# Arthur NikoghossianByurakan Astrophysical Observatory | BAO · Theoretical astrophysics

Arthur Nikoghossian

Doctor of Physical and Mathematical Sciences

## About

115

Publications

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383

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Citations since 2017

## Publications

Publications (115)

This is a continuation of the author’s research on the temporal variations in the spectral line profiles formed in absorbing and scattering atmospheres illuminated by non-stationary energy sources. The line radiation transfer in an atmosphere of finite optical thickness is considered under assumption of complete frequency redistribution. We treat t...

The time-dependent problem of spectral line formation in a one-dimensional semi-infinite scattering and absorbing atmosphere containing a local energy source radiating equally in both directions is considered. The energy release is assumed to be non-stationary and to be either δ(t)-shaped or of the unit jump form given by the Heaviside H(t) functio...

The solution of several one-dimensional problems in nonstationary radiative transfer at frequencies in a spectral line is given. An approach based on searching for the unknowns in the form of Neumann series expansions is applied. The evolution of the line profile formed during reflection from a semi-infinite atmosphere is studied both with coherent...

Some new results from our study of the effect of continuum scattering on different observed characteristics of spectral lines are presented. The main purpose is to study temporal variations in line profiles and some statistical averages describing the diffusion of radiation in a medium. For simplicity the one dimensional problem of the formation of...

We consider the classical time-dependent problem of diffuse reflection of the line-radiation from a semi-infinite absorbing and scattering atmosphere. By the example of the simplest 1D problem it is shown how its solution is constructed in the general case, when both the photon lifetime in the absorbed state and the time of its travel between two c...

The purpose of this review is to give a general idea of the results on theoretical astrophysics investigations carried out at the Byurakan Observatory during the period of its existence. A special place in the review is given to the research work in the Department of "Theoretical Astrophysics", founded in 1965. The review consists of two parts. The...

This is a theoretical study of changes in several characteristics of the spectra of early-type supergiants owing to scattering of continuum radiation. Thomson scattering on free electrons is treated as the scattering mechanism, although the approach used here is quite general. The reason for changes in the spectral class of a star with constant bol...

The effect of radiation scattering in continuum on the frequency distribution of the observed flux in atmospheres of supergiants of the late B and A spectral classes is treated. Thomson scattering on free electrons, which is important for hydrodynamic balance and wind dynamics in extended atmospheres of these stars, is considered as a specific scat...

The effect of radiation scattering in continuum on the spectral lines formation is treated. Compared to the early works in this direction, the conditions of emission lines appearance in the atmosphere with the temperature gradient with depth is discussed in detail. For simplicity's sake, the coherent scattering in the spectral line frequencies is a...

The inverse scalar problem of radiative transfer in a ID atmosphere is solved analytically. It is shown, in particular, that not all combinations of the reflection and transmission coefficients r, q ≤1 can be physically significant.

This is the last of a series of papers devoted to the application of group theoretical methods for astrophysical radiative transfer problems. The superdeterminant of the supermatrix-operators of the composition and translation groups for inhomogeneous absorbing and scattering media of finite thickness is proved to equal unity. The feasibility of ap...

We consider frequency and directional features of the reflectance and transmittance of an inhomogeneous scattering and absorbing atmosphere with the scattering albedo periodically varying with depth. The dependence of global optical properties of such media on optical thickness is found. Two different kind of problems are considered dependent on th...

The paper presents some new results of investigation developing the approach aimed at applying the group theory methods to radiation transfer problems. It consists of two separate parts. In the first part we derive new properties of supersymmetry of fundamental supermatrices - representations of composition and translation groups. It is shown that...

The report is a brief overview of our results in the theory of spectral line formation in atmospheres with complex fine structure. The research motivation was due to interpretation of the EUV spectrum of solar quiescent prominences observed with the SUMER spectrograph as a part of the SOHO space program. We describe the methods proposed for solving...

The model problem of the formation of spectral lines in an absorbing and scattering atmosphere of finite optical depth with developed turbulence is stated and solved. The purpose of this paper is to clarify the influence of different kinds of spatial correlated nonthermal motions on observed line profiles. The method of invariant imbedding is used;...

Luminous Blue Variables are among the brightest objects and lie at the Humphreys-Davidson empirical limit of maximum luminosity (Lamers 1986b). A distinct feature of these stars is the higher rate of mass loss and the relatively lower terminal velocity than for other stars of the same spectral type. LBV stars show moderate photometric variations wi...

The paper presents the basic ideas of pinch-model for UV Cet star flares. The observational consequences and predictions are discussed.

A group theoretical approach is developed for solving astrophysical radiative transfer problems described in a previous series of papers. Addition laws for observed radiative intensities are derived for the case in which atmospheres not only absorb and scatter radiation incident on them, but radiate themselves because of energy sources contained wi...

The operator-matrix description of radiative transfer developed in the first part of this paper is used to find the radiation field inside an inhomogeneous atmosphere. An important result along the way is separation of the variables of optical thickness and depth. The concept of a group of optical depth translation is introduced. It is shown that t...

The invariant imbedding technique is applied to the problems of radiation transfer in a plane-parallel inhomogeneous atmosphere. All the parameters which describe the elementary event of scattering and the distribution of the energy sources are allowed to vary with depth. Mathematically, the considered standard problems of the theory are reduced to...

Recent results by the author on radiative transfer problems in
inhomogeneous atmospheres are analyzed. With three of the most often
encountered astrophysical problems as examples, the advantages of this
approach, which is based on the concept of invariant embedding, are
demonstrated. Particular attention is devoted to the important role
played by t...

The objective of this review is to provide some insight into trends in the development of one of the most important fields of theoretical astrophysics – the theory of radiative transfer. The foundation of the field goes back to the pioneering works of Schuster, Schwarzschild, Milne and Eddington appearing at the beginning of the last century in con...

A new method for determining various quantities describing the radiation field in an inhomogeneous, plane-parallel atmosphere is proposed in this two-part paper. The essence of this method is the reduction of the boundary value problems which arise during the customary statement of various astrophysical problems associated with solving the radiativ...

It is shown that the group of problems in the theory of radiative transfer that are reducible to the sourcefree problem admits a class of integrals involving quadratic moments of the intensity of arbitrarily high orders. Based on a variational principle, it is found that these integrals, which include the R-integral, follow from the corresponding c...

Group theory is used to describe a procedure for adding inhomogeneous absorbing and scattering atmospheres in a one-dimensional
approximation. The inhomogeneity originates in the variation of the scattering coefficient with depth. Group representations
are derived for the composition of media in three different cases: inhomogeneous atmospheres in w...

The classical method for determining the velocities of microturbulent motions in solar prominences is generalized to account
for the possible opacity of the spectral lines. A new characteristic of a line is introduced which, for a given line formation
mechanism, can be used to determine the optical thickness of the emitting region. The method is ap...

Eclipse observations of the W-L corona show linear rays above active regions at times of solar maximum. We show that these linear rays are also observed in the field-of-view of the C2-LASCO coronagraph, in perfect correspondence with the eclipse results. A selected prominent case taken from the 2001 eclipse observation in Angola is analysed with se...

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.

In this paper, elaborating the statistical approach recently proposed by one of the present authors (A.G.N) has been elaborated for interpretation of the spatial brightness variations of solar quiescent prominences. The mean statistical characteristics of the LTE line radiation such as the mean intensity and its Relative Mean Square Deviation (RelM...

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...

This paper is devoted to a determination of the statistical mean quantities describing spectral line radiation of dynamically
active stochastic and multicomponent atmospheres. The lines in LTE are considered so that the effects of multiple scattering
can be neglected. Two types of problem are discussed. In the first it is assumed that the realizati...

The diffuse reflection of line radiation from a one dimensional semi-infinite turbulent atmosphere is examined in two limiting
regimes of micro-and macroturbulence. Ambartsumyan’s invariance principle is used to solve this problem. In addition to the
observed spectral line profile, statistical averages describing the diffusion process in the atmosp...

The formation of spectral lines in a turbulent atmosphere with a spatially correlated velocity field is examined. A new approach
for solving this problem is proposed which is not based on the Fokker-Planck formalism. The invariant imbedding method makes
it possible to reduce the problem of finding the mean radiant intensity observed in a line to so...

In the first part we used Ambartsumains method of addition of layers to show that various problems of radiative transfer in a plane-parallel inhomogeneous atmosphere may be reduced to the solution of the cauchy problems for linear differential equations. The idea of the approach is that we start with the determining of the reflection and transmissi...

The problem of finding statistical averages which characterize the radiation field in a semi-infinite atmosphere has been
discussed by the author in a series of papers. In this paper some of the results obtained there are generalized to media of
finite optical thickness. The average number of scattering events undergone by a photon during its rando...

This paper reports on the first measurements of the color index of the coronal suprathermal streams. Observations performed during the total eclipse of 2001 in Angola imply that the color index of streams differs markedly from that for adjacent parts of the surrounding corona taken as reference. The registered drifts to the longwave and shortwave d...

This part of a series of papers examines the more general problem in which it is assumed that the fluctuations in the intensity of radiation emerging from a medium are caused by random variations in both the optical thickness of the structural elements and the power of the energy sources contained in them. The frequency dependence of the relative m...

We use Ambartsumian's method of addition of layers to show that various problems, including the standard ones, of radiation transfer in a plane-parallel inhomogeneous atmosphere may be reduced to the solution of the Cauchy problems for linear differential equations. This allows avoiding the known difficulties arising in solving the boundary-value p...

The approach proposed in the previous parts of this series of papers is used to solve the radiative transfer problem in scattering and absorbing multicomponent atmospheres. Linear recurrence relations are obtained for both the reflectance and transmittance of these kinds of atmospheres, as well as for the emerging intensities when the atmosphere co...

This paper is a continuation of a study of radiative transfer in one-dimensional inhomogeneous atmospheres. Two of the most important characteristics of multiple scattering in these media are calculated: the photon escape probability and the average number of scattering events. The latter is determined separately for photons leaving the medium and...

This series of papers is devoted to multiple scattering of light in plane parallel, inhomogeneous atmospheres. The approach proposed here is based on Ambartsumyan's method of adding layers. The main purpose is to show that one can avoid difficulties with solving various boundary value problems in the theory of radiative transfer, including some sta...

The paper aims at revealing the role of the Compton effect when considering the W-L scattering of the solar photospheric radiation by coronal supra-thermal streams. We first document several examples of well observed cases of linear W-L coronal threads extending above flaring active regions which are good candidates to give the signature known from...

This is the second part of the series of two articles which consider the effect of Compton scattering of the photospheric radiation on fast electrons of the coronal suprathermal streams. As compared to the previous part, a more realistic height-dependent model problem is treated. The results of numerical calculations for the mean frequency change a...

Linear structures of the Solar Corona seen on eclipse images above active regions after the flaring activity are good candidates for being produced by primary supra-thermal streams of electrons. We propose a method to compute the Compton scattering effect applied to this case and compute the corresponding diagrams. This predicted effect will be sea...

This is the first part of a paper devoted to a theoretical investigation of intensity fluctuations of radiation at the frequencies of a spectral line formed in a multicomponent stochastic atmosphere. It is assumed that the optical depth of structural elements and the power of the energy sources contained in them undergo random variations. The frequ...

This is the first part of a series of two articles aimed at revealing the role of the Compton effect in scattering of the solar photospheric radiation by coronal suprathermal streams. The simplest situation of a single beam of electrons gyrating around the strength lines of magnetic field is considered. Attention is focused on the height-independen...

This paper is devoted to a determination of the dependence of the filling factor and its fluctuations on the number and geometrical sizes of structural elements of the radiating medium that occupy a spectrometer's observing aperture. The consequences of the results obtained in the interpretation of observed spatial fluctuations in the surface brigh...

An analytical procedure is explored to construct the profiles of the
spectral lines formed in a stochastic multicomponent atmosphere. A model
one- dimensional problem is solved for the atmospheres in both local
thermodynamic equilibrium (LTE) and non-LTE. The results previously
obtained by the authors for the LTE atmosphere are generalized to
inclu...

The Lagrangian formalism is used to obtain nonlinear equations for frequently encountered statistical characteristics of radiative
transfer. Under the assumption of complete frequency redistribution, the average number of scatterings and the average time
of stay of a photon in a one-dimensional, semi-infinite atmosphere are analyzed. Two typical pr...

We discuss the question whether the way of finding the conservation laws based on variational formalism is applicable to the multilevel problems of radiative transfer in a homogeneous atmosphere. For expository reasons, the simplest one-dimensional model case is considered. For the special three-level problem treated in the paper the Lagrangian app...

The paper treats the quadratic and bilinear integrals of the transfer equation for the homogeneous plane-parallel atmospheres. The method for derivation of these integrals differs from that developed previously by the author with use of Ambartsumian’s invariance principle. The variational formalism is used to show that they are a direct consequence...

In this paper we continue studying the spatial brightness variations of
solar quiescent prominences. The theory is developed to determine the
mean intensity and the relative mean square deviation (RelMSD) for the
line radiation emerging from an one-dimensional atmosphere with randomly
distributed inhomogeneities. Both the local thermodynamic equili...

A statistical approach is proposed for the study of the prominence fine
structure in EUV lines (T=3D10(4) to 3x 10(5) K). For different regions
of several solar prominences, the distributions of the observed
intensities, and their averaged characteristics (mean intensity and
relative variance) are determined. We develop a statistical model,
assumin...

A statistical approach is proposed for the study of the prominence fine structure. Intensity variations in several EUV lines are measured in different regions of prominences. The expected number of fine structural elements is derived.

We consider the problem of determining the statistical properties
(intensity distribution, mean value and relative mean square deviation)
of radiation which emerges from a medium with randomly distributed
inhomogeneities when scattering is taken into account. The analytical
formalism we propose is based on Ambartsumian's method of addition of
layer...

This is the Ðrst of two papers devoted to physical interpretation of RybickiÏs quadratic and bilinear relations. The fundamental equations obtained on the basis of AmbartsumianÏs invariance principle and regarded as its extension to all depths in the atmosphere, imply that the Q and R relations have with more general structures than those known up...

The numerical solution of the idealized, but self-consistent, LTE atmosphere problem posed by the authors has been used to introduce the effect of multiple scattering on the temperature gradient, the spectral distribution of the observed flux and the intensity discontinuities. In the theoretical consideration of the problem, the nature of scatterin...

We investigate the influence of scattering processes in the continuum on the formation of lines in a static, one-dimensional atmosphere. It is shown that the Schuster mechanism of emission line formation can work under certain physical conditions. We derive formulae for the calculation of the line profiles in atmospheres with an ad hoc temperature...

The paper is devoted to the application of the variational approach to the radiative transfer problems in the one-dimensional case. To this end the results of the rigorous mathematical theory are used. It is shown that the conservation law due to the form-invariance of Lagrangian with respect to the axes translation transformation entails both the...

This paper presents some new results concerning the generalization and physical interpretation of Rybicki's quadratic and bilinear relations. The fundamental equations obtained on the basis of Ambartsumian's invariance principle and regarded as its extension to all depths in the atmosphere, imply Q- and R-relations have a more general structure tha...

In a series of papers of You, Cheng, et al. (cf., for example, [2–4]) the Cherenkov mechanism of radiation was proposed as the explanation of the wide emission lines in the spectra of quasars. This mechanism acts because the refractive index of the plasma in a certain frequency range near a spectral line can exceed one. It has been found that the r...

Luminous Blue Variables are among the brightest objects and lie at the Humphreys-Davidson empirical limit of maximum luminosity (Lamers 1986b). A distinct feature of these stars is the higher rate of mass loss and the relatively lower terminal velocity than for other stars of the same spectral type. LBV stars show moderate photometric variations wi...

The main ideas of the pinch model for UV Cet stellar flares are
summarized. The observational consequences of the model for the time
scale and energetics of the flare process, the flare maxima in optics
and X-rays, the energy spectrum of particles and the X-ray spectrum of
the nonthermal phase, the asymmetry and central depression of emission
lines...

A pinch-model of primary energy release of solar and stellar flares is suggested. This model is based on the effect of transformation of floating magnetic field energy into kinetic and thermal energy in stellar plasma atmosphere. This mechanism is similar to the energy transformation process in pinch discharge. Numerical modeling of closed magnetic...

The attempt to use the principle of invariance to solve the problem of multiple scattering of radiation by free electrons is made. The mechanism of the photon-electron interaction is discussed firstly and the expressions of the redistribution functions for various cases are given. Then the nonstationary problem of the reflection of radiation from a...

A brief review of a series of the authors' papers concerning the solution of the problems of non-coherent and non-isotropic scattering is given. The approach proposed in the general case of non-coherent scattering is based on representation of the redistribution functions in the form of bilinear expansion. Some of these expansions are obtained with...

Numerical calculations are presented for the mean number of scatterings
and the mean travel time of a photon in semiinfinite and infinite
atmospheres. It is shown that these quantities depend on the scattering
mechanism, the initial photon characteristics, and values of lambda and
beta (lambda is the probability of photon reradiation during element...

The invariance principle is used to determine the 'reflection function'
for an infinite homogeneous atmosphere in the case of incoherent
scattering. This function characterizes the angular and frequency
dependence of the intensity of the diffuse radiation in the plane of the
source. In the general case, the problem reduces to the determination of
a...

The method suggested in the previous papers, which is based on the
invariance principle and the applications of generating and
characteristic functions, is used to determine the mean time of photon
travel in a three-dimensional semi-infinite medium. Under general
assumptions concerning the elementary act of scattering new equations
and formulae are...

A new approach to the determination of the mean number of scatterings is proposed on the basis of Ambartsumian's invariance principle and systematic use of the method of generating functions. The average quantities found in the paper refer to the case when the medium is illuminated from without. Photons that perish in the medium during diffusion an...

Under general assumptions concerning the elementary act of scattering equations for determining the mean number of scatterings of photons in a three-dimensional semi-infinite medium are obtained. For illustration the case of complete frequency redistribution, including the absorption and emission in a continuum, is examined in more detail. In this...

It has been shown by Arutiunian and Nikogosian (1978) that the use of the invariance principle makes it possible to establish a relationship between spectral line profiles formed in a semiinfinite atmosphere and various distributions of internal energy sources on the one hand and line profiles calculated in the diffuse scattering problem on the oth...

The classical results of Doppler broadening of the line absorption profile are generalized to a relativistic gas in thermal equilibrium by taking into account the relativistic variance of the volume absorption coefficients of the gas, as derived by L. H. Thomas. This variance produces a small correction, even in the non-relativistic approximation.

The mechanism of Compton scattering by electrons is widely used in the interpretation of observation data on such astrophysical phenomena as the emission of quasars and sources of short X-ray bursts. The present analysis deals with the distortion of emission spectra, owing to multiple scattering by electrons. Particular attention is given to scatte...

The problem of determining the radiation field, in a medium in which the energy sources are distributed in depth according to the power law, under broad assumptions concerning the elementary scattering process, has been reduced to a solution of similar problem in an isothermal atmosphere. In addition, the radiation intensity at a specified point in...

Some relatively simple radiative-transfer problems are considered in the general case of non-coherent scattering. The approach proposed in the paper is based on representation of the redistribution function in the form of a bilinear expansion with respect to an appropriately orthonormalized set of functions. The new simple, but at the same time rat...

The problem of diffuse reflection of light from a semiinifinite medium is considered for the case of an arbitrary scattering phase function. In particular, the components of the reflection coefficient are analyzed as functions of the number of the harmonic. Results of numerical calculations are given for the special case of the Henyey-Greenstein ph...