共查询到20条相似文献，搜索用时 828 毫秒
1.
Radiative transfer 总被引：1，自引：0，他引：1
George B. Rybicki 《Journal of Astrophysics and Astronomy》1996,17(3-4):95-112
Chandrasekhar’s work in radiative transfer theory began in 1944 and culminated with the publication of his influential treatiseRadiative Transfer in 1950. In this review his major contributions to radiative transfer will be recounted and evaluated. These include his
development of the discrete ordinates method, the invariance principles, and his formulation and solution of the transfer
equation for polarized light. 相似文献
2.
Jones Hancock Lasenby Davies Gutiérrez Rocha Watson & Rebolo 《Monthly notices of the Royal Astronomical Society》1998,294(4):582-594
The complete set of data from the Tenerife 10-GHz (8° FWHM) twin-horn, drift scan experiment is described. These data are affected by both long-term atmospheric baseline drifts and short-term noise. A new maximum entropy procedure, utilizing the time invariance and spatial continuity of the astronomical signal, is used to achieve a clean separation of these effects from the astronomical signal, and to deconvolve the effects of the beam-switching. We use a fully positive/negative algorithm to produce two-dimensional maps of the intrinsic sky fluctuations. Known discrete sources and Galactic features are identified in the deconvolved map. The data from the 10-GHz experiment, after baseline subtraction with MEM, are then analysed using conventional techniques, and new constraints on Galactic emission are made. 相似文献
3.
The processes by which energetic electrons lose energy in a weakly ionized gas of molecular hydrogen are analysed and calculations are carried out taking into account the discrete nature of the excitation processes. The excitation, ionization and heating efficiencies are computed for electrons with energies up to 100 eV absorbed in a gas with fractional ionizations up to 10?2 and the mean energy per neutral hydrogen atom pair is calculated. 相似文献
4.
H. Domke 《Astrophysics and Space Science》1974,29(2):379-386
It is shown that it is always possible to expand the scattering matrix for an isotropic medium in generalized spherical functions.
Additional physical symmetry properties of the scattering process, e.g., time-reversal invariance or reflection invariance,
give rise to simple symmetry relations for the expansion coefficents. The expansion in generalized spherical functions naturally
leads to decomposition of variables and appears to be the simplest way to include the various symmetry relations for the scattering
matrix discussed in the literature. 相似文献
5.
The processes by which energetic electrons lose energy in a weakly ionized gas of argon are analysed and calculations are carried out taking into account the discrete nature of the excitation processes. The excitation, ionization and heating efficiences are computed for energies up to 200 eV absorbed in a gas with fractional ionizations varying up to 10?2. 相似文献
6.
The constancy of light speed is a basic assumption in Einstein’s special relativity, and consequently the Lorentz invariance is a fundamental symmetry of space–time in modern physics. However, it is speculated that the speed of light becomes energy-dependent due to the Lorentz invariance violation (LV) in various new physics theories. We analyse the data of the energetic photons from the gamma-ray bursts (GRBs) by the Fermi Gamma-Ray Space Telescope, and find more events to support the energy dependence in the light speed with both linear and quadratic form corrections. We provide two scenarios to understand all the new-released Pass 8 data of bright GRBs by the Fermi-LAT Collaboration, with predictions from such scenarios being testable by future detected GRBs. 相似文献
7.
We study the dipole anisotropy in the arrival directions of high energy CR electrons and positrons (CRE) of dark matter (DM) origin. We show that this quantity is very weakly model dependent and offers a viable criterion to discriminate among CRE from DM or from local discrete sources, like e.g. pulsars. In particular, we find that the maximum anisotropy which DM can provide is to a very good approximation a universal quantity and, as a consequence, if a larger anisotropy is detected, this would constitute a strong evidence for the presence of astrophysical local discrete CRE sources, whose anisotropy, instead, can be naturally larger than the DM upper limit. We further find that the main source of anisotropy from DM is given by the fluctuation in the number density of DM sub-structures in the vicinity of the observer and we thus devote special attention to the study of the variance in the sub-structures realization implementing a dedicated Montecarlo simulation. Such scenarios will be probed in the next years by Fermi-LAT, providing new hints, or constraints, about the nature of DM. 相似文献
8.
John A. Adam 《Astrophysics and Space Science》1985,114(2):249-258
A class of singular differential equations that often occurs in problems in magnetohydrodynamics, plasma physics, and astrophysics is studied; first by means of the specific and fairly straightforward example of motions in a stratified magnetofluid, and secondly in terms of the nature of the eigenvalue spectrum for the more general class of problems. It is shown that in the absence of discontinuities in the basic profiles, the eigenvalue spectrum is real and continuous, and no discrete spectrum exists. Bounds are obtained on the location of the continuous spectrum. 相似文献
9.
Sebastian Heinz Andrea Merloni Tiziana Di Matteo Rashid Sunyaev 《Astrophysics and Space Science》2005,300(1-3):15-21
The scale invariance model (Heinz, S. and Sunyaev, R.A.: 2003, MNRAS 343, L59) can be used to derive robust scaling relations between the radio luminosity from accreting black holes and the black
hole mass and accretion rate. These relations agree well with the recently found “fundamental plane” of black hole activity
(Merloni, A., Heinz, S. and Di Matteo, T.: 2003, MNRAS 345, 1057). This relation provides a new, powerful tool for the comparison of jets from black holes of different masses and accretion
rates. The regression coefficients of this relation contain information about the nature of the X-ray emission mechanism driving
the correlation. We argue that X-ray synchrotron emission from the base of the jets is unlikely to be the dominant contribution
to the X-ray spectrum in most of the sources. 相似文献
10.
Lorentz invariance is such an important principle of fundamental physics that it should constantly be subjected to experimental scrutiny as well as theoretical questioning. Distant astrophysical sources of energetic photons with rapid time variations, such as active galactic nuclei (AGNs) and gamma-ray bursters (GRBs), provide ideal experimental opportunities for testing Lorentz invariance. The Čerenkov Telescope Array (CTA) is an excellent experimental tool for making such tests with sensitivities exceeding those possible using other detectors. 相似文献
11.
Recently, detection of discrete features in the X-ray afterglow spectra of GRB 970508 and GRB 970828 was reported. The most natural interpretation of these features is that they are redshifted Fe K emission complexes. The identification of the line emission mechanism has drastic implications for the inferred mass of radiating material and hence the nature of the burst site. X-ray spectroscopy provides a direct observational constraint on these properties of gamma-ray bursters. We briefly discuss how these constraints arise in the context of an application to the spectrum of GRB 970508. 相似文献
12.
The redshift (z) dependence of the dispersion relations for free particles is analyzed by taking into account the Lorentz invariance violation. A nonlinear algebraic equation is derived for the momenta of the particles involved in the annihilation reaction of a hard photon from a γ-ray source with a soft cosmic microwave background (CMB) photon near the threshold of this reaction. The solutions of this threshold equation are constructed and analyzed as a function of the redshift. We show that the threshold of the reaction under consideration tends to decrease with increasing z; the energy spectra of γ-ray sources at energies of ～10 TeV must be cut off in accordance with the calculated z dependence. We also calculate the time delay of the light signals from γ-ray sources that corresponds to the Lorentz invariance violation for photons. We discuss the possibility of improving the standard constraints on the Lorentz invariance violation parameters for fields of various physical natures. 相似文献
13.
A recent model of torsion-modified electrodynamics which breaks the electromagnetic conformal invariance is shown to lead to the generation of magnetic fields in the Early Universe. 相似文献
14.
A. G. Nikoghossian 《Astrophysics》2009,52(1):1-23
The purpose of this article is to provide some insight into Ambartsumian’s methods in the theory of radiative transfer, their
applications, and further development. Two of these methods are emphasized--the invariance principle and the method of addition
of layers, proposed by Ambartsumian in the 1940’s. The difference between these methods and the classical approach for solving
radiative transfer problems is discussed. We discuss only a small portion of the subsequent work by others that we believe
reveals, in a more intuitive way, the essence and significance of Ambartsumian’s methods and their efficiency for applications.
Thus, for example, a separate section is devoted to applications of the Lagrangian formalism to radiative transfer and it
is shown that the invariance principle is a special case of a more general variational principle that reflects an invariance
with respect to translational transformation of the optical depth. Our discussion of the method of addition of layers points
out its generality and the major role it has played in the later creation of such methods as Bellman’s invariant imbedding
method and the method for solving radiative transfer problems in inhomogeneous media. The latter method has yielded a number
of new analytic results. The concluding section is a brief summary of Ambartsumian’s results in the nonlinear theory of radiative
transfer, where he was a pioneer in the study of the class of multilevel problems. This article also sets out to demonstrate
the place and role of Ambartsumian’s methods in the theory of radiative transfer, which, to a great extent, set the path along
which this theory developed for many years to come.
Translated from Astrofizika, Vol. 52, No. 1, pp. 5–27 (February 2009). 相似文献
15.
A. G. Nikoghossian 《Astrophysics》2009,52(3):431-439
This paper is devoted to one of the methods proposed by Ambartsumian in radiative transfer theory— the invariance principle.
The possible connection of several well known nonlinear relations in the theory to a variational principle involving a translational
transformation of the optical depth is discussed. 相似文献
16.
We analyse the period distribution of eclipsing and spectroscopic binary systems, using various methods to take into account selection effects on observational data, with the purpose of deriving the actual distribution curve. The discussion of results shows the presence of some secondary maxima in the distribution which are probably of a non-statistical character. They could be regarded as an indirect clue to the discrete nature of the star formation process, according to the spiral density wave theory of galactic evolution by Linet al. (1969). 相似文献
17.
The Very Large Array and the Soft X-ray Telescope (SXT) aboard the Yohkoh satellite jointly observed the rapid growth and decay of a so-called anemone active region on 3–6 April, 1992 (AR 7124). The VLA obtained maps of the AR 7124 at 1.5, 4.7, and 8.4 GHz. In general, discrete coronal loop systems are rarely resolved at 1.5 GHz wavelengths because of limited brightness contrast due to optical depth effects and wave scattering. Due to its unusual anemone-like morphology, however, several discrete loops or loop systems are resolved by both the VLA at 1.5 GHz and the SXT in AR 7124.Using extrapolations of the photospheric field and the radio observations at 4.7 and 8.4 GHz, we find that the microwave emission is the result of gyroresonance emission from a hot, rarefied plasma, at the second and/or third harmonic. The decimetric source is complex -1.5 GHz emission from the leading part of AR 7124 is due to free-free emission, while that in the trailing part of the active region is dominated by gyroresonance emission. We also examine an interesting case of a discrete radio loop with no soft X-ray (SXR) emission adjacent to a hot SXR loop. This observation clearly shows the multithermal nature of the solar corona. 相似文献
18.
C. Sivaram 《Astrophysics and Space Science》1986,125(1):189-199
The problem of the Universe emerging out of the Planck epoch is discussed. It is pointed out that an earlier exponential expansion phase well before the onset of the GUTs phase transition is essential. Such an expansion can occur owing to the breaking of scale invariance at Planck energies in a unified theory of gravity with other interactions. 相似文献
19.
Tao Kiang Yong-Feng Wu Xing-Fen Zhu Dunsink Observatory Dublin Institute for Advanced Studies Dublin Ireland Center for Astrophysics University of Science Technology of China Hefei 《中国天文和天体物理学报》2004,4(3):209-220
Rhombic cell analysis as outlined in the first paper of the present seriesis applied to samples of varying depths and liming luminosities of the IRAS/PSCzCatalogue. Numerical indices are introduced to summarize essential information.Because of the discrete nature of the analysis and of the space distribution of galax-ies, the indices for a given sample must be regarded as each having an irreduciblescatter. Despite the scatter, the mean indices show remarkable variations acrossthe samples. The underlying factor for the variations is shown to be the limitingluminosity rather than the sampling depth. As samples of more and more luminousgalaxies are considered over a range of some 2.5 magnitudes (a factor of some 75in space density), the morphology of the filled and empty regions defined by thegalaxies degrades steadily towards insignificance, and the degrading is faster for thefilled than the empty region. 相似文献
20.
J. M. Pittard J. E. Dyson S. A. E. G. Falle T. W. Hartquist 《Monthly notices of the Royal Astronomical Society》2005,361(3):1077-1090
Because of its key role in feedback in star formation and galaxy formation, we examine the nature of the interaction of a flow with discrete sources of mass injection. We show the results of two-dimensional numerical simulations in which we explore a range of configurations for the mass sources and study the effects of their proximity on the downstream flow. The mass sources act effectively as a single source of mass injection if they are so close together that the ratio of their combined mass injection rate is comparable to or exceeds the mass flux of the incident flow into the volume that they occupy. The simulations are relevant to many diffuse sources, such as planetary nebulae and starburst superwinds, in which a global flow interacts with material evaporating or being ablated from the surface of globules of cool, dense gas. 相似文献