arXiv subjects

A. Lazarian

Publications and source records attributed to A. Lazarian.

At least 163 records · Page 9Linked to original sources

arXiv2010

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.

astro-ph.CO↗

arXiv2010

Acceleration of small astrophysical grains due to charge fluctuations

We discuss a novel mechanism of dust acceleration which may dominate for particles smaller than $\sim0.1~μ$m. The acceleration is caused by their direct electrostatic interactions arising from fluctuations of grain charges. The energy source for the acceleration are the irreversible plasma processes occurring on the grain surfaces. We show that this mechanism of charge-fluctuation-induced acceleration likely affects the rate of grain coagulation and shattering of the population of small grains.

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arXiv2010

Magnetic reconnection as the cause of cosmic ray excess from the heliospheric tail

The observation of a broad excess of sub-TeV cosmic rays compatible with the direction of the heliospheric tail (Nagashima et al. 1998) and the discovery of two significant localized excess regions of multi-TeV cosmic rays by the MILAGRO collaboration (Abdo et al. 2008), also from the same region of the sky, have raised questions on their origin. In particular, the coincidence of the most significant localized region with the direction of the heliospheric tail and the small angular scale of the observed anisotropy (~ 10deg) is suggestive a local origin and of a possible connection to the low energy broad excess. Cosmic ray acceleration from magnetic reconnection in the magnetotail is proposed as a possible source of the energetic particles.

astro-ph.CO↗

arXiv2010

Acceleration of primary and secondary particles in galaxy clusters by compressible MHD turbulence: from radio halos to gamma rays

Radio observations discovered large scale non thermal sources in the central Mpc regions of dynamically disturbed galaxy clusters (radio halos). 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 inter-galactic-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 clysters. Following Brunetti and Lazarian (2007), in this paper we consider the advances in the theory of 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 reacceleration 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 halos 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 halos.

astro-ph.CO↗

arXiv2010

Galactic foregrounds: Spatial fluctuations and a procedure of removal

Present day cosmic microwave background (CMB) studies require more accurate removal of Galactic foreground emission. In this paper, we consider a way of filtering out the diffuse Galactic fluctuations on the basis of their statistical properties, namely, the power-law spectra of fluctuations. We focus on the statistical properties of two major Galactic foregrounds that arise from magnetized turbulence, namely, diffuse synchrotron emission and thermal emission from dust and describe how their power laws change with the Galactic latitude. We attribute this change to the change of the geometry of the emission region and claim that the universality of the turbulence spectrum provides a new way of removing Galactic foregrounds. We discuss and demonstrate how we can make use of our findings to remove Galactic foregrounds using a template of spatial fluctuations. In particular, we consider examples of spatial filtering of a foreground at small scales, when the separation into CMB signal and foregrounds is done at larger scales. We demonstrate that the new technique of spatial filtering of foregrounds may be promising for recovering the CMB signal in a situation when foregrounds are known at a scale different from the one under study. It can also improve filtering by combining measurements obtained at different scales.

astro-ph.CO↗

arXiv2010

Improving the model of emission from spinning dust: effects of grain wobbling and transient spin-up

Observations continue to support the interpretation of the anomalous microwave foreground as electric dipole radiation from spinning dust grains as proposed by Draine and Lazarian (1998ab). In this paper we present a refinement of the original model by improving the treatment of a number of physical effects. First, we consider a disk-like grain rotating with angular velocity at an arbitrary angle with respect to the grain symmetry axis and derive the rotational damping and excitation coefficients arising from infrared emission, plasma-grain interactions and electric dipole emission. The angular velocity distribution and the electric dipole emission spectrum for grains is calculated using the Langevin equation, for cases both with and without fast internal relaxation. Our results show that, the peak emissivity of spinning dust, compared to earlier studies, increases by a factor of ~2 for the Warm Neutral Medium (WNM), the Warm Ionized Medium (WIM), the Cold Neutral Medium (CNM) and the Photodissociation Region (PDR), and by a factor ~4 for Reflection Nebulae (RN). The frequency at the emission peak also increases by factors ~1.4 to ~2 for these media. The increased emission and peak frequency result from the non-sphericity of grain shape and from the anisotropy in damping and excitation along directions parallel and perpendicular to the grain symmetry axis. Second, we provide a detailed numerical study including transient spin-up of grains by single-ion collisions. The impulses broaden the emission spectrum and increase the peak emissivity for the CNM, WNM and WIM. In addition, we present an improved treatment of rotational excitation and damping by infrared emission.

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arXiv2010

Scaling laws and diffuse locality of balanced and imbalanced MHD turbulence

The search of ways to generalize the theory of strong MHD turbulence for the case of non-zero cross-helicity (or energy imbalance) has attracted considerable interest recently. In our earlier publications we performed three-dimensional numerical simulations and showed that some of existing models are inconsistent with numerics. In this paper we focused our attention on low-imbalance limit and performed new high-resolution simulations. The results strongly suggest that in the limit of small imbalances we smoothly transition to a standard Goldreich-Sridhar (1995) balanced model. We also claim that Perez-Boldyrev (2009) model that predicts the same nonlinear timescale for both components due to so-called "dynamic alignment" strongly contradicts numerical evidence.

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arXiv2010

Diffusion of magnetic field and removal of magnetic flux from clouds via turbulent reconnection

The diffusion of astrophysical magnetic fields in conducting fluids in the presence of turbulence depends on whether magnetic fields can change their topology via reconnection in highly conducting media. Recent progress in understanding fast magnetic reconnection in the presence of turbulence is reassuring that the magnetic field behavior in computer simulations and turbulent astrophysical environments is similar, as far as magnetic reconnection is concerned. Our studies of magnetic field diffusion in turbulent medium reveal interesting new phenomena. In the presence of gravity and turbulence, our 3D simulations show the decrease of the magnetic flux-to-mass ratio as the gaseous density at the center of the gravitational potential increases. We observe this effect both in the situations when we start with equilibrium distributions of gas and magnetic field and when we follow the evolution of collapsing dynamically unstable configurations. Thus the process of turbulent magnetic field removal should be applicable both to quasi-static subcritical molecular clouds and cores and violently collapsing supercritical entities. The increase of the gravitational potential as well as the magnetization of the gas increases the segregation of the mass and magnetic flux in the saturated final state of the simulations, supporting the notion that the reconnection-enabled diffusivity relaxes the magnetic field + gas system in the gravitational field to its minimal energy state. This effect is expected to play an important role in star formation, from its initial stages of concentrating interstellar gas to the final stages of the accretion to the forming protostar.

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arXiv2010

Reconnection Diffusion and Star Formation

The diffusion of astrophysical magnetic fields in conducting fluids in the presence of turbulence depends on whether magnetic fields can change their topology or reconnect in highly conducting media. Recent progress in understanding fast magnetic reconnection in the presence of turbulence is reassuring that the magnetic field behavior in computer simulations and turbulent astrophysical environments is similar, as far as the magnetic reconnection is concerned. This makes it meaningful to perform MHD simulations of turbulent flows in order to understand the diffusion of magnetic field in astrophysical environments. These simulations support the concept of reconnection diffusion, which describes the ability of magnetic fields to get removed from magnetized clouds and cores in the process of star formation.

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arXiv2010

Turbulent magnetic reconnection in 2D and 3D

Magnetic field embedded in a perfectly conducting fluid preserves its topology for all time. Although ionized astrophysical objects, like stars and galactic disks, are almost perfectly conducting, they show indications of changes in topology, `magnetic reconnection', on dynamical time scales. Reconnection can be observed directly in the solar corona, but can also be inferred from the existence of large scale dynamo activity inside stellar interiors. Solar flares and gamma ray busts are usually associated with magnetic reconnection. Previous work has concentrated on showing how reconnection can be rapid in plasmas with very small collision rates. Here we present numerical evidence, based on three dimensional simulations, that reconnection in a turbulent fluid occurs at a speed comparable to the rms velocity of the turbulence, regardless of the value of the resistivity. In particular, this is true for turbulent pressures much weaker than the magnetic field pressure so that the magnetic field lines are only slightly bent by the turbulence. These results are consistent with the proposal by Lazarian and Vishniac (1999) that reconnection is controlled by the stochastic diffusion of magnetic field lines, which produces a broad outflow of plasma from the reconnection zone. This work implies that reconnection in a turbulent fluid typically takes place in approximately a single eddy turnover time, with broad implications for dynamo activity and particle acceleration throughout the universe. In contrast, the reconnection in 2D configurations in the presence of turbulence depends on resistivity, i.e. is slow.

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arXiv2010

Fast Magnetic Reconnection and Energetic Particle Acceleration

Our numerical simulations show that the reconnection of magnetic field becomes fast in the presence of weak turbulence in the way consistent with the Lazarian and Vishniac (1999) model of fast reconnection. We trace particles within our numerical simulations and show that the particles can be efficiently accelerated via the first order Fermi acceleration. We discuss the acceleration arising from reconnection as a possible origin of the anomalous cosmic rays measured by Voyagers.

astro-ph.GA↗

arXiv2010

Damping of MHD turbulence in partially ionized gas and the observed difference of velocities of neutrals and ions

Theoretical and observational studies on the turbulence of the interstellar medium developed fast in the past decades. The theory of supersonic magnetized turbulence, as well as the understanding of projection effects of observed quantities, are still in progress. In this work we explore the characterization of the turbulent cascade and its damping from observational spectral line profiles. We address the difference of ion and neutral velocities by clarifying the nature of the turbulence damping in the partially ionized. We provide theoretical arguments in favor of the explanation of the larger Doppler broadening of lines arising from neutral species compared to ions as arising from the turbulence damping of ions at larger scales. Also, we compute a number of MHD numerical simulations for different turbulent regimes and explicit turbulent damping, and compare both the 3-dimensional distributions of velocity and the synthetic line profile distributions. From the numerical simulations, we place constraints on the precision with which one can measure the 3D dispersion depending on the turbulence sonic Mach number. We show that no universal correspondence between the 3D velocity dispersions measured in the turbulent volume and minima of the 2D velocity dispersions available through observations exist. For instance, for subsonic turbulence the correspondence is poor at scales much smaller than the turbulence injection scale, while for supersonic turbulence the correspondence is poor for the scales comparable with the injection scale. We provide a physical explanation of the existence of such a 2D-3D correspondence and discuss the uncertainties in evaluating the damping scale of ions that can be obtained from observations. However, we show that the statistics of velocity dispersion from observed line profiles can provide the spectral index and the energy transfer rate of turbulence. Also, comparing two similar simulations with different viscous coefficients it was possible to constrain the turbulent cut-off scale. This may especially prove useful since it is believed that ambipolar diffusion may be one of the dominant dissipative mechanism in star-forming regions. In this case, the determination of the ambipolar diffusion scale may be used as a complementary method for the determination of magnetic field intensity in collapsing cores. We discuss the implications of our findings in terms of a new approach to magnetic field measurement proposed by Li & Houde (2008).

astro-ph.GA↗

arXiv2010

Magnetic Field Effects near the launching region of Astrophysical Jets

One of the fundamental properties of astrophysical magnetic fields is their ability to change topology through reconnection and in doing so, to release magnetic energy, sometimes violently. In this work, we review recent results on the role of magnetic reconnection and associated heating and particle acceleration in jet/accretion disk systems, namely young stellar objects (YSOs), microquasars, and active galactic nuclei (AGNs).

astro-ph.HE↗

arXiv2010

Numerical study of Cosmic Ray Diffusion in MHD turbulence

We study diffusion of Cosmic Rays (CRs) in turbulent magnetic fields using test particle simulations. Electromagnetic fields are produced in direct numerical MHD simulations of turbulence and used as an input for particle tracing, particle feedback on turbulence being ignored. Statistical transport coefficients from the test particle runs are compared with earlier analytical predictions. We find qualitative correspondence between them in various aspects of CR diffusion. In the incompressible case, that we consider in this paper, the dominant scattering mechanism occurs to be the non-resonant mirror interactions with the slow-mode perturbations. Perpendicular transport roughly agrees with being produced by magnetic field wandering.

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arXiv2010

Extending Big Power Law in the Sky with Turbulence Spectra from WHAM data

We use the data of Wisconsin H$α$ Mapper (WHAM) to test the hypothesis of whether the amplitudes and spectrum of density fluctuations measured by WHAM can be matched to the data obtained for interstellar scintillations and scattering. To do this, first of all, we adjusted the mean level of signal in the adjacent patches of the data. Then, assuming that the spectrum is Kolmogorov, we successfully matched the amplitudes of turbulence obtained from the WHAM data and the interstellar density fluctuations reported in the existing literature. As a result, we conclude that the existing data is consistent with the Kolmogorov cascade which spans from $10^6$ to $10^{17}$ $m$.

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arXiv2010

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.

astro-ph↗

arXiv2009

Tsallis statistics as a tool for studying interstellar turbulence

We used magnetohydrodynamic (MHD) simulations of interstellar turbulence to study the probability distribution functions (PDFs) of increments of density, velocity, and magnetic field. We found that the PDFs are well described by a Tsallis distribution, following the same general trends found in solar wind and Electron MHD studies. We found that the PDFs of density are very different in subsonic and supersonic turbulence. In order to extend this work to ISM observations we studied maps of column density obtained from 3D MHD simulations. From the column density maps we found the parameters that fit to Tsallis distributions and demonstrated that these parameters vary with the sonic and Alfvén Mach numbers of turbulence. This opens avenues for using Tsallis distributions to study the dynamical and perhaps magnetic states of interstellar gas.

astro-ph.GA↗

arXiv2009

Turbulence and the formation of filaments, loops and shock fronts in NGC 1275 in the Perseus Galaxy Cluster

NGC1275, the central galaxy in the Perseus cluster, is the host of gigantic hot bipolar bubbles inflated by AGN jets observed in the radio as Perseus A. It presents a spectacular $Hα$-emitting nebulosity surrounding NGC1275, with loops and filaments of gas extending to over 50 kpc. The origin of the filaments is still unknown, but probably correlates with the mechanism responsible for the giant buoyant bubbles. We present 2.5 and 3-dimensional MHD simulations of the central region of the cluster in which turbulent energy, possibly triggered by star formation and supernovae (SNe) explosions is introduced. The simulations reveal that the turbulence injected by massive stars could be responsible for the nearly isotropic distribution of filaments and loops that drag magnetic fields upward as indicated by recent observations. Weak shell-like shock fronts propagating into the ICM with velocities of 100-500 km/s are found, also resembling the observations. The isotropic outflow momentum of the turbulence slows the infall of the intracluster medium, thus limiting further starburst activity in NGC1275. As the turbulence is subsonic over most of the simulated volume, the turbulent kinetic energy is not efficiently converted into heat and additional heating is required to suppress the cooling flow at the core of the cluster. Simulations combining the MHD turbulence with the AGN outflow can reproduce the temperature radial profile observed around NGC1275. While the AGN mechanism is the main heating source, the supernovae are crucial to isotropize the energy distribution.

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