterion when the velocity centroids may provide trustworthy velocity statistics. To enhance the scope of centroids applications, we construct a measure that we term ``modified velocity centroids'' (MVCs) and derive an analytical solution that relates the 2D spectra of the modified centroids with the underlying 3D velocity spectrum. We test our results using synthetic maps constructed with data obtained through simulations of compressible magnetohydrodynamical (MHD) turbulence. We show that the modified velocity centroids (MVCs) are complementary to the the Velocity Channel Analysis (VCA) technique. Employed together, they make determining of the velocity spectral index more reliable and for wider variety of astrophysical situations.Comment: 4 pages, 1 figure, Accepted for publication in ApJ Letters. minor change

Velocity Spectrum for HI at High Latitudes

In this paper we present the results of the statistical analysis of high-latitude HI turbulence in the Milky Way. We have observed HI in the 21 cm line, obtained with the Arecibo L-Band Feed Array (ALFA) receiver at the Arecibo radio telescope. For recovering of velocity statistics we have used the Velocity Coordinate Spectrum (VCS) technique. In our analysis we have used direct fitting of the VCS model, as its asymptotic regimes are questionable for Arecibo's resolution and given the restrictions from thermal smoothing of the turbulent line. We have obtained a velocity spectral index $3.87 \pm 0.11$, an injection scale of $140 \pm 80$ pc, and an HI cold phase temperature of $52 \pm 11$ K. The spectral index is steeper than the Kolmogorov index and can be interpreted as being due to shock-dominated turbulence.Comment: Accepted to Ap

Velocity Modification of Power Spectrum from Absorbing Medium

Quantitative description of the statistics of intensity fluctuations within spectral line data cubes introduced in our earlier work is extended to the absorbing media. A possibility of extracting 3D velocity and density statistics from both integrated line intensity as well as from the individual channel maps is analyzed. We find that absorption enables the velocity effects to be seen even if the spectral line is integrated over frequencies. This regime that is frequently employed in observations is characterized by a non-trivial relation between the spectral index of velocities and the spectral index of intensity fluctuations. For instance when density is dominated by fluctuations at large scales, i.e. when correlations scale as r^{-\gamma}, \gamma<0, the intensity fluctuations exhibit a universal spectrum of fluctuations ~K^{-3} over a range of scales. When small scale fluctuations of density contain most of the energy, i.e. when correlations scale as r^{-\gamma}, \gamma>0, the resulting spectrum of the integrated lines depends on the scaling of the underlying density and scales as K^{-3+\gamma}. We show that if we take the spectral line slices that are sufficiently thin we recover our earlier results for thin slice data without absorption. As the result we extend the Velocity Channel Analysis (VCA) technique to optically thick lines enabling studies of turbulence in molecular clouds. In addition, the developed mathematical machinery enables a quantitative approach to solving other problems that involved statistical description of turbulence within emitting and absorbing gas.Comment: 51 page, 3 figures. Accepted to Astrophysical Journa

MHD Turbulence as a Foreground for CMB Studies

Measurements of intensity and polarization of diffuse Galactic synchrotron emission as well as starlight polarization reveal power law spectra of fluctuations. We show that these fluctuations can arise from magnetohydrodynamic (MHD) turbulence in the Galactic disk and halo. To do so we take into account the converging geometry of lines of sight for the observations when the observer is within the turbulent volume. Assuming that the intensity of turbulence changes along the line of sight, we get a reasonable fit to the observed synchrotron data. As for the spectra of polarized starlight we get a good fit to the observations taking into account the fact that the observational sample is biased toward nearby stars.Comment: 10 pages, 6 figures, Astrophyscal J., submitte