Nonresonant generation of MHD disturbances during supernova explosions in an inhomogeneous interstellar medium

We analyze the nonresonant generation of large-scale magnetic inhomogeneities near a shock front by accelerated particles. The MHD disturbances are generated by the electric current excited by relativistic particles in the preshock medium in the presence of weak large-scale density inhomogeneities. The MHD modes considered can be amplified by other resonant and nonresonant mechanisms related to the presence of relativistic particles. We estimate the magnetic fields and the energies to which charged particles can be accelerated in different phases of the interstellar medium by taking into account the random magnetic fields generated by the mechanism considered.     

Long-wavelength MHD instability in the prefront of collisionless shocks with accelerated particles

Collisionless shocks in turbulent space plasmas accelerate particles by the Fermi mechanism to ultrarelativistic energies. The interaction of accelerated particles with the plasma inflow produces extended supersonic MHD flows of multicomponent plasma. We investigate the instabilities of a flow of three-component turbulent plasma with relativistic particles against long-wavelength perturbations with scales larger than the accelerated particle transport mean free path and the initial turbulence scales. The presence of turbulence allows us to formulate the system of single-fluid equations, the equation of motion for the medium as a whole, and the induction equation for the magnetic field with turbulent magnetic and kinematic viscosities. The current of accelerated particles enters into the induction equation with an effective magnetic diffusion coefficient. We have calculated the local growth rates of the perturbations related to the nonresonant long-wavelength instability of the current of accelerated particles for MHD perturbations in the WKB approximation. The amplification of long-wavelength magnetic field perturbations in the flow upstream of the shock front can affect significantly the maximum energies of the particles accelerated by a collisionless shock and can lead to the observed peculiarities of the synchrotron X-ray radiation in supernova remnants.     

Angular spectra of polarized Galactic foregrounds

It is believed that magnetic field lines are twisted and bend by turbulent motions in the Galaxy. Therefore, both Galactic synchrotron emission and thermal emission from dust reflects statistics of Galactic turbulence. Our simple model of Galactic turbulence, motivated by results of our simulations, predicts that Galactic disk and halo exhibit different angular power spectra. We show that observed angular spectra of synchrotron emission are compatible with our model. We also show that our model is compatible with the angular spectra of star-light polarization for the Galactic disk. Finally, we discuss how one can estimate polarized microwave emission from dust in the Galactic halo using star-light polarimetry.     

Milestones in the Observations of Cosmic Magnetic Fields

Magnetic fields are observed everywhere in the universe. In this review, we concentrate on the observational aspects of the magnetic fields of Galactic and extragalactic objects. Readers can follow the milestones in the observations of cosmic magnetic fields obtained from the most important tracers of magnetic fields, namely, the star-light polarization, the Zeeman effect, the rotation measures (RMs, hereafter) of extragalactic radio sources, the pulsar RMs, radio polarization observations, as well as the newly implemented sub-mm and mm polarization capabilities. The magnetic field of the Galaxy was first discovered in 1949 by optical polarization observations. The local magnetic fields within one or two kpc have been well delineated by starlight polarization data. The polarization observations of diffuse Galactic radio background emission in 1962 confirmed unequivocally the existence of a Galactic magnetic field. The bulk of the present information about the magnetic fields in the Galaxy comes from anal     

The Converter Mechanism of Particle Acceleration and Its Applications to the Unidentified Egret Sources

We discuss the properties of gamma-ray radiation accompanying the acceleration of cosmic rays via the converter mechanism. The mechanism exploits multiple photon-induced conversions of high-energy particles from charged into neutral state (namely, protons to neutrons and electrons to photons) and back. Because a particle in the neutral state can freely cross the magnetic field lines, this allows to avoid both particle losses downstream and reduction in the energy gain factor, which normally takes place due to highly collimated distribution of accelerated particles. The converter mechanism efficiently operates in relativistic outflows under the conditions typical for Active Galactic Nuclei, Gamma-Ray Bursts, and microquasars, where it outperforms the standard diffusive shock acceleration. The accompanying radiation has a number of distinctive features, such as an increase of the maximum energy of synchrotron photons and peculiar radiation beam-pattern, whose opening angle is much wider at larger photon energies. This provides an opportunity to observe off-axis relativistic jets in GeV–TeV energy range. One of the implications is the possibility to explain high-latitude unidentified EGRET sources as off-axis but otherwise typical relativistic-jet sources, such as blazars.     

Double-layer disk and magnetic field dynamics in the Crab Nebula

The mechanism of the large-scale magnetic field generation in the Crab Nebula is proposed. The basis for the considered fast mechanism is the model of the central region of Crab Nebula amorphous part having the form of slightly divergent double-layer disk consisting of the relativistic electron-positron plasma.The nebula toroidal magnetic field generation occurs in the double-layer disk in the immediate neighbourhood of the light cylinder of pulsar PSR 0531+21 due to the differential rotation by means of dynamo-mechanism. The generated field is transferred into the nebula by the pulsar wind which forms the double-layer disk.By use of the known parameters of pulsar PSR 0531+21, the considered mechanism yields the strength of magnetic fieldB=10^–3 G observed in the nebula. The disk structure must be destroyed toward the edges of the nebula.     

Ejector and bipolar outflow of the radio galaxy M87

The superfine structure of the jet formation region in the radio galaxy M87 has been investigated. An accretion disk and high- and low-velocity jet and counterjet components have been identified. The high-velocity bipolar outflow is ejected from the central disk region, a nozzle 4 mpc in diameter, while the low-velocity one is ejected from a ring 60 mpc in diameter and 14 mpc in width. The low-velocity plasma flow is a hollow tube with a built-in helix. The observed helical structure of the high-velocity jet is determined by precession. The components of the structure, its disk and bipolar outflow, suggest solid-body rotation. Ring currents and aligned magnetic fields are generated in them under the action of an external magnetic field. The bipolar outflows are ejected coaxially but in opposite directions—along and opposite to the disk field. As a result, the jet flow accelerates, while the counterjet one decelerates. This causes the extent of the region of radiative cooling of the ejected relativistic electrons in the counterjet to decrease and maintains their “afterglow” at large distances in the jet. The high collimation of the rotating flows is determined by their interaction with the environment.     

Dynamo saturation in direct simulations of the multi‐phase turbulent interstellar medium

The ordered magnetic field observed via polarised synchrotron emission in nearby disc galaxies can be explained by a mean‐field dynamo operating in the diffuse interstellar medium (ISM). Additionally, vertical‐flux initial conditions are potentially able to influence this dynamo via the occurrence of the magnetorotational instability (MRI). We aim to study the influence of various initial field configurations on the saturated state of the mean‐field dynamo. This is motivated by the observation that different saturation behaviour was previously obtained for different supernova rates. We perform direct numerical simulations (DNS) of three‐dimensional local boxes of the vertically stratified, turbulent interstellar medium, employing shearing‐periodic boundary conditions horizontally. Unlike in our previous work, we also impose a vertical seed magnetic field. We run the simulations until the growth of the magnetic energy becomes negligible. We furthermore perform simulations of equivalent 1D dynamo models, with an algebraic quenching mechanism for the dynamo coefficients. We compare the saturation of the magnetic field in the DNS with the algebraic quenching of a mean‐field dynamo. The final magnetic field strength found in the direct simulation is in excellent agreement with a quenched α) dynamo. For supernova rates representative of the Milky Way, field losses via a Galactic wind are likely responsible for saturation. We conclude that the relative strength of the turbulent and regular magnetic fields in spiral galaxies may depend on the galaxy's star formation rate. We propose that a mean field approach with algebraic quenching may serve as a simple sub‐grid scale model for galaxy evolution simulations including a prescribed feedback from magnetic fields. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Acceleration of charged particles in the magnetosphere of a collapsing star and their nonthermal electromagnetic radiation

We investigate a transformation of a magnetic field and plasma in nonhomogeneous magnetospheres of collapsing stars with a dipole initial magnetic field and certain initial energy distributions of particles in the magnetosphere as the power low, relativistic Maxwell and Boltzmann. The betatron mechanism of the charged particles acceleration in a collapsing star’s magnetosphere is considered. When a magnetized star is compressed in the stage of the gravitational collapse, the magnetic field increases strongly. This variable magnetic field generates a vortical electric field. Our calculations show that this electric field will accelerate charged particles up to relativistic velocities. Thus, collapsing stars may be sources of high energy cosmic rays in our galaxy as in others. The acceleration of particles during the collapse happens mostly in polar regions of the magnetosphere that leads to polar relativistic streams (jets) formation. When moving in a magnetic field, these particles will generate nonthermal electromagnetic radiation in a broad electromagnetic wavelength band from radioto gamma rays. Thus, in the stage of the gravitational collapse, relativistic jets are formed in stellar magnetospheres. These jets are powerful sources of the nonthermal electromagnetic radiation.     

On the possibility of observing the Shapiro effect for pulsars in globular clusters

For pulsars in globular clusters, we suggest using observations of the relativistic time delay of their radiation in the gravitational field of a massive body (the Shapiro effect) located close to the line of sight to detect and identify invisible compact objects and to study the distribution of both visible and dark matter in globular clusters and different components of the Galaxy. We have derived the dependences of the event probability on the Galactic latitude and longitude of sources for two models of the mass distribution in the Galaxy: the “classical” Bahcall—Soneira model and the more recent Dehnen—Binney model. Using three globular clusters (M15, 47 Tuc, Terzan 5) as an example, we show that the ratios of the probability of the events due to the passages of massive Galactic objects close to the line of sight to the parameter f ² for pulsars in the globular clusters 47 Tuc and M 15 are comparable to those for close passages of massive objects in the clusters themselves and are considerably higher than those for the cluster Terzan 5. We have estimated the rates of such events. We have determined the number of objects near the line of sight toward the pulsar that can produce a modulation of its pulse arrival times characteristic of the effect under consideration; the population of brown dwarfs in the Galactic disk, whose concentration is comparable to that of the disk stars, has been taken into account for the first time.     

On the origin of highest energy cosmic rays

In this paper we show that the conventional diffusive shock acceleration mechanism for cosmic rays associated with relativistic astrophysical shocks in active galactic nuclei (AGNs) has severe difficulties to explain the highest energy cosmic ray events. We show that protons above around 2 x 10²⁰ eV could have marginally been produced by this mechanism in an AGN or a rich galaxy cluster not further away than around 100 Mpc. However, for the highest energy Fly's Eye and Yakutsk events this is inconsistent with the observed arrival directions. Galactic and intergalactic magnetic fields appear unable to alter the direction of such energetic particles by more than a few degrees. We also discuss some other options for these events associated with relativistic particles including pulsar acceleration of high Z nuclei. At the present stage of knowledge the concept of topological defects left over from the early universe as the source for such events appears to be a promising option. Such sources are discussed and possible tests of this hypothesis are proposed.     

Linear and Circular Polarization Properties of Jets

I discuss the transfer of polarized synchrotron radiation in relativistic jets. I argue that the main mechanism responsible for the circular polarization properties of compact synchrotron sources is likely to be Faraday conversion and that, contrary to common expectation, a significant rate of Faraday rotation does not necessarily imply strong depolarization. The long-term persistence of the sign of circular polarization, observed in some sources, is most likely due to a small net magnetic flux generated in the central engine, carried along the jet axis and superimposed on a highly turbulent magnetic field. I show that the mean levels of circular and linear polarizations depend on the number of field reversals along the line of sight and that the gradient in Faraday rotation across turbulent regions can lead to`correlation depolarization'. The model is potentially applicable to a wide range of synchrotron sources. In particular, I demonstrate how the model can naturally explain the excess of circular over linear polarization in the Galactic Center (SgrA^*) and the low-luminosity AGN M81^*.     

Hydrostatic equilibrium in a magnetized, warped Galactic disc

Hydrostatic equilibrium of the multiphase interstellar medium in the solar vicinity is reconsidered, with the regular and turbulent magnetic fields treated separately. The regular magnetic field strength required to support the gas is consistent with independent estimates, provided that energy equipartition is maintained between turbulence and random magnetic fields. Our results indicate that a mid-plane value of B ₀=4 μG for the regular magnetic field near the Sun leads to more attractive models than B ₀=2 μG . The vertical profiles of both the regular and random magnetic fields contain disc and halo components, the parameters of which we have determined. The layer at 1≲| z |≲4 kpc can be overpressured and an outflow at a speed of about 50 km s⁻¹ may occur there, presumably associated with a Galactic fountain flow, if B ₀≃2 μG .
We show that hydrostatic equilibrium in a warped disc must produce asymmetric density distributions in z , in rough agreement with H  i observations in the outer Galaxy. This asymmetry may be a useful diagnostic of the details of the warping mechanism in the Milky Way and other galaxies. We find indications that gas and magnetic field pressures are different above and below the warped midplane in the outer Galaxy, and quantify the difference in terms of turbulent velocity and/or magnetic field strength.     

Synchro－Curvature Radiation──弯曲磁场中一种新的、更普遍的辐射

Ｓｙｎｃｈｒｏ－ＣｕｒｖａｔｕｒｅＲａｄｉａｔｉｏｎ──弯曲磁场中一种新的、更普遍的辐射张家铝（中国科技大学天体物理中心合肥２３００２６）郑广生（香港大学物理系）关键词同步辐射，曲率辐射，脉冲星磁层在脉冲星的辐射模型中，同步辐射和曲率辐射占有极其重要...     

Evolution of the Radial Abundance Gradient and Cold Gas of the Galactic Disk

In order to understand the forming mechanism of the radial abun- dance gradient of the Galactic disk and the evolution of cold gas, we have con- structed a chemical evolution model of the Galactic disk, in which the star for- mation law concerned with molecular hydrogens is adopted, and the evolution of mass surface density is calculated for the molecular and atomic hydrogens separately, then the model predictions and the observed radial distributions of some physical quantities are compared. The result indicates that the model prediction is sensitive to the adopted infall timescale, the model which adopts the star formation law concerned with the molecular hydrogens can agree well with the major observed properties of the Galactic disk, especially can obtain naturally the radial oxygen abundance gradient of the Galactic disk, and the radial surface density profile of cold gas. The assumption of instantaneous or non-instantaneous recycling approximation has a small effect on the evolution of cold gas, especially in the case of rather low gas density.     

Parker instability in a self-gravitating magnetized gas disk: II. Competition between gravitational and convective instabilities

Under influence of external gravity generated by Galactic all components excluding ISM, a magnetized gas disk may experience both Parker and convective instabilities. Growth rate of the convective instability increases with decreasing perturbation wavelength, and the convective motion makes sheet-like structures all over before the Parker instability forms structures of any meaningful size in the disk. Yet the Parker instability is thought to be an ideal route to form large-scale condensations in the Galaxy. In search of a means to curb convective activities in the Galactic ISM disk, the external gravity is replaced by self-gravity as a driving force of the Parker instability and the gravitational instability is invoked to reinforce the Parker instability. Perturbation of interchange mode is known to trigger convective instability in such disk and the one of undular mode to activate the Parker instability, while the gravitational instability can be triggered by both modes. Therefore, the resulting Jeans instability would help the Parker instability to overcome disrupting behavior of the convection. Dynamical properties of the disk can be characterized by ratio α of magnetic to gas pressure, adiabatic exponent γ, scale height H of the ISM, and disk thickness z_a. A linear stability analysis has been done to the disk, and the maximum growth rate of the Parker–Jeans instability is compared with that of the convective instability. The latter may or may not be higher than the former, depending on the disk parameters. The Parker–Jeans instability has chances to override the convective instability, when the disk is thicker than a certain value. In the disk thinner than the critical one, the Jeans instability can always suppress the convection. Since the growth rate of the convective instability is proportional to local gravitational acceleration, thereby in the general Galactic gravity, the convective instability works actively only in upper regions, we expect chaotic features to appear in regions of low density far from Galactic mid-plane.     

The connection between the mean and local coefficients of diffusion for cosmic rays

The diffusion of charged particles in an external large-scale arbitrary magnetic field is considered. The diffusion coefficient averaged over a large volume of space which results from the local diffusion tensor is calculated.     

Polarization of radiation from a strongly magnetized accretion disk: The asymptotic spectral distribution

We calculate the polarization of the radiation from an optically thick accretion disk with a vertical averaged magnetic field. The polarization arises from the scattering of light by free electrons in a magnetized disk plasma. The Faraday rotation of the polarization plane during the propagation of a photon in a medium with a magnetic field is considered as the main effect. We discuss various models of optically thick accretion disks with a vertical averaged magnetic field. Our main goal is to derive simple asymptotic formulas for the polarization of radiation in the case where the Faraday rotation angle Ψ ≫ 1 at the Thomson optical depth τ = 1. The results of our calculations allow the magnetic field strength in the region of the marginally stable orbit near a black hole to be estimated from polarimetric observations, including X-ray observations expected in the future. Since the polarization spectrum of the radiation strongly depends on the accretion disk model, a realistic physical model of the accretion disk can be determined from data on the polarization of its radiation.     

脉冲星的星系磁场

脉冲星的偏振信息是理解脉冲星辐射区的重要手段，利用澳大利亚的64m射电望远镜进行大量的脉冲星观测，得到了一批脉冲星的偏振轮廓和偏振参数，编辑了几乎所有发表的脉冲星轮廓资料，系统总结了脉冲星圆偏辐射的规律，为理论上解释脉冲星辐射这一重要难题提供观测依据和物理限制，利用脉冲星作为探针，研究了银河系磁场结构和模型，确定了银河系BS磁场模型，发现了银河系上下反对称的环向磁场，并首次对星系尺度的发电机类型进行判别，证认出A0型发电机运行于银河系，发现了银晕中的垂直磁场和M31及银盘中的非常延展的磁场，探测到NGC2997星系中由内到外的旋涡磁场，并提出可能有两种发电机在这个星系的不同区域运行。     

Linkage between accretion disks and blazars

The magnetic field in an accretion disk is estimated assuming that all of the angular momentum within prescribed accretion disk radii is removed by a jet. The magnetic field estimated at the base of the jet is extrapolated to the blazar emission region using a model for a relativistic axisymmetric jet combined with some simplifying assumptions based on the relativistic nature of the flow. The extrapolated magnetic field is compared with estimates based upon the synchrotron and inverse Compton emission from three blazars, MKN 501, MKN 421 and PKS 2155-304. The magnetic fields evaluated from pure synchrotron self-Compton models are inconsistent with the magnetic fields extrapolated in this way. However, in two cases inverse Compton models in which a substantial part of the soft photon field is generated locally agree well, mainly because these models imply magnetic field strengths consistent with an important Poynting Flux component. This comparison is based on estimating the mass accretion rate from the jet energy flux. Further comparisons along these lines will be facilitated by independent estimates of the mass accretion rate in blazars and by more detailed models for jet propagation near the black hole.