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

A model for the high-energy emission of Cyg X-1

We construct a model of Cyg X-1 which describes self-consistently its emission from soft X-rays to MeV gamma rays. Instead of a compact pair-dominated gamma-ray emitting region, we consider a hot optically thin and spatially extended proton-dominated cloud surrounding the whole accretion disc. The gamma-ray emission is due to the bremsstrahlung, Comptonization, and positron annihilation, while the corona-disc model is retained for the X-ray emission. We show that the Cyg X-1 spectrum accumulated by OSSE, BATSE, and COMPTEL in 1991--95, as well as the HEAO-3 gamma1 and gamma2 spectra can be well fitted by our model. The derived parameters are in qualitative agreement with the picture in which the spectral changes are governed by the mass flow rate in the accretion disc. In this context, the hot outer corona could be treated as the advection-dominated flow co-existing with a standard thin accretion disc.Comment: 5 pages including 2 figures, latex, aipproc.sty, aipproc.cls, epsfig.sty. To be published in Proc. 4th Compton Symp., 1997 (27-30 April, Williamsburg, Virginia

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