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Overview Antiprotons Diffuse Emission Elementary Processes Electrons and Positrons Extrgalactic Background Interstellar Radiation Field Magnetic Fieldsnew Nucleons in Cosmic Rays Numerical Scheme Propagation Reviews Sources Wave-Particle Interactions WebRun Paper Download PDF Supplementary Material

Recent results

An overview of our previous results is given in Strong & Moskalenko (1999), Moskalenko & Strong (2000), and details of the numerical method are presented in Strong & Moskalenko (1998). See also GALPROP-related talks for additional information.

PROPAGATION OF STABLE AND RADIOACTIVE NUCLEI IN COSMIC RAYS

The study of secondary nuclei allows one to determine the diffusion coefficient and the halo size. It can also serve as a probe of the models of cosmic-ray transport. We have evaluated the boron/carbon (B/C) and 10Be/9Be ratios using our GALPROP code Strong & Moskalenko (1998) in the framework of diffusion/convection and reacceleration models. It was found that simple diffusion/convection models have difficulty accounting for the observed form of the B/C ratio without special assumptions chosen to fit the data, while the analysis confirmed the conclusion of other authors that models with reacceleration account naturally for the energy dependence over the whole observed range. The Ulysses data on 10Be/9Be ratio (Connell 1998) were used to obtain limits on the halo size. The halo height was derived as 4-12 kpc. The limit on the convection velosity gradient was obtained as dV/dz < 7 km s-1 kpc-1. The gradient of protons derived from gamma rays is found smaller than expected for supernova remnant sources.

Development of a new 3D model, which employs a full nuclear reaction network, enabled us to include all nuclei H-Ni in the propagation runs (Strong & Moskalenko 2001). The focus was made on B/C and sub-Fe/Fe ratios, radioactive isotopes 10Be, 26Al, 36Cl, 54Mn, and isotopic source abundances. The conclusions were that (i) reacceleration fits B/C and sub-Fe/Fe quite well over the whole energy range, (ii) the halo size indicated by the four radioactive isotope ratios measured by ACE (Yanasak ) is in the range 3 - 7 kpc, but (iii) the dispersion between the isotopes is large presumably due to cross-section inaccuracies.

An attempt to clarify the halo size limits based on cosmic-ray measurements of 26Al, 36Cl, 54Mn was made (Moskalenko et al. 2001) using the T16 Los Alamos compilation of experimental cross sections by Mashnik () together with calculations using the improved Cascade-Exciton Model code CEM2k (cem,cem2k). Only the main production channels for isotopes of Al, Cl, Mn were chosen for the more accurate calculation. The preliminary conclusion to be drawn from all radioactive nuclei is that, at least within the context of the present propagation model, halo size is 4-6 kpc based on the ACE data (Yanasak) and Ulysses elemental abundances (Ulysses ). This is consistent with our previous result zh=3-7 kpc (Strong & Moskalenko 2001) and supports our previous conclusion that the large dispersion between the isotopes is mostly due to cross-section inaccuracies.

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SECONDARY ANTIPROTONS AND POSITRONS AS A PROBE OF PROPAGATION MODELS

Secondary antiprotons and positrons are produced in collisions of cosmic-ray nuclei with interstellar gas atoms. This is a closely related process to that where diffuse gamma rays are produced (via neutral pion decay). Therefore, accurate measurements of the antiproton and positron fluxes, especially at high energies, could provide a diagnostic of the interstellar nucleon spectrum complementary to that provided by gamma rays (Moskalenko et al. 1998, Strong et al. 2000).

The excess of continuum gamma-ray emission from the Galaxy above 1 GeV (Hunter et al. 1997) found by EGRET may indicate that the local interstellar spectra of nucleons or electrons are not representative (hard_nucleons1, hard_nucleons2, hard_electrons1, hard_electrons2), as could be the case if a local source of cosmic rays were to dominate the nearby flux. Using the antiproton and positron data available at that time (Hof et al. , Basini et al. , Barwick et al. 1998) we concluded that the hard nucleon spectrum required to match the excess in GeV gamma rays can be excluded at the few sigma level.

A new accurate calculation of the antiproton flux (Moskalenko et al. 2002) has been triggered by the new data with larger statistics on both low and high energy fluxes (Orito et al. 2000, Bergstroumlm et al. 2000, Maeno et al. 2001, Boezio et al. 2001). The distinguishing spectral shape with a maximum at 2 GeV and a sharp decrease towards lower energies makes antiprotons a unique probe of the models of particle propagation in the Galaxy and modulation in the heliosphere. Our detailed analysis shows that there is no simple model capable of accurately describing the whole variety of data: boron/carbon and sub-iron/iron ratios, spectra of protons, helium, antiprotons, positrons, electrons. The reacceleration model was shown not to reproduce primary proton and He spectra, and, more important, secondary positrons and antiprotons. A plain diffusion model (no reacceleration, no convection) has a problem with the low energy part of nuclear secondary/primary ratios and overestimates the antiproton flux. We found that only a model with a break in the diffusion coefficient plus convection can reproduce measurements of cosmic-ray species, and the reproduction of primaries (p, He) can be further improved by introducing a break in the primary injection spectra. For the best-fit model we made predictions of proton and antiproton fluxes near the Earth for different modulation levels and polarity using a steady-state drift model of propagation in the heliosphere.

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DIFFUSE GAMMA RAYS AS A PROBE OF THE LARGE-SCALE NUCLEON AND ELECTRON SPECTRA

An extensive work (Strong et al. 2000) has been devoted to study of the origin of the excess in Galactic continuum gamma-ray emission above 1 GeV (Hunter et al. 1997) found by EGRET. As mentioned before, this was taken as an indication that the local interstellar spectra of protons and/or electrons may be not representative. The study included an evaluation of the interstellar radiation field and the effect of anisotropy on the inverse Compton scattering of cosmic-ray electrons (Moskalenko & Strong 2000).

It was found (Strong et al. 2000) that models based on locally measured electron and nucleon spectra and synchrotron constraints are consistent with gamma-ray measurements only in the range 30 MeV - 500 MeV. A harder nucleon spectrum was considered but fitting to gamma rays causes it to violate limits from positrons and antiprotons. A harder interstellar electron spectrum allows the gamma-ray spectrum to be fitted above 1 GeV, and this can be further improved when combined with a modified nucleon spectrum (consistent with the limits from antiprotons and positrons). Halo sizes in the range 4-10 kpc favoured by the gamma-ray analysis are consistent with that deduced from cosmic rays. The halo contribution to the high-latitude Galactic gamma-ray emission is large, with implications for the study of the diffuse extragalactic component and signatures of dark matter.

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SECONDARY POSITRONS AND DARK MATTER

Positrons in cosmic rays supposedly consist of two components, secondary positrons, produced in cosmic-ray interactions with gas, and primary positrons, which may consist of particles accelerated by pulsars and those produced in the annihilations of hypothetical particles (WIMPs) constituting the dark matter. The calculation of secondary positrons is important as it provides a "background" in searches for primary component; secondary positrons may also serve as a diagnostic of models of galactic propagation and modulation in the heliosphere.

Our calculation of the cosmic-ray secondary positron spectrum using a diffusive halo model with reacceleration is given in (Moskalenko & Strong 1998). The same propagation model was used to make a calculation (Moskalenko & Strong 1999) of the propagation of positrons from WIMP annihilation in the Galaxy in different models of the dark matter halo distribution and derive the propagation Green's functions. Our predictions of the primary positron flux from annihilations of neutralino with mass in the range 5 - 400 GeV were compared with cosmic-ray positron spectra computed for the "conventional" cosmic-ray nucleon spectrum which matches the local measurements, and a modified spectrum constructed to match the measurements of diffuse Galactic gamma rays, antiprotons, and positrons.

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