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I. V. Moskalenko

Publications and source records attributed to I. V. Moskalenko.

141 records · Page 8Linked to original sources
arXiv1999

Galactic cosmic rays & gamma rays: a synthesis

We have developed a model which aims to reproduce observational data of many kinds related to cosmic-ray origin and propagation: direct measurements of nuclei, antiprotons, electrons and positrons, gamma-rays, and synchrotron radiation. Our main results include evaluation of diffusion/convection and reacceleration models, estimates of the halo size, calculations of the interstellar positron and antiproton spectra, evaluation of alternative hypotheses of nucleon and electron interstellar spectra, and computation of the Galactic diffuse gamma-ray emission. We show that combining information from classical cosmic-ray studies with gamma-ray and other data leads to tighter constraints on cosmic-ray origin and propagation.

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arXiv1998

Galactic cosmic rays and gamma rays: a unified approach

We are constructing a model which aims to reproduce observational data of many kinds related to cosmic-ray origin and propagation: direct measurements of nuclei, antiprotons, electrons and positrons, gamma-rays, and synchrotron radiation. These data provide many independent constraints on any model. Propagation of primary and secondary nucleons, primary and secondary electrons and positrons are calculated self-consistently. Fragmentation and energy losses are computed using realistic distributions for the interstellar gas and radiation fields, and diffusive reacceleration is also incorporated. The models are adjusted to agree with the observed cosmic-ray B/C and 10Be/9Be ratios. Our main results include evaluation of diffusion/convection and reacceleration models, estimates of the halo size, calculations of the interstellar positron and antiproton spectra, evaluation of alternative hypotheses of hard nucleon and hard electron interstellar spectra, and computation of the Galactic diffuse gamma-ray emission.

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arXiv1998

Puzzles of Galactic continuum gamma rays

Inverse Compton scattering appears to play a more important role in the diffuse Galactic continuum emission than previously thought, from MeV to GeV energies. We compare models having a large inverse Compton component with EGRET data, and find good agreement in the longitude and latitude distributions at low and high energies. We test an alternative explanation for the >1 GeV gamma-ray excess, the hard nucleon spectrum, using secondary antiprotons and positrons. At lower energies to fit the COMPTEL and OSSE data as diffuse emission requires either a steep upturn in the electron spectrum below 200 MeV or a population of discrete sources.

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arXiv1998

A pair plasma model for PKS 0208-512

Assuming that the enhanced MeV emission from the MeV blazar PKS 0208-512 observed by COMPTEL on occasions is due to annihilation in a pair plasma, we estimate parameters of the annihilation region: its size, the number density of particles, and the variability timescale. The values we find are in accord with our present-day knowledge of emission mechanisms operating in AGNs. The constructed model implies that the blueshifted annihilation emission is an intrinsic property of all AGN jet models, and predicts an anticorrelation between the high-energy emission and the annihilation flux.

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arXiv1998

Deciphering diffuse Galactic continuum gamma rays

Inverse Compton scattering appears to play a more important role in the diffuse Galactic continuum emission than previously thought, from MeV to GeV energies. We compare models having a large inverse Compton component with EGRET data, and find good agreement in the longitude and latitude distributions at low and high energies. We test an alternative explanation for the >1 GeV gamma-ray excess, the hard nucleon spectrum, using secondary antiprotons and positrons.

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arXiv1998

New constraints on Galactic cosmic-ray propagation

An extensive program for the calculation of galactic cosmic-ray propagation has been developed. Primary and secondary nucleons, primary and secondary electrons, secondary positrons and antiprotons are included. Fragmentation and energy losses are computed using realistic distributions for the interstellar gas and radiation fields. Models with diffusion and convection only do not account naturally for the observed energy dependence of B/C, while models with reacceleration reproduce this easily. The height of the halo propagation region is determined, using recent 10Be/9Be measurements, as greater than 4 kpc. The radial distribution of cosmic-ray sources required is broader than current estimates of the SNR distribution for all halo sizes. Our results include an estimate of cosmic-ray antiproton and positron spectra, and the Galactic diffuse gamma-ray emission (see accompanying paper: Moskalenko 1998b).

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arXiv1998

Diffuse Galactic gamma rays, cosmic-ray nucleons and antiprotons

The excess of continuum gamma-ray emission from the Galaxy above 1 GeV is an unsolved puzzle. It may indicate that the interstellar nucleon or electron spectra are harder than local direct measurements, as could be the case if a local source of cosmic rays were to dominate the nearby flux. It is however difficult to distinguish between the two cases. Cosmic-ray secondary antiprotons provide a way to resolve this issue. We have made a calculation of the cosmic-ray secondary antiproton spectrum in our model, which computes self-consistently propagation of primary and secondary nucleons, and electrons. Fragmentation and energy losses are computed using realistic distributions for the interstellar gas and radiation fields, and diffusive reacceleration is also incorporated. Our study shows that accurate measurements of the antiproton flux, especially at high energies, could provide a diagnostic of the interstellar nucleon spectrum allowing us to test the hard nucleon spectrum hypothesis. Present antiproton data above 3 GeV indicate that it can already be excluded at the few sigma level.

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arXiv1998

Propagation of cosmic-ray nucleons in the Galaxy

We describe a method for the numerical computation of the propagation of primary and secondary nucleons, primary electrons, and secondary positrons and electrons. Fragmentation and energy losses are computed using realistic distributions for the interstellar gas and radiation fields, and diffusive reacceleration is also incorporated. The models are adjusted to agree with the observed cosmic-ray B/C and 10Be/9Be ratios. Models with diffusion and convection do not account well for the observed energy dependence of B/C, while models with reacceleration reproduce this easily. The height of the halo propagation region is determined, using recent 10Be/9Be measurements, as >4 kpc for diffusion/convection models and 4-12 kpc for reacceleration models. For convection models we set an upper limit on the velocity gradient of dV/dz < 7 km/s/kpc. The radial distribution of cosmic-ray sources required is broader than current estimates of the SNR distribution for all halo sizes. Full details of the numerical method used to solve the cosmic-ray propagation equation are given.

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arXiv1998

A combined model for the X-ray to gamma-ray emission of Cyg X-1

We use recent data obtained by three (OSSE, BATSE, and COMPTEL) of four instruments on board the Compton Gamma Ray Observatory, to construct a model of Cyg X-1 which describes its emission in a broad energy range from soft X-rays to MeV gamma-rays self-consistently. The gamma-ray emission is interpreted to be the result of Comptonization, bremsstrahlung, and positron annihilation in a hot optically thin and spatially extended region surrounding the whole accretion disk. For the X-ray emission a standard corona-disk model is applied. We show that the Cyg X-1 spectrum accumulated by the CGRO instruments during a ~4 year time period between 1991 and 1995, as well as the HEAO-3 gamma1 and gamma2 spectra can be well represented by our model. The derived parameters match the observational results obtained from X-ray measurements.

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arXiv1997

Production and propagation of cosmic-ray positrons and electrons

We have made a new calculation of the cosmic-ray secondary positron spectrum using a diffusive halo model for Galactic cosmic-ray propagation. The code computes self-consistently the spectra of primary and secondary nucleons, primary electrons, and secondary positrons and electrons. The models are first adjusted to agree with the observed cosmic-ray Boron/Carbon ratio, and the interstellar proton and Helium spectra are then computed; these spectra are used to obtain the source function for the secondary positrons/electrons which are finally propagated with the same model parameters. The primary electron spectrum is evaluated, again using the same model. Fragmentation and energy losses are computed using realistic distributions for the interstellar gas and radiation fields, and diffusive reacceleration is also incorporated. Our study includes a critical re-evaluation of the secondary decay calculation for positrons. The predicted positron fraction is in good agreement with the measurements up to 10 GeV, beyond which the observed flux is higher than that calculated. Since the positron fraction is now accurately measured in the 1-10 GeV range our primary electron spectrum should be a good estimate of the true interstellar spectrum in this range, of interest for gamma ray and solar modulation studies. We further show that a harder interstellar nucleon spectrum, similar to that suggested to explain EGRET diffuse Galactic gamma ray observations above 1 GeV, can reproduce the positron observations above 10 GeV without requiring a primary positron component.

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arXiv1997

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.

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arXiv1997

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.

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arXiv1997

Numerical models for cosmic ray propagation and gamma ray production

An extensive program for the calculation of galactic cosmic-ray propagation has been developed. Primary and secondary nucleons, primary and secondary electrons, and secondary positrons are included. The basic spatial propagation mechanisms are (momentum-dependent) diffusion, convection, while in momentum space energy loss and diffusive reacceleration are treated. Fragmentation and energy losses are computed using realistic distributions for the interstellar gas and radiation fields. Predictions of the diffuse Galactic gamma ray and synchrotron emission are made. Our new estimate of the cosmic-ray positron spectrum is compared with the latest experimental data.

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arXiv1997

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.

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arXiv1997

Positron propagation in semi-relativistic plasmas: particle spectra and the annihilation line shape

By solving the Fokker-Planck equation directly we examine effects of annihilation, particle escape and injection on the form of a steady-state positron distribution in thermal hydrogen plasmas with $kT<m_ec^2$. We show that the escape of positrons in the form of electron-positron pairs and/or pair plasma has an effect on the positron distribution causing, in some cases, a strong deviation from a Maxwellian. Additionally, we present accurate formulas for energy exchange rates, and losses due to Coulomb scattering, bremsstrahlung, and Compton scattering in the Klein-Nishina regime. Suggesting that annihilation features observed by SIGMA telescope from Nova Muscae and the 1E 1740.7-2942 are due to the positron/electron slowing down and annihilation in thermal plasma, the electron number density and the size of the emitting regions have been estimated. We show that in the case of Nova Muscae the observed radiation is coming from a pair plasma stream ($n_{e^+}\approx n_{e^-}$) rather than from a gas cloud. We argue that two models are probably relevant to the 1E 1740.7-2942 source: annihilation in (hydrogen) plasma $n_{e^+}\la n_{e^-}$ at rest, and annihilation in the pair plasma stream, which involves matter from the source environment.

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