Polarization effects in the radiation of magnetized envelopes and extended accretion structures

We have calculated the degree and position angle of the polarization of radiation scattered in a magnetized, optically thin or optically thick envelope around a central source, taking into account Faraday rotation of the plane of polarization during the propagation of the scattered radiation and the finite size of the radiation source. The wavelength dependence of the degree of polarization can be used to estimate the magnetic field of the source (a star, the region around a neutron star, or a black hole), and we have used our calculations to estimate the magnetic fields in a number of individual objects: several hot O and Wolf-Rayet stars, compact objects in X-ray close binaries with black holes (SS 433, Cyg X-1), and supernovae. The spectrum of the linear polarization can be used to determine the magnetic field in the vicinity of a central supermassive black hole, where the polarized optical radiation is generated. In a real physical model, this value can be extrapolated to the region of the last stable orbit. In the future, the proposed technique will make it possible to directly estimate the magnetic field in the region of the last stable orbit of a supermassive black hole using X-ray polarimetry.     

The formation of polarized lines: Allowance for the Hanle effect

The paper formulates the standard theory for the transport of polarized radiation in the presence of resonance scattering in an atmosphere with a weak magnetic field, so that the Zeeman splitting is small compared to the Doppler line width. For an atmosphere with conservative scattering, this reduces to the Milne problem, which consists of computing a polarized radiation field in a medium with sources lying in infinitely deep layers. In the approximation of complete frequency redistribution, the problem reduces to solving a Wiener-Hopf integral equation for a (6×6)-matrix source function. Asymptotic and numerical solutions for the standard problem are obtained, including solutions for the Milne problem, for the case of a Doppler absorption profile. The line polarization profiles for the emergent flux at various angular distances from the disk center are derived, and the dependence of the limiting degree of polarization (at the line center at the disk edge) on the direction of the magnetic field is computed. For nearly conservative scattering with photon destruction probability ε?1, the limiting degree of polarization varies with ε in accordance with a simple empirical law similar to that found earlier for a medium with zero magnetic field.     

Polarization in resonance lines: Diffuse reflection

The linear polarization of resonance radiation diffusely reflected from a semi-infinite atmosphere is analyzed, including a full account of the azimuthal dependence. The radiating atom is assumed to have two levels and the radiation to be fully redistributed in frequency. A new approach to the transformation of the equations of polarized radiative transport is proposed based on factorization of the full (3×3) Rayleigh phase matrix. The emergent radiation field is expressed in terms of a matrix analog for the Chandrasekhar H function. The polarization properties of radiation at the frequencies of resonance lines diffusely reflected from a scattering atmosphere are discussed. The degree of polarization of the reflected radiation can be very high.     

Broad-band multicolor photometry and polarimetry of spotted stars

We have confirmed the BY Dra-type variability of the active spotted stars MS Ser, LQ Hya, VY Ari, and EK Dra using simultaneous UBVRI photometric and polarimetric observations. We have also reliably detected the intrinsic linear polarization of their radiation and its rotational modulation in U due to the inhomogeneous distribution of active magnetized regions over the surfaces of the stars. Modeling of the linear polarization based on the Zeeman effect indicates that all the stars display strong local magnetic fields (about 2 kG, similar to those in sunspots), with filling factors of up to 40% of the total stellar surface. The magnetized regions coincide with cool photospheric spots detected in photoelectric observations.     

Linear polarization of the radiation from active galactic nuclei and the redshift dependence of their main parameters

We consider the observed continuum linear polarization of extragalactic objects with various redshifts z, most of which have degrees of polarization p ≤ 10%. We propose that this polarization is due to multiple scattering of the radiation in magnetized accretion disks around the Active Galactic Nuclei (AGN; the Milne problem in an optically thick atmosphere). The structure of the accretion disks and the polarization of the emergent radiation depend on the main parameters of the AGN—the mass of the central body M _BH , accretion rate $ \dot M $ \dot M , magnetic field at the black-hole event horizon B _H , angular momentum a _*, and the explicit form of the magnetic-field distribution in the accretion disk. Theoretical expressions for the degree of polarization are averaged over all angles of the disks to the line of sight, and the resulting formula compared with the mean observed polarizations in redshift intervals Δz = 0.25. The dependence of the observed degree of polarization and the main parameters on the redshift z is derived. The degrees of polarization of 305 objects from the catalog of Hutsemekers et al. with redshifts from zero to z = 2.25 are used for the analysis.     

Curvature radiation for intermediate pitch angles

The radiation by a charged particle in a non-uniform magnetic field is analyzed when the particle’s trajectory forms a curved spiral. The case when the pitch angle is less than the inverse of the Lorentz factor is considered. It is shown that the spectral angular distribution of the radiation consists of two parts that are substantially separated in frequency. One of these components represents curvature radiation, which is primarily linearly polarized, while the other possesses the typical properties of undulatory radiation, with a high degree of circular polarization. The derived formulas can be used to analyze the radiation of pulsars.     

基于屏方法的弹性波叠前深度偏移

弹性波成像是多分量地震勘探需要解决的关键技术之一。借鉴声波方程中傅立叶有限差分算子中的有限差分补偿方法,对弹性波相屏传播算子进行了大角度校正。在求取成像值时,采用了矢量成像条件,即通过极化矢量,将各分量投影极化方向,然后再成像,从而使波场中的信息得到充分利用,同时解决了P-SV波成像过程中的极性不一致问题。这样,便可以叠加多炮的成像结果,从而提高成像质量。利用模型数据,对大角度相位校正后的弹性屏延拓算子和矢量成像条件进行了试验,成像效果理想。      

The polarization of the solar corona on March 29, 2006

The effect of the auroral ring on the polarization of the solar corona during the solar eclipse of March 29, 2006 is studied. The angle and degree of polarization for emission arising from the combination of two partially polarized components is calculated. The emission of each component is described in terms of the Stokes parameters, and the corresponding parameters added. The position angle and degree of polarization are found for all cases realized in observations of the polarized corona with polarizing filter positions of 0°, 60°, 120° and 0°, 45°, and 90°. These calculations indicate that singular polarization points (saddles with index ?1/2) arise at a distance of about 1R_⊙ from the limb. A model for the total emission of the corona is constructed, which is used to calculate deviations of the polarization plane from the tangential direction (with respect to the limb).     

Detection of prolonged, extremely faint decimeter bursts on the sun

We report the detection of long-lived sources of radio bursts accompanied by polarized background emission in solar active regions. Both types of radio sources were detected at several decimeter wavelengths in observations on the RATAN-600 radio telescope in one-dimensional scans in intensity and circular polarization with a sensitivity of about 5–10 Jy. The degree of polarization is from 70 to 100%. The microburst and background sources exist for several days and appear at sites of prolonged energy release. The typical duration of an individual microburst is about 1–2 s, and the time interval between microbursts is about 3–5 s. A negative microburst frequency drift of about 100 MHz/s or more is also observed. Some interpretations of the microburst and background sources are discussed. The most probable microburst model involves the generation of radio emission via the plasma mechanism, with the upward propagation of fast electrons above an active region. In this case, the required energy of the Langmuir waves is 2×10^?8 of the heat energy of the background plasma. Microbursts appear in different places in an active region. New methods for determining the magnetic-field intensity in the regions of generation of the decimeter-wavelength emission are proposed.     

Solution of the radiative-transfer equations with an azimuth-averaged Hanle phase matrix

The system of radiative-transfer equations for polarized radiation is solved analytically using an azimuth-averaged Hanle phase matrix. A Milne-Eddington model with a constant ratio of the line and continuum absorption coefficients and a linear dependence of the source function on optical depth is adopted for the atmosphere. The vector magnetic field is taken to be constant with optical depth. The dependence of the linear polarization obtained during observations at the solar limb on the magnitude and inclination of the magnetic field to the normal of the atmosphere is presented as an illustration of the theoretical computations. The measured polarization corresponds to two magnetic-field values and several possible field inclinations. The measured polarization is directly proportional to the quantum coefficient W ₂ determining the resonance polarization in lines.     

Peculiarities of polarized radio emission of solar active regions

Spectra of solar-flare active regions displaying peculiarities in their polarized radio emission observed on the RATAN-600 radio telescope at 2–16 GHz are considered. An appreciable dip of the circularly polarized emission (Stokes parameter V) in the middle of the microwave range (6–12 GHz), sometimes with a reversal of the sign of the polarization, is unusual. In some cases, the ordinary emission also dominates at long microwave wavelengths. Expected peculiarities of the frequency structure of microwave sources are calculated in simple models with loops in the form of hot and cool tori. Numerical calculations of these spectra show that the above features of the polarized emission can be explained by the presence of a hot region in the solar corona. It is shown that the parameters of the spectrum of the polarized emission can be used to determine the magnetic field in this hot region and the product of the relative magnetic-field gradient and the loop thickness.     

The “X component” of the radio background

The recent publication of evidence for a new mechanism producing background radio emission of the Galaxy at centimeter wavelengths (in addition to synchrotron radiation, free—free transitions in ionized gas, and the weak radio emission of standard dust) gave rise to a strong reaction among observers, and requires independent experimental verification. This signal is of special concern in connection with studies of the polarization of the cosmic microwave background (CMB) using new-generation experiments. We have derived independent estimates of the validity of the “spinning-dust” hypothesis (dipole emission of macromolecules) using multi-frequency RATAN-600 observations. Test studies in the Perseus molecular cloud show evidence for anomalous extended emission in the absence of strong radio sources (compact HII regions) that could imitate an anomalous radio spectrum in this region. A statistical analysis at centimeter wavelengths over the Ratan Zenith Field shows that the upper limit for the polarized noise from this new component in the spinning-dust hypothesis is unlikely to exceed 1 µK at wavelengths of 1 cm or shorter on the main scales of the EE mode of Sakharov oscillations. Thus, this emission should not hinder studies of this mode, at least to within several percent of the predicted level of polarization of the CMB emission.     

Behavior of pulsar B0329+54 pulse characteristics in the immediate vicinity of mode switch times at 111.4 MHz

Times of switches from the normal to the abnormal radiation mode have been recorded in observations of individual pulses of pulsar B0329+54 using the Large Phased Array of the Pushchino Radio Astronomy Observatory at 111.4 MHz. The variations in the amplitudes of the outer components that accompany the switch to the abnormal pulse profile occurred simultaneously in only half the cases. The phase of component IV of the integrated pulse does not vary during mode switches. In half the cases, instantaneous variations of the phases of component I and the central component during mode switches may be preceeded by additional smooth variations of the phases of individual pulses occuring over several minutes. We detected a decrease in the linear polarization of the central component by, on average, 8% in the abnormal mode for the integrated pulse, due to variations in the relative intensities of two orthogonally polarized modes of the pulsar radiation.     

利用磁异常模量进行磁性体边界检测

地下磁性体具有较强的剩磁或退磁会给磁异常的解释带来困难。由于磁异常模量对磁化方向不敏感,将磁异常转换为磁异常模量并用来分析和处理,可很大程度上提高磁异常解释的准确性和可靠性。本文介绍了五种磁异常模量,并且利用改进的Tilt梯度方法对各磁异常模量数据做边界检测,确定地下磁性体的水平位置。理论模型试验表明,在存在强剩磁的情况下,不同模量数据对磁性体的反应程度也不一样,运用改进的Tilt梯度方法对磁异常模量进行处理可不同程度地加强对异常体的识别,其中模量E最为准确,可较清晰地识别磁性体边界。笔者还将该方法运用到坦桑尼亚基戈马地区的航磁异常处理,并推测了该地区的构造带及磁性体边界,为深入了解基戈马成矿区带的地质和成矿背景及可能矿床分布提供了重要信息。     

Variability of the Blazar 4C 38.41 in 2006–2017

The results of multicolor photometric and polarization observations of the blazar 4C 38.41 (Q 1633+382) performed at St. Petersburg State University, the Pulkovo Astronomical Observatory of the Russian Academy of Sciences, Boston University, and Steward Observatory in 2006–2017 are analyzed. Separate variable sources responsible for the observed activity are distinguished, whose power-lawspectra and high degree of polarization confirm their synchrotron nature. The observed color variability of the object (redder when brighter) can be explained by an increase in the contribution to the total emission of a red component with variable flux and a constant relative spectral energy distribution. A close correlation between the optical and gamma-ray light curves is identified, suggesting the same location of the variable sources responsible for the radiation in these bands.

     

New measurements of the circular polarization of the radio emission of Active Galactic Nuclei on parsec scales

We describe and compare methods for reducing radio circular-polarization data for Active Galactic Nuclei on parsec (VLBI) scales, and present results derived by applying these methods to 15 GHz observations of 29 AGN obtained on the Very Long Baseline Array. Circular polarization was detected in six of these objects, with characteristic degrees of several tenths of a percent. According to results obtained in other studies and the new results presented here, the most probable mechanism for the formation of the circular polarization is Faraday conversion of linear polarization when the radio emission passes through a magnetized plasma. For the objects OJ 287, 1334-127, and 3C 279, which were observed at several epochs, we find a constant degree and sign of the circular polarization on time scales of several years, testifying to the presence of stable ordered magnetic fields in the corresponding regions.     

The possible escape of electron cyclotron maser radiation from hot stellar coronas through a window of transparency

The main argument against the idea that the intense radio emission observed from active regions on the Sun and flare stars is electron-cyclotron maser (ECM) radiation is that such radiation should be strongly absorbed in higher-lying layers where the condition for the cyclotron resonance at harmonics of the electron gyrofrequency is fulfilled. Cyclotron absorption lowers the efficiency of ECM radiation virtually to zero for a broad range of angles between the direction of propagation of the radiation and the magnetic field. Less severe absorption is possible only in narrow angular “windows” along (for ordinary and extraordinary waves) and perpendicular to (for ordinary waves) the magnetic field. However, the ECM radiation that is generated does not fall into these windows of transparency due to the kinematic conditions corresponding to coronal magnetic traps. We investigate the efficiency of induced scattering of ECM radiation on ions in the equilibrium plasma in the source. Under certain conditions, induced scattering leads to the formation of a condensate of ECM radiation with the direction of its wave vectors approximately along the magnetic field, enabling the escape of the radiation through windows of transparency. The most favorable conditions for this phenomenon are realized for ordinary waves. We estimate the optical depths of the sources of the ECM radiation to the scattering and the angular width of the condensate for ordinary and extraordinary waves for the cases of the flare radio emission of the star AD Leo and the sources of type I noise storms in the solar corona. In both cases, the polarization of the emergent radiation should correspond to the ordinary wave.     

Validation of an elastoplastic model for chalk

A new elastoplastic model for porous rocks (chalk) has been developed [1]. It is based on the concept of two plastic deformation mechanisms[2]. In this paper, we present the validation of this model for both homogenous tests and a typical boundary condition problem, hollow cylinder test. Comparisons between experimental results and predictions by the model are presented for each test. Influences of parameter perturbations on numerical simulation are also studied.     

The role of satellite altimetry in gravity field modelling in coastal areas

During recent years altimetry from the two geodetic missions of GEOSAT and ERS-1 has enabled the derivation of high resolution near global gravity field from altimetry [Andersen and Knudsen, 1995, 1996; Sandwell and Smith, 1997].Altimetric gravity fields are unique in the sense that they provide global uniform gravity information with very high resolution, and these global marine gravity fields are registered on a two by two minute grid corresponding to 4 by 4 kilometres at the equator.In this presentation several coastal complications in deriving the marine gravity field from satellite altimetry will be investigated using the KMS98 gravity field. Comparison with other sources of gravity field information like airborne and marine gravity observations will be carried out and two fundamentally different test areas (Azores and Skagerak) will be studied to investigated the different role of these different sources of gravity information.     

Possible observations of the epoch of reionization

We consider possibilities for presently operating and planned infrared, submillimeter, and millimeter telescopes for observations of the epoch of reionization in the Universe, i.e., of the sources of ultraviolet radiation that have caused the reionization (galaxies, quasars, etc.) and the ionized intergalactic medium. Along with direct observations of such sources in the optical and infrared, we analyze the feasibility of observations of the intergalactic gas, as well as fluctuations of the temperature and polarization of the cosmic microwave background due to inhomogeneities in the reionization process.