arXiv:0804.1774v1 [-ph] 10 Apr 2008 fcniumhr -and X- hard continuum of ucl ta.19;Wrale l 92,adms re- The and 1986; XMM-Newton. 1982), al. et and al. Koyama et Chandra Worrall by 2001; 1996; cently 1999, al. et al. Purcell et (Kinzer RXTE, , OSSE ASCA, (ASTRO-B), observed Tenma 1972), been HEAO-1, al. subsequently by et has (Bleach emission 1972 interstellar and in discovered was emission ray etdi h adXryand X-ray hard the in pected its to respect 2008) with in forward experiments launched leap these predecessor. significant of be Each a (to represents 2007). since LAT Ritz orbit GLAST 2007; (in (Michelson the INTEGRAL is for emission and diffuse performed objective 2002) Galactic study be The to major studies a detail. With such greater allow much to in qual- (1991–2000). sufficient data CGRO in was improvement ity the the EGRET on and COMPTEL EGRET COMP- and and (1975–1982) fol- TEL COS-B 1968, 1972, in in satellite SAS-2 by OSO-III lowed the with started servations 06.Frtennpstoimcnium adX- hard continuum, non-positronium the Y¨uksel For & (Beacom (continuum) inter- range flight MeV the 2006). few in the with annihilation in nuclei contribute Positron pions may CR neutral gas. of of stellar decay interactions (CR) via by cosmic-ray and produced positrons, from and bremsstrahlung electrons (IC) inverse-Compton and positronium), scattering interme- of (through formation annihilation diate positron of processes ical tnodUiest,Safr,C 94309 CA Stanford, University, Stanford h aatcrdei nw ob nitnesource intense an be to known is ridge Galactic The otnu msino iue neselrntr sex- is nature interstellar diffuse, of emission Continuum rpittpstuigL using 2018 typeset 24, Preprint October version Draft 1 NES OPO RGNO H ADXRYADSF AM-A E -RAY SOFT AND X-RAY HARD THE OF ORIGIN COMPTON INVERSE loKviIsiuefrPril srpyisadCosmolo and Astrophysics Particle for Institute Kavli Also n oetal pn e idwo aatccsi rays. cosmic Galactic on window myst headings: new long-standing Subject a a solve opens model may GALPROP This potentially sec the and field. and uses radiation prediction electrons interstellar the The primary of emission. cosmic-ray the Both to contribute medium. the interstellar on instrument the SPI the using ratory ridge, Galactic the eetr-eemnto ftenntemlcmoeto the of component non-thermal the of re-determination recent A ITGA)Osraoy ssont ewl erdcda inver as reproduced well be to shown is Observatory, (INTEGRAL) a-lnkIsiu ¨retaersrsh hsk Pos Physik, f¨ur extraterrestrische Max--Institut 1. A at rzIsiuefrPril hsc,Uiest fCa of University Physics, Particle for Institute Cruz Santa T introduction E asnEprmna hsc aoaoy tnodUnivers Stanford Laboratory, Physics Experimental Hansen tl mltajv 3/25/03 v. emulateapj style X lmnayprils—csi as—gmary:theory gamma-rays: — rays cosmic — particles elementary γ ERCR,9A.d ooe oh,308Tuos ee 4 F 04, Cedex Toulouse 31028 Roche, Colonel du Av. 9 CESR-CNRS, ryeiso.Tehr X- hard The emission. -ray γ ryrgm rmtephys- the from regime -ray nrwW togadEeaOrlando Elena and Strong W. Andrew rf eso coe 4 2018 24, October version Draft γ grV Moskalenko V. Igor ryob- -ray AATCRIDGE GALACTIC ryA Porter A. Troy ABSTRACT .Bouchet L. gy, and NEntoa am-a srpyisLabo- Astrophysics Gamma-Ray INTErnational eatiue oapplto fsucstowa to truly weak be general, too not In sources would hence of and interstellar. population individually, a detected be to attributed be uncertain also is 2000). emission al. the et of (Strong origin At the 2000b). the energies al. MeV or et supernovae (Valinia turbulence in com- interstellar pro- accelerated ambient nonthermal e.g. electrons been and (see have from thermal ponents models large incorporate Composite which unacceptably posed 2002a). is al. luminosity which et a Dogiel electrons imply CR emission in bremsstrahlung from rays inmdl a encridotb toge l (2000, al. et Strong by out carried been has models tion aibe;seas envsv&Sznv(07 and (2007) Galactic Sazonov & (2007). Revnivtsev al. cataclysmic et also magnetic Revnivtsev Galac- see mainly a by all sources, variables; for keV of accounted 50 population be below tic can emission that “diffuse” convincingly and the argue INTEGRAL/IBIS data, using data, re- PCA (2007), RXTE RXTE more al. with et However, the (2006), Krivonos sources, and al. nature. Galactic et in by Revnivtsev cently for extragalactic and accounted being diffuse, be rest probably the can of 9% is 80% only emission that of 2004) analysis X-ray al. an Galactic-ridge et from compo- (Hands claimed diffuse has data been truly 2006) XMM-Newton has a it 2001, of Similiarly, existence (2– al. nent. the et X-rays prove (Ebisawa to Chandra in claimed 1997). with imaging al. X- keV) et high-resolution 10 For (Yamasaki Ginga recently, and ASCA More issue. by 1997) made al. was this et area this (Kaneda clarify in progress to important rays, resolution spatial quate lentvl,teoii fterdeeiso could emission ridge the of origin the Alternatively, thge nris netniesuyo h diffuse the of study extensive an energies, higher At fc 32 -54 acig Germany Garching, D-85741 1312, tfach 1 γ iona at rz A95064 CA Cruz, Santa lifornia, adXryt soft to X-ray hard ryeiso ntecneto Rpropaga- CR of context the in emission -ray t,Safr,C 94305 CA Stanford, ity, r fteoii fti emission, this of origin the of ery nayeetosadpositrons and electrons ondary n nldsanwcalculation new a includes and eCmtneiso from emission se-Compton rance γ rytlsoe aeinade- have telescopes -ray γ ryeiso from emission -ray ISO RMTHE FROM MISSION 2 Porter et al.

2004b). This study confirmed that models based on use the GALPROP model together with a new model for locally measured electron and nuclei spectra and syn- the Galactic interstellar radiation field (ISRF) to solve a chrotron constraints are consistent with γ-ray measure- long-standing mystery of the origin of the hard X-ray ments in the 30 MeV – 500 MeV range; outside this emission – IC emission from CR electrons and positrons range deviations from the data are apparent. The puz- – and to build a model of the Galactic diffuse emission in zling excess in the EGRET diffuse emission data above the energy range from keV to TeV energies, thus covering 1 GeV relative to that expected (Hunter et al. 1997; more that 10 orders of magnitude in energy. Strong et al. 2000) has shown up in all models that Primary CR electrons are directly accelerated in CR are tuned to be consistent with the locally measured sources like supernova remnants or pulsars. Secondary CR nuclei and electron spectra (Moskalenko et al. 2004; electrons and positrons are produced via interactions of Strong et al. 2004b). The excess has shown up in all di- energetic nuclei with interstellar gas, and are usually con- rections, not only in the Galactic plane. This implies that sidered a minor component of CRs. This is indeed the the GeV excess is not a feature restricted to the Galactic case in the heliosphere where the positron to all-electron + + − ridge or the gas-related emission. A simple re-scaling of ratio is small at all energies, e /(e + e )tot ∼ 0.1. the components (π0-decay, IC, bremsstrahlung) does not However, as we will show, the combined secondary elec- improve the fit in any region, since the observed peak tron/positron flux in the interstellar medium (ISM) is is at an energy higher than the π0-peak. For recent re- more than half of the primary CR electron flux at ∼1 views see Moskalenko et al. (2004), Strong et al. (2007), GeV energies and below. This leads to a considerable and references therein. contribution by secondary positrons and electrons to the Assuming that the GeV excess is not an instrumen- diffuse γ-ray flux via IC scattering and bremsstrahlung tal artefact2, the so-called “optimised model,” which ex- and significantly, by up to a factor of ∼2, increases the plains the GeV excess in terms of CR intensity varia- flux of diffuse Galactic emission below ∼100 MeV. Sec- tions in the Galaxy, has been proposed by Strong et al. ondary positrons and electrons are, therefore, directly (2004b). It reproduces the spectrum of the diffuse γ-rays seen in hard X-rays and γ-rays. in all directions, as well as the latitude and longitude profiles for the whole EGRET energy range 30 MeV – 50 2. galprop code GeV at the cost of relaxation of the restrictions imposed The GALPROP code (Strong & Moskalenko 1998) was by the measurements of local CR proton and electron created to enable simultaneous predictions of all rel- spectra. At lower energies, the predictions of this model evant observations including CR nuclei, electrons and have never been tested because of the lack of good data. positrons, γ-rays and synchrotron radiation. The study of the Galactic-ridge continuum X-ray emis- We give a very brief summary of GALPROP; for sion is a key goal of the INTEGRAL mission. The details we refer the reader to the relevant papers high spectral resolution combined with its imaging ca- (Moskalenko & Strong 1998, 2000; Moskalenko et al. pabilities promises new insights into the nature of this 2002; Ptuskin et al. 2006; Strong & Moskalenko 1998; enigmatic radiation. Previous work based on initial, Strong et al. 2000, 2004b) and the dedicated website3. smaller sets of INTEGRAL/SPI observations have re- The GALPROP code solves the CR transport equation ported the detection of diffuse emission at a level con- with a given source distribution and boundary condi- sistent with previous experiments (Strong et al. 2003; tions for all CR species. This includes a galactic Strong 2003). However, statistical and systematic errors (convection), diffusive reacceleration in the ISM, energy were large, due in part to the uncertainty in the point- losses, nuclear fragmentation, radioactive decay, and pro- source contribution. Meanwhile, a new analysis of IN- duction of secondary particles and isotopes. The nu- TEGRAL/IBIS data (Lebrun et al. 2004; Terrier et al. merical solution of the transport equation is based on 2004) showed that, up to 100 keV, indeed a large frac- a Crank-Nicholson (Press et al. 1992) implicit second- tion of the total emission from the inner Galaxy is due order scheme. The spatial boundary conditions assume to sources. Strong et al. (2004b) used the source cat- free particle escape. Since the grid involves a 3D (R,z,p) alogue from this work (containing 91 sources) as input or 4D (x,y,z,p) problem (spatial variables plus momen- to SPI model fitting, giving a more solid basis for the tum) “operator splitting” is used to handle the implicit contribution of point sources in such an analysis. This solution. For a given halo size the diffusion coefficient, exploited the complementarity of the instruments on IN- as a function of momentum and the reacceleration or TEGRAL for the first time in the context of diffuse emis- convection parameters, is determined by the boron-to- sion. The SPI analysis by Bouchet et al. (2005) gave carbon ratio data. If reacceleration is included, the a rather lower 50–1000 keV power-law continuum than momentum-space diffusion coefficient Dpp is related to δ Strong et al. (2005), but the errors were large in the early the spatial coefficient Dxx (= βD0ρ ) (Berezinskii et al. datasets. Now, a new analysis (Bouchet et al. 2008) with 1990; Seo & Ptuskin 1994), where δ = 1/3 for a Kol- 3 times as much SPI exposure gives better statistics, mogorov spectrum of interstellar turbulence or δ = 1/2 background handling, and point-source subtraction. for a Kraichnan cascade, ρ is the magnetic rigidity, In the present paper we focus on energies above 50 keV D0 is a constant, and β = v/c. Production of sec- where sources do not appear to be important because of ondary positrons and electrons is calculated using a for- the rapid cut-off in the spectra of the majority, and the malism described in Moskalenko & Strong (1998) with relatively small number of hard-spectrum sources. We a correction by Kelner et al. (2006). The γ-rays are calculated using the propagated CR distributions, in- 2 A discussion of uncertainties and possible sources of error asso- ciated with determining the diffuse Galactic γ-ray emission using cluding a contribution from secondary particles such EGRET data is available in Moskalenko et al. (2007) 3 http://galprop.stanford.edu IC Origin of the Galactic Ridge Emission 3

10 10

) 1 ) 1 -1 -1 m m µ µ

-3 -3

10-1 10-1 m eV cm m eV cm µ µ ( ( λ λ u u λ λ

10-2 10-2

10-1 1 10 102 103 10-1 1 10 102 103 λ (µm) λ (µm) Fig. 1.— Spectral energy distribution of the MW ISRF in the Galactic plane. Line colouring: black, R = 0 kpc; blue, R = 4 kpc; red, R = 8 kpc; magenta, R = 12 kpc. Left: maximum metallicity gradient; Right:, minimum metallicity gradient. The cosmic microwave background (CMB) is included in both figures and dominates the SED for wavelengths λ & 600 µm. as positrons and electrons from inelastic processes in output in standard astronomical formats for comparison the ISM that increases the γ-ray flux at MeV ener- with data. Recent extensions to GALPROP include gies (Strong et al. 2004b). Gas-related γ-ray intensi- non-linear wave damping (Ptuskin et al. 2006) and a ties are computed from the emissivities as a function of dark matter package to allow for the propagation of (R,z,Eγ) using the column densities of H i and H2 for WIMP annihilation products and calculation of the galactocentric annuli based on 21-cm and CO surveys corresponding synchrotron and γ-ray skymaps; an included in the GALPROP model. Neutral pion pro- interface between GALPROP and the DarkSUSY code duction is calculated using the method given by Dermer (Gondolo et al. 2004) will be implemented in the near (1986a,b) as described in Moskalenko & Strong (1998), future to allow direct calls of GALPROP from within or using a parameterisation developed by Kamae et al. DarkSUSY. (2005); bremsstrahlung is calculated using a formalism The optimised model (Strong et al. 2004b) is used to by Koch & Motz (1959) as described in Strong et al. calculate the diffuse emission in the range 10 keV – TeV (2000). The IC scattering is treated using the appro- energies. The CR source distribution is based on the priate cross section for an anisotropic radiation field de- Galactic pulsar distribution (Lorimer 2004), while the veloped by Moskalenko & Strong (2000) using the full XCO-factors, XCO = N(H2)/WCO, are variable, increas- angular distribution of the ISRF. ing towards the outer Galaxy, and fully compatible with Cross-sections are based on the extensive LANL the expected variations based on the metallicity gradi- database, nuclear codes, and parameterisations ent and COBE data (Strong et al. 2004c). Such a model (Mashnik et al. 2004). Starting with the heaviest reproduces the diffuse Galactic γ-ray emission for the primary nucleus considered (e.g. 64Ni) the propagation whole sky as well as the radial gradient of diffuse Galac- solution is used to compute the source term for its tic γ-ray emissivity. spallation products, which are then propagated in turn, and so on down to protons, secondary electrons and 3. interstellar radiation field positrons, and antiprotons. The inelastically scat- The Galactic ISRF is the result of emission by stars, tered protons and antiprotons are treated as separate and the scattering, absorption, and re-emission of ab- components (secondary protons, tertiary antiprotons). sorbed starlight by dust in the ISM. The most de- In this way secondaries, tertiaries, etc., are included. tailed calculation to date (Strong et al. 2000), which (Production of 10B via the 10Be-decay channel is includes spatial and wavelength dependence over the important and requires a second iteration of this pro- whole Galaxy, has been widely used. The Strong et al. cedure.) GALPROP includes K-capture and electron (2000) model uses emissivities based on stellar popu- stripping processes, where a nucleus with an electron lations based on COBE/DIRBE fits by Freudenreich (H-like) is considered a separate species because of the (1998) and the SKY model of Wainscoat et al. (1992) to- difference in lifetime, and knock-on electrons. Primary gether with COBE/DIRBE derived infrared emissivities electrons are treated separately. Normalisation of (Dwek et al. 1997; Sodroski et al. 1997). Subsequent to protons, alphas, and electrons to experimental data this work new relevant astronomical information on stel- is provided (all other isotopes are determined by the lar populations, Galactic structure, and interstellar dust source composition and propagation). Gamma rays has become available, motivating a re-evaluation of the are computed using interstellar gas data (for π0-decay ISRF. We briefly describe our calculation of the ISRF; and bremsstrahlung) and the ISRF model (for IC). The further details can be found in Moskalenko et al. (2006) synchrotron emission is computed using the Galactic and Porter et al. (2006). magnetic field model. Spectra of all species on the The fundamental factors influencing the ISRF are the chosen grid and the γ-ray and synchrotron skymaps are luminosity distribution from the stellar populations of 4 Porter et al.

101 ELECTRONS 101 IS POSITRONS

❍ AMS I

-1 -1 ● CAPRICE 94 0 IS 0 sr ❑ sr 10 10 HEAT 94-95 -1 -1 ∆ MASS 91 s s -2 -2

10-1 10-1

❍ AMS I = 600 MV ∇ CAPRICE 94 Φ

Flux, MeV cm = 600 MV Flux, MeV cm -2 -2 ❑ 2 2 HEAT 94-95 10 10 Φ E E ▲ MASS 91 ● Sanriku

10-3 52±6002029RG 10-3 52±6002029RG 101 102 103 104 105 101 102 103 104 105 Kinetic energy, MeV Kinetic energy, MeV Fig. 2.— Spectra of CR electrons and positrons in the Galactic plane, as predicted by the adopted optimised GALPROP model. Left: Total (primary + secondary) and secondary electrons; Right: Secondary positrons. Interstellar spectra (IS): R = 0 kpc (red long dashes), R = 4 kpc (blue short dashes), R = 8.5 kpc (black solid), also shown modulated to 600 MV. Secondary electrons are shown separately as magenta lines (IS and modulated) on the left panel at R = 8.5 kpc. the Galaxy and the radiative transport of the star light the Galactic metallicity gradient. Estimates for the through the ISM. The interstellar dust absorbs and scat- Galactic [O/H] gradient vary in the range 0.04 − 0.07 ters the star light in the ultraviolet (UV) and optical, dex kpc−1 (Strong et al. 2004c, and references therein). and re-emits the absorbed radiation in the infrared. The variation of the metallicity gradient influences the In our model, we represent the stellar distribu- redistribution of the mainly UV and blue component of tion by four spatial components: the thin and thick the ISRF into the infrared: increased metallicity implies disc, the bulge, and the spheroidal halo. We follow more dust, which enhances the absorption of the star Garwood & Jones (1987) and Wainscoat et al. (1992) light. The variation in the infrared component affects and use a table of stellar spectral types comprising nor- the emission in the hard X-rays (see below). Therefore, mal stars and exotics to represent the luminosity function we consider two ISRFs corresponding to a maximal case (LF) for each of the spatial components. The spectral of 0.07 dex kpc−1 and a minimal case with no gradient. templates for each stellar type are taken from the semi- The ISRF is calculated for a cylindrical geometry with empirical library of Pickles (1998). The normalisations azimuthal symmetry. The maximum radial extent is per stellar type are obtained by adjusting the space densi- Rmax = 20 kpc with the maximum height above the ties to reproduce the observed LFs in the V- and K-band galactic plane zmax = 5 kpc. The radiative transport for the thin disc. The LFs for the other spatial compo- is performed using the so-called partial intensity method nents are obtained by adjusting weights per component (Baes & Dejonghe 2001; Kylafis & Bahcall 1987). for each of the stellar types relative to the normalisations Figure 1 shows the spectral energy distributions obtained for the thin disc LF. (SEDs) in the Galactic plane for selected galactocentric We assume a dust model including graphite, poly- radii for the maximal metallicity gradient (left) and no cyclic aromatic hydrocarbons (PAHs), and silicate. Dust metallicity gradient (right). An increased metallicity gra- grains in the model are spherical and the absorption dient reduces the UV in the inner Galaxy significantly – and scattering efficiencies for graphite, PAHs, and sili- by up to a factor of 3 for λ . 0.3 µm – which is redis- cate grains are taken from Li & Draine (2001). The dust tributed into the infrared. The infrared emission for the grain abundance and size distribution are taken from inner Galaxy for the maximum metallicity gradient is a Weingartner & Draine (2001) (their best fit Galactic factor of ∼ 2 higher than for the case of no metallicity model). We assume a purely neutral ISM. We consider gradient. For the outer Galaxy the ISRFs calculated for only coherent scattering, and a Henyey-Greenstein angu- the two cases differ less dramatically. For the case of the lar distribution function (Henyey & Greenstein 1941) is maximal metallicity gradient the UV emission is higher used in the scattering calculation. The stochastic heat- than the no gradient case because there is less dust in ing of grains smaller than ∼0.1 µm is treated using the the outer Galaxy. In turn, this results in less emission in “thermal continuous” approach of Draine & Li (2001); the infrared than for the maximal gradient case. we calculate the equilibrium heating of larger dust grains 4. by balancing absorption with re-emission as described by results Li & Draine (2001). We compare our results for the diffuse emission in the Dust follows the Galactic gas distribution and we inner Galaxy with the new SPI data, and COMPTEL assume uniform mixing between the two in the ISM and EGRET data. The spectrum for each instrument is (Bohlin et al. 1978). The dust-to-gas ratio scales with obtained by integrating over deconvolved skymaps. Since IC Origin of the Galactic Ridge Emission 5

galdef ID 53_6102029RH galdef ID 53_6102039RG

0.25

MeV -1 MeV -1

-1 10 -1 10 s s -1 -1 sr sr -2 -2

10-2 10-2 Intensity, cm Intensity, cm × ×

2 2 E E

10-3 10-3

10-4 10-4 10-2 10-1 1 10 102 103 104 105 106 10-2 10-1 1 10 102 103 104 105 106 Energy, MeV Energy, MeV

Fig. 3.— The spectrum of the diffuse emission for 330◦

galdef ID 53_6102029RG galdef ID 53_6202029RG

0.25

MeV -1 MeV -1

-1 10 -1 10 s s -1 -1 sr sr -2 -2

10-2 10-2 Intensity, cm Intensity, cm × ×

2 2 E E

10-3 10-3

10-4 10-4 10-2 10-1 1 10 102 103 104 105 106 10-2 10-1 1 10 102 103 104 105 106 Energy, MeV Energy, MeV

Fig. 4.— The total spectrum of the diffuse emission as calculated Fig. 5.— The spectrum of the diffuse emission as calculated in the optimised model for 330◦

galdef ID 53_6102029RG galdef ID 53_6102029RG

0.25

MeV -1 MeV -1

-1 10 -1 10 s s -1 -1 sr sr -2 -2

10-2 10-2 Intensity, cm Intensity, cm × ×

2 2 E E

10-3 10-3

10-4 10-4 10-2 10-1 1 10 102 103 104 105 106 10-2 10-1 1 10 102 103 104 105 106 Energy, MeV Energy, MeV

Fig. 6.— The spectrum of the diffuse emission as calculated in the optimised model with contribution of secondary electron and positrons for different latitude ranges. Left: Region 330◦

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