Andrew W. Strong

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 that 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 reevaluation 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 measured accurately 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.

Cosmic-ray Propagation and Interactions in the Galaxy

We survey the theory and experimental tests for the propagation of cosmic rays in the Galaxy up to energies of 10 15 eV. A guide to the previous reviews and essential literature is given, followed by an exposition of basic principles. The basic ideas of cosmic-ray propagation are described, and the physical origin of its processes is explained. The various techniques for computing the observational consequences of the theory are described and contrasted. These include analytical and numerical techniques. We present the comparison of models with data, including direct and indirect—especially γ-ray—observations, and indicate what we can learn about cosmic-ray propagation. Some important topics, including electron and antiparticle propagation, are chosen for discussion.

DIFFUSE CONTINUUM GAMMA RAYS FROM THE GALAXY

A new study of the diUuse Galactic c-ray continuum radiation is presented, using a cosmic-ray propa- gation model which includes nucleons, antiprotons, electrons, positrons, and synchrotron radiation. Our treatment of the inverse Compton scattering includes the eUect of anisotropic scattering in the Galactic interstellar radiation —eld (ISRF) and a new evaluation of the ISRF itself. Models based on locally mea- sured electron and nucleon spectra and synchrotron constraints are consistent with c-ray measurements in the 30¨500 MeV range, but outside this range excesses are apparent. A harder nucleon spectrum is considered but —tting to c-rays causes it to violate limits from positrons and antiprotons. A harder inter- stellar electron spectrum allows the c-ray spectrum to be —tted above 1 GeV as well, and this can be further improved when combined with a modi—ed nucleon spectrum which still respects the limits imposed by antiprotons and positrons. A large electron/inverse Compton halo is proposed which repro- duces well the high-latitude variation of c-ray emission; this is taken as support for the halo size for nucleons deduced from studies of cosmic-ray composition. Halo sizes in the range 4¨10 kpc are favored by both analyses. The halo contribution of Galactic emission to the high-latitude c-ray intensity is large, with implications for the study of the diUuse extragalactic component and signatures of dark matter. The constraints provided by the radio synchrotron spectral index do not allow all of the c-ray emission at less than 30 MeV to be explained in terms of a steep electron spectrum unless this takes the form of a sharp upturn below 200 MeV. This leads us to prefer a source population as the origin of the excess low-energy c-rays, which can then be seen as a continuation of the hard X-ray continuum measured by OSSE, Ginga, and RXT E. Subject headings: cosmic raysdiUuse radiationGalaxy: generalgamma rays: observations ¨ gamma rays: theoryISM: general

SPI: The spectrometer aboard INTEGRAL

SPI is a high spectral resolution gamma-ray telescope on board the ESA mission INTEGRAL (International Gamma Ray Astrophysics Laboratory). It consists of an array of 19 closely packed germanium detectors surrounded by an active anticoincidence shield of BGO. The imaging capabilities of the instrument are obtained with a tungsten coded aperture mask located 1.7 m from the Ge array. The fully coded field-of-view is 16degrees, the partially coded field of view amounts to 31degrees, and the angular resolution is 2.5degrees. The energy range extends from 20 keV to 8 MeV with a typical energy resolution of 2.5 keV at 1.3 MeV. Here we present the general concept of the instrument followed by a brief description of each of the main subsystems. INTEGRAL was successfully launched in October 2002 and SPI is functioning extremely well.

Diffuse galactic continuum gamma rays: A model compatible with egret data and cosmic-ray measurements

We present a study of the compatibility of some current models of the diffuse Galactic continuum � -rays with EGRET data. A set of regions sampling the whole sky is chosen to provide a comprehensive range of tests. The range of EGRET data used is extended to 100 GeV. The models are computed with our GALPROP cosmic-ray propagation and � -ray production code. We confirm that the ‘‘conventional model’’ based on the locally observed electron and nucleon spectra is inadequate, for all sky regions. A conventional model plus hard sources in the inner Galaxy is also inadequate, since this cannot explain the GeVexcess away from the Galactic plane. Models with a hard electron injection spectrum are inconsistent with the local spectrum even considering the expected fluctuations; they are also inconsistent with the EGRET data above 10 GeV. We present a new model that fits the spectrum in all sky regions adequately. Secondary antiproton data were used to fix the Galactic average proton spectrum, while the electron spectrum is adjusted using the spectrum of diffuse emission itself. The derived electron and proton spectra are compatible with those measured locally considering fluctuations due to energy losses, propagation, or possibly details of Galactic structure. This model requires a much less dramatic variation in the electron spectrum than models with a hard electron injection spectrum, and moreover it fits the � -ray spectrum better and to the highest EGRET energies. It gives a good representation of the latitude distribution of the � -ray emission from the plane to the poles, and of the longitude distribution. We show that secondary positrons and electrons make an essential contribution to Galactic diffuse � -ray emission. Subject heading gs: cosmic rays — diffusion — Galaxy: general — gamma rays: observations — gamma rays: theory — ISM: general

Secondary Antiprotons and Propagation of Cosmic Rays in the Galaxy and Heliosphere

High-energy collisions of cosmic-ray nuclei with interstellar gas are believed to be the mechanism producing the majority of cosmic-ray antiprotons. Because of the kinematics of the process, they are created with a nonzero momentum; the characteristic spectral shape with a maximum at D2 GeV and a sharp decrease toward lower energies makes antiprotons a unique probe of models for particle propagation in the Galaxy and modulation in the heliosphere. On the other hand, accurate calculation of the secondary antiproton —ux provides a ii background ˇˇ for searches for exotic signals from the annihilation of supersymmetric particles and primordial black hole evaporation. Recently, new data with large statistics on both low- and high-energy antiproton —uxes have become available which allow such tests to be performed. We use our propagation code GALPROP to calculate interstellar cosmic-ray propagation for a variety of models. We show 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, and diUuse c-rays. We —nd that only a model with a break in the diUusion 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 our best-—t model we make predictions of proton and antiproton —uxes near the Earth for diUerent modulation levels and magnetic polarity using a steady state drift model of propagation in the heliosphere.

The INTEGRAL Science Data Centre (ISDC)

The INTEGRAL Science Data Centre (ISDC) provides the INTEGRAL data and means to analyse them to the scientific community. The ISDC runs a gamma ray burst alert system that provides the position of gamma ray bursts on the sky within seconds to the community. It operates a quick-look analysis of the data within few hours that detects new and unexpected sources as well as it monitors the instruments. The ISDC processes the data through a standard analysis the results of which are provided to the observers together with their data.

A new determination of the extragalactic diffuse gamma-ray background from egret data

We use the GALPROP model for cosmic-ray propagation to obtain a new estimate of the Galactic component of γ-rays and show that away from the Galactic plane it gives an accurate prediction of the observed EGRET intensities in the energy range 30 MeV to 50 GeV. On this basis we reevaluate the extragalactic γ-ray background. We find that for some energies previous work underestimated the Galactic contribution at high latitudes and hence overestimated the background. Our new background spectrum shows a positive curvature similar to that expected for models of the extragalactic emission based on the blazar population.

Radioactive 26Al from massive stars in the Galaxy.

Gamma-rays from radioactive 26Al (half-life ∼7.2 × 105 years) provide a ‘snapshot’ view of continuing nucleosynthesis in the Galaxy. The Galaxy is relatively transparent to such γ-rays, and emission has been found concentrated along its plane. This led to the conclusion that massive stars throughout the Galaxy dominate the production of 26Al. On the other hand, meteoritic data show evidence for locally produced 26Al, perhaps from spallation reactions in the protosolar disk. Furthermore, prominent γ-ray emission from the Cygnus region suggests that a substantial fraction of Galactic 26Al could originate in localized star-forming regions. Here we report high spectral resolution measurements of 26Al emission at 1808.65 keV, which demonstrate that the 26Al source regions corotate with the Galaxy, supporting its Galaxy-wide origin. We determine a present-day equilibrium mass of 2.8 (± 0.8) solar masses of 26Al. We use this to determine that the frequency of core collapse (that is, type Ib/c and type II) supernovae is 1.9 (± 1.1) events per century.

Dissipation of magnetohydrodynamic waves on energetic particles: impact on interstellar turbulence and cosmic ray transport

The physical processes involved in diffusion of Galactic cosmic rays in the interstellar medium are addressed. We study the possibility that the nonlinear MHD cascade sets the power-law spectrum of turbulence which scatters charged energetic particles. We find that the dissipation of waves due to the resonant interaction with cosmic ray particles may terminate the Kraichnan-type cascade below wavelengths 10{sup 13} cm. The effect of this wave dissipation has been incorporated in the GALPROP numerical propagation code in order to asses the impact on measurable astrophysical data. The energy-dependence of the cosmic-ray diffusion coefficient found in the resulting self-consistent model may explain the peaks in the secondary to primary nuclei ratios observed at about 1 GeV/nucleon.