共查询到20条相似文献，搜索用时 31 毫秒
1.
A. V. Dementyev 《Astrophysics》2009,52(4):545-564
Multiple resonance scattering of radiation in a spectrum line with absorption in the continuum is examined. It is assumed
that the scattering atmosphere is semi-infinite and that there is no magnetic field or continuum emission at the frequencies
of the spectrum line. The polarization characteristics of the emerging radiation are determined for unpolarized primary radiation
sources distributed uniformly within an atmosphere in the case of a Voigt absorption profile. The calculations employ an iterative
solution of the Ambartsumyan-Chandrasekhar matrix integral equation. 相似文献
2.
A. V. Dementyev 《Astrophysics》2010,53(3):419-433
A matrix transfer equation for multiple resonance scattering of radiation in a spectral line in a semiinfinite atmosphere
with a uniform distribution of primary radiation sources is examined. A nonlinear matrix integral is obtained for this equation
as a generalization of the Rybicki two-point Q-integral. One special case of the matrix [^(Q)] {\mathbf{\hat{Q}}} -integral is the Volterra equation for the matrix source function of the problem discussed here. The Volterra equation is
solved numerically for a Doppler profile of the absorption coefficient. Several polarization characteristics of the emerging
radiation are obtained. 相似文献
3.
Equipartition magnetic fields can dramatically affect the polarization of radiation emerging from accretion disc atmospheres in active galactic nuclei. We extend our previous work on this subject by exploring the interaction between Faraday rotation and absorption opacity in local, plane-parallel atmospheres with parameters appropriate for accretion discs. Faraday rotation in pure scattering atmospheres acts to depolarize the radiation field by rotating the polarization planes of photons after last scattering. Absorption opacity in an unmagnetized atmosphere can increase or decrease the polarization compared to the pure scattering case, depending on the thermal source function gradient. Combining both Faraday rotation and absorption opacity, we find the following results. If absorption opacity is much larger than scattering opacity throughout the atmosphere, then Faraday rotation generally has only a small effect on the emerging polarization because of the small electron column density along a photon mean free path. However, if the absorption opacity is not too large and it acts alone to increase the polarization, then the effects of Faraday rotation can be enhanced over those in a pure scattering atmosphere. Finally, while Faraday rotation often depolarizes the radiation field, it can in some cases increase the polarization when the thermal source function does not rise too steeply with optical depth. We confirm the correctness of the analytic calculation by Silant'ev of the high magnetic field limit of the pure scattering atmosphere, which we incorrectly disputed in our previous paper. 相似文献
4.
J. Patrick Harrington 《Astrophysics and Space Science》1970,8(2):227-242
An accurate numerical method is presented for the solution of the transfer equations in a plane-parallel atmosphere in which scattering occurs according to Rayleigh's law. Some results are given for the polarization and limb darkening of both integrated and monochromatic radiation emerging from grey atmospheres with various ratios of scattering to absorption. The method is equally applicable to non-grey atmospheres. 相似文献
5.
The Milne problem is solved numerically for a magnetized semi-infinite electron atmosphere in the case where the magnetic field is directed along the normal to the medium. The calculated angular distribution, degree of linear polarization, and positional angle of inclination of the plane of polarization of the emerging radiation are given in tables for a number of values of the Faraday rotation parameter and for degrees of intrinsic optical absorption, q=0, 0.2, and 0.4. It is assumed that the magnetic field B106 G, so that scattering in the optical range is purely Thomson scattering. 相似文献
6.
K. L. Coulson 《Planetary and Space Science》1959,1(4):277-284
The solution of the equation of radiative transfer in a medium exhibiting Rayleigh scattering, as developed by S. Chandrasekhar, has been used for an extensive series of computations(3) of the characteristics of the scattered and diffusely reflected radiation emerging from the top of an atmospheric model which corresponds in many respects to the sunlit portion of the earth's atmosphere. The first part of this two-part discussion dealt with the intensity, degree of polarization, plane of polarization and the neutral points of the emergent light as functions of sun elevation, direction in the downward hemisphere, optical thickness of the model atmosphere and reflectivity of the underlying surface. This second part is concerned with the upward flux obtained by an integration of the intensity over the entire hemisphere, for the incident radiation (a) being independent of wavelength or (b) having the spectral distribution of the extra-terrestrial solar radiation. Integration with respect to wavelength in the latter case, together with an approximation for the sphericity of the atmosphere, yields a value of 7.6 per cent for the earth's planetary albedo due to scattering by the clear atmosphere. An approximation for ozone absorption decreases the computed albedo to 6.9 per cent. 相似文献
7.
The problem of determining the intensity and the degree of polarization of radiation emerging from an inhomogeneous finite plane medium for the case of Rayleigh scattering with internal energy source is considered. A system of coupled integral equations are obtained and solved by the Galerkin method. The degree of polarization for homogeneous and inhomogeneous media are calculated for uniform and nonuniform sources. 相似文献
8.
V. M. Loskutov 《Astrophysics》2011,54(3):421-428
The limiting polarization of a resonance line is examined for standard radiative transfer of polarized radiation in a semi-infinite
scattering atmosphere with complete frequency redistribution. Two families of profiles of the line absorption coefficient,
which are generalizations of Lorentz and Doppler profiles, are examined. It is shown that for both families this parameter
approaches the Sobolev-Chandrasekhar limit when the fraction of absorption within the frequency interval (expressed in appropriate
units) from −1 to 1 relative to the total absorption in the line approaches unity. 相似文献
9.
Freely oriented small anisotropic grains and molecules depolarize radiation both in single scattering and in the process of multiple scattering. Especially large depolarization occurs for resonant scattering corresponding to the electron transitions between the energy levels with very different quantum numbers. The existence of light absorption also changes essentially the angular distribution and polarization of radiation, outgoing from an atmosphere. In the present paper we consider these effects in detail both for continuum radiation and for resonant lines. Because the term describing the depolarization deals with isotropic radiation, we consider the axially symmetric part of radiation. We derived the formulas for observed intensity and polarization using the invariance-principles both for continuum and resonant scattering. We confine ourselves to two problems—the diffuse reflection of the light beam from semi-infinite atmosphere, and the Milne problem. 相似文献
10.
A Monte Carlo model designed to compute both the input and output radiation fields from spherical-shell cometary atmospheres has been developed. The code is an improved version of that by H. Salo (1988, Icarus76, 253-269); it includes the computation of the full Stokes vector and can compute both the input fluxes impinging on the nucleus surface and the output radiation. This will have specific applications for the near-nucleus photometry, polarimetry, and imaging data collection planned in the near future from space probes. After carrying out some validation tests of the code, we consider here the effects of including the full 4×4 scattering matrix in the calculations of the radiative flux impinging on cometary nuclei. As input to the code we used realistic phase matrices derived by fitting the observed behavior of the linear polarization as a function of phase angle. The observed single scattering linear polarization phase curves of comets are fairly well represented by a mixture of magnesium-rich olivine particles and small carbonaceous particles. The input matrix of the code is thus given by the phase matrix for olivine as obtained in the laboratory plus a variable scattering fraction phase matrix for absorbing carbonaceous particles. These fractions are 3.5% for Comet Halley and 6% for Comet Hale-Bopp, the comet with the highest percentage of all those observed.The errors in the total input flux impinging on the nucleus surface caused by neglecting polarization are found to be within 10% for the full range of solar zenith angles. Additional tests on the resulting linear polarization of the light emerging from cometary nuclei in near-nucleus observation conditions at a variety of coma optical thicknesses show that the polarization phase curves do not experience any significant changes for optical thicknesses τ?0.25 and Halley-like surface albedo, except near 90° phase angle. 相似文献
11.
It is usually assumed that the ions of cosmic rays contribute nothing to the observable electromagnetic radiation. However, this is true only when these ions are moving in a vacuum or a quiet (nonturbulent) plasma. In the case of fast ions in a turbulent plasma, there is an effective nonlinear mechanism of radiation which is discussed in this paper. The fast ion (relativistic or nonrelativistic) moving in the plasma creates a polarization cloud around itself which also moves with the particles. The turbulent plasma waves may scatter on the moving electric field of this polarization cloud. In the process of this scattering an electromagnetic wave with frequency (2.7) is generated. Let 1 and k1 be the frequency and wave vector of turbulent plasma waves,V is the velocity of the ion, and is the angle between the wave vector of electromagnetic radiation and the direction of the ion velocity. The method of calculating the probability of the conversion of plasma waves (k1) into electromagnetic waves (k) by scattering on an ion with velocityV is described in detal in Section 2 (Equation (2.14)).The spectral coefficients of spontaneous radiation in the case of scattering of plasma waves on polarization clouds created by fast nonrelativistic ions are given in (3.6) for an ion energy distribution function (3.4) and in (3.8) for more general evaluations. The Equations (3.9)–(3.13) describe the spectral coefficients of spontaneous emission for different modes of plasma turbulence (Langmuir (3.9), electron cyclotron in a weak (3.10) or strong (3.11) magnetic field and ion acoustic (3.12)–(3.13) waves). The coefficients of reabsorption or induced emission are given by Equations (3.14) and (3.16)–(3.19). There is a maser effect in the case of scattering of plasma waves on a stream of ions. The effective temperature of the spontaneous emission is given by Equation (3.15). The spectral coefficients of radiation due to scattering of plasma waves on relativistic ions are calculated in the same manner (Equations (4.14)–(4.15)). The total energy loss due to this radiation is given in Equations (4.23)–(4.25). The coefficients of induced emission are given in (4.26)–(4.28).The results are discussed in Section 5. It is shown that the loss of energy by nonlinear plasma radiation is much smaller than the ionization loss. However, the coefficients of synchrotron radiation of electrons and nonlinear radiation of ions under cosmic conditions may be comparable in the case of a weak magnetic field and fairly low frequencies (5.5)–(5.6). Usually the spectrum of nonlinear plasma radiation is steeper than in the case of synchroton radiation. Equation (5.10) gives the condition for nonlinear radiation to prevail over thermal radiation.Translated by D. F. Smith. 相似文献
12.
Hyo Jeong Kim Hee-Won Lee Suna Kang 《Monthly notices of the Royal Astronomical Society》2007,374(1):187-195
Thomson scattering is often invoked to explain broad wing features that are seen in various objects including active galactic nuclei and symbiotic stars. Despite the wavelength-independent scattering cross-section of Thomson scattering, the line flux may exhibit wavelength-dependent linear degree of polarization, because various parts of emission wings are contributed by photons with different scattering numbers. Specifically, more scattered and hence more weakly polarized photons tend to fill the farther wing parts from the line centre, while the neighbourhood of the line centre is dominated by less-scattered photons with higher degree of polarization. Using a Monte Carlo technique, we investigate the polarization structure of Thomson-scattered line radiation. A detailed analysis of polarization structure formation is conducted by investigating the dependence of the polarization and profile width on the scattering number for various finite electron scattering slabs. Significantly varying degree of polarization is obtained when the scattering medium has Thomson optical depth τTh ≥ 1 . We present our high-resolution spectrum of the symbiotic star V1016 Cyg obtained with the Bohyunsan Optical Echelle Spectrograph (BOES) in order to fit the broad profile around Hα by electron scattering wings adopting an oblate spheroidal geometry with Thomson optical depth τTh = 0.5 and electron temperature T e = 6.2 × 104 K . Local maxima in the linear degree of polarization of Thomson-scattered line radiation are expected to appear in the spectral regions characterized by the average scattering number ≃1. 相似文献
13.
We consider the transfer of radiation and calculate the force of its pressure in the electron gyroresonance line in the atmospheres of magnetic degenerate stars. We specify the atmospheric parameters for which an outflow of plasma is possible under radiation pressure in the cyclotron line. We show that the permittivity tensor of a mildly relativistic plasma in a strong magnetic field found by applying relativistic corrections to the cyclotron resonance condition and by taking into account the vacuum polarization and recoil effects during photon scattering should be used to obtain proper results. We have determined the real and imaginary parts of the refractive indices and the polarization coefficients for normal electromagnetic waves when scattering dominates over absorption. Relativistic effects, which change greatly the dispersion and resonant absorption of waves propagating almost perpendicular to the magnetic field, and vacuum polarization have been found to change qualitatively the gyroresonance radiation spectrum and pressure for a wide range of parameters of stellar magnetospheres. 相似文献
14.
Atmospheres and spectra of strongly magnetized neutron stars 总被引：1，自引：0，他引：1
We construct atmosphere models for strongly magnetized neutron stars with surface fields and effective temperatures . The atmospheres directly determine the characteristics of thermal emission from isolated neutron stars, including radio pulsars, soft gamma-ray repeaters, and anomalous X-ray pulsars. In our models, the atmosphere is composed of pure hydrogen or helium and is assumed to be fully ionized. The radiative opacities include free–free absorption and scattering by both electrons and ions computed for the two photon polarization modes in the magnetized electron–ion plasma. Since the radiation emerges from deep layers in the atmosphere with , plasma effects can significantly modify the photon opacities by changing the properties of the polarization modes. In the case where the magnetic field and the surface normal are parallel, we solve the full, angle-dependent, coupled radiative transfer equations for both polarization modes. We also construct atmosphere models for general field orientations based on the diffusion approximation of the transport equations and compare the results with models based on full radiative transport. In general, the emergent thermal radiation exhibits significant deviation from blackbody, with harder spectra at high energies. The spectra also show a broad feature around the ion cyclotron resonance , where Z and A are the atomic charge and atomic mass of the ion, respectively; this feature is particularly pronounced when . Detection of the resonance feature would provide a direct measurement of the surface magnetic fields on magnetars. 相似文献
15.
A. V. Dementyev 《Astronomy Letters》2008,34(8):574-580
Multiple resonance scattering of radiation in a spectral line is considered in the case of a Voigt absorption profile. The scattering is assumed to take place in a nonmagnetic semi-infinite atmosphere with uniformly distributed sources of unpolarized radiation. Polarization characteristics have been obtained for the emergent radiation by numerically solving the Ambartsumian-Chandrasekhar matrix integral equation. 相似文献
16.
K. E. Rangarajan 《Monthly notices of the Royal Astronomical Society》1999,308(4):1053-1060
The polarized radiative transfer equation is solved when angle-dependent partial redistribution and non-coherent electron scattering are included as line-scattering mechanisms. A static atmosphere with plane parallel symmetry is assumed. Test calculations are used to illustrate the effects of the electron-scattering coefficient, the thermalization parameter and the continuous absorption coefficient on the line polarization. Results of angle-averaged and angle-dependent redistribution functions are compared and it is shown that angle-dependent functions should be used to model the wing polarization of optically thin lines. The lower the continuous absorption compared with the electron scattering, the higher the wing polarization. 相似文献
17.
Tsutomu Takashima 《Astrophysics and Space Science》1974,30(2):309-313
If the atmosphere is simulated by a number of homogeneous sublayers, it was shown (Takashima, 1973a) that the intensity and polarization parameters emerging from any boundary of internal sublayer's field can be determined, provided that the diffuse reflection and transmission matrices of radiation of sublayers are known. Furthermore, if the surface (ground) is assumed to reflect light in accordance with the Lambert law, it is shown in this paper that the internal radiation field at boundary of any sublayer can be also computed in terms of the diffuse radiation matrices of sublayers rather than in terms of that of the entire atmosphere (Sekera, unpublished). The effect of polarization is included. 相似文献
18.
V. Krishan 《Journal of Astrophysics and Astronomy》1988,9(4):231-236
The occurrence of superluminal motion in extragalactic radio sources is believed to be quite common. Among others, the geometrical
scattering of radio radiation can also cause superluminal expansion and or motion and halo formation, In this paper, the effectiveness
of the stimulated Raman scattering in producing these features is investigated. The scattering medium is a plasma whose position,
density and temperature decide the rate and angle of scattering. When the radiation from a stationary and constant source
gets scattered from a stationary plasma, a halo is formed around the source. However, the scattering of a rotating radiation
beam does produce superluminal motion of the virtual source. It is found that the plasma should have the characteristics of
the emission-line regions and the intercloud medium in order to Raman scatter the radiation. Since the scattering is polarization
dependent, it is possible to estimate the rotation of the electric vector along the direction of the apparent motion of a
radio source. 相似文献
19.
Circularly polarized radio radiation maintains its polarization even where the magnetic field reverses its sign relative to the ray (QT region) if the reversal is sufficiently abrupt (strong QT region). Bastian (1995) suggested that coronal turbulence scatters radiation, such as type I bursts, sufficiently to make the reversal abrupt where it would otherwise not be. However, the observed directivity of type I bursts sets an upper limit on the scattering. This limit implies that the turbulent scattering is not sufficient to maintain the circular polarization as in a strong QT region. The conclusion is strengthened by an analytical calculation of the polarization. Apparently, the fully polarized type I bursts, near disk center, encounter no horizontal magnetic fields, at least not until high enough in the corona that the QT region is strong anyway. 相似文献
20.
Dinh-V-Trung † 《Monthly notices of the Royal Astronomical Society》2009,399(3):1495-1505
In this paper, we investigate the polarization property of the radiation amplified by astronomical masers in the presence of a strong magnetic field. Our model explicitly takes into account the broad-band nature of the radiation field and the interaction of the radiation with the maser transition J = 1–0 . The amplification of different realizations of the background continuum radiation by the maser is directly simulated and the Stokes parameters of the radiation field are then obtained by averaging over the ensemble of emerging maser radiation. For isotropic pumping and partially saturated masers, we find that the maser radiation is linearly polarized in two representative cases where the magnetic field B makes an angle θ= 30° and 90° to the maser axis. The linear polarization for maser radiation obtained in our simulations for both cases is in agreement with the results of the standard model. Furthermore, no instability during amplification is seen in our simulations. Therefore, we conclude that there is no problem with the previous numerical investigations of maser polarization in the unsaturated and partially saturated regime. 相似文献