1,325 research outputs found
Observational constraints on annihilation sites in 1E 1740.7-2942 and Nova Muscae
The region of the Galactic center contains several sources which demonstrate
their activity at various wavelengths and particularly above several hundred
keV. Escape of positrons from such a source or several sources into the
interstellar medium, where they slow down and annihilate, can account for the
511 keV narrow line observed from this direction. 1E 1740.7-2942 object has
been proposed as the most likely candidate to be responsible for this variable
source of positrons. Besides, Nova Muscae shows a spectrum which is consistent
with Comptonization by a thermal plasma kT<100 keV in its hard X-ray part,
while a relatively narrow annihilation line observed by SIGMA on January 20-21,
1991 implies that positrons annihilate in a much colder medium.
We estimate the electron number density and the size of the emitting regions
suggesting that annihilation features observed by SIGMA from Nova Muscae and 1E
1740.7-2942 are due to the positron slowing down and annihilation in thermal
plasma. We show that in the case of Nova Muscae the observed radiation is
coming from a pair plasma stream, N(e+)~N(e-), rather than from a gas cloud. We
argue that two models are probably relevant to the 1E source: annihilation in
(hydrogen) plasma N(e+)<~N(e-) at rest, and annihilation in the pair plasma
stream, which involves matter from the source environment.Comment: 5 pages including 2 figures, latex, aipproc.sty, aipproc.cls,
epsfig.sty. To be published in Proc. of 4th Compton Symp., 1997 (27-30 April,
Williamsburg, Virginia
Interaction of strongly correlated electrons and acoustical phonons
We investigate the interaction of correlated electrons with acoustical
phonons using the extended Hubbard-Holstein model in which both, the
electron-phonon interaction and the on-site Coulomb repulsion are considered to
be strong. The Lang-Firsov canonical transformation allows to obtain mobile
polarons for which a new diagram technique and generalized Wick's theorem is
used. This allows to handle the Coulomb repulsion between the electrons emerged
into a sea of phonon fields (\textit{phonon clouds}). The physics of emission
and absorption of the collective phonon-field mode by the polarons is discussed
in detail. Moreover, we have investigated the different behavior of optical and
acoustical phonon clouds when propagating through the lattice. In the
strong-coupling limit of the electron-phonon interaction, and in the normal as
well as in the superconducting phase, chronological thermodynamical averages of
products of acoustical phonon-cloud operators can be expressed by one-cloud
operator averages. While the normal one-cloud propagator has the form of a
Lorentzian, the anomalous one is of Gaussian form and considerably smaller.
Therefore, the anomalous electron Green's functions can be considered to be
more important than corresponding polarons functions, i.e., pairing of
electrons without phonon-clouds is easier to achieve than pairing of polarons
with such clouds.Comment: : 28 pages, 9 figures, revtex4. Invited paper for a special issue of
Low Temperature Physics dedicated to the 20th anniversary of HTS
Designing Algorithms for Optimization of Parameters of Functioning of Intelligent System for Radionuclide Myocardial Diagnostics
The influence of the number of complex components of Fast Fourier transformation in analyzing the polar maps of radionuclide examination of myocardium at rest and stress on the functional efficiency of the system of diagnostics of pathologies of myocardium was explored, and there were defined their optimum values in the information sense, which allows increasing the efficiency of the algorithms of forming the diagnostic decision rules by reducing the capacity of the dictionary of features of recognition.The information-extreme sequential cluster algorithms of the selection of the dictionary of features, which contains both quantitative and category features were developed and the results of their work were compared. The modificatios of the algorithms of the selection of the dictionary were suggested, which allows increasing both the search speed of the optimal in the information sense dictionary and reducing its capacity by 40 %. We managed to get the faultless by the training matrix decision rules, the accuracy of which is in the exam mode asymptotically approaches the limit.It was experimentally confirmed that the implementation of the proposed algorithm of the diagnosing system training has allowed to reduce the minimum representative volume of the training matrix from 300 to 81 vectors-implementations of the classes of recognition of the functional myocardium state
Modelling cosmic rays and gamma rays in the Galaxy
An extensive program for the calculation of galactic cosmic-ray propagation
has been developed. This is a continuation of the work described in Strong &
Youssefi (1995). The main motivation for developing this code is the prediction
of diffuse Galactic gamma rays for comparison with data from the CGRO
instruments EGRET, COMPTEL, and OSSE. The basic spatial propagation mechanisms
are (momentum-dependent) diffusion, convection, while in momentum space energy
loss and diffusive reacceleration are treated. Primary and secondary nucleons,
primary and secondary electrons, and secondary positrons are included.
Fragmentation and energy losses are computed using realistic distributions for
the interstellar gas and radiation fields.
This study indicates that it is possible to construct a model satisfying a
wide range of observational constraints and provides a basis for future
developments.Comment: 5 pages including 7 figures, latex, aipproc.sty, aipproc.cls,
epsfig.sty. To be published in Proc. 4th Compton Symp., 1997 (27-30 April,
Williamsburg, Virginia). Details can be found at
http://www.gamma.mpe-garching.mpg.de/~aws/aws.htm
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