cosmic rays: interstellar gamma-ray and radio emission
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Cosmic rays: interstellar gamma-ray and radio emission
Elena Orlandoa, Andrew Strongb
aW.W. Hansen Experimental Physics Laboratory, Kavli Institute for Particle Astrophysics and Cosmology, Stanford University, Stanford, CA,94305, USA
bMax-Planck-Institut fuer extraterrestrische Physik, Postfach 1312, 85741, Garching, Germany
We discuss the relation between cosmic-ray induced interstellar gamma-ray and radio emission from our Galaxy,and emphasize the importance of their parallel study. We give an overview of results from Fermi-LAT on the inter-stellar gamma-ray emission, and then focus on complementary studies of the radio emission from the Galaxy.
Keywords:Interstellar gamma rays; interstellar radio emission - Cosmic rays
Cosmic rays (CRs) propagate in the Galaxy and inter-act with the interstellar medium (ISM). From their inter-actions with the gas and the interstellar radiation eld(ISRF), gamma-ray diuse emission is produced. Themechanisms of this diuse emission are: neutral piondecay from hadronic interactions of CR protons andheavier nuclei with gas, inverse-Compton (IC) scatter-ing from CR electron and positron interactions with theISRF and the cosmic microwaves background (CMB),and bremsstrahlung from CR electron interactions withgas (see eg. ). The same CR electrons and positronsproducing gamma rays also generate diuse radio emis-sion, via synchrotron radiation in Galactic magneticelds (B-eld). Interstellar synchrotron emission ex-tends from a few MHz to tens of GHz and is one of themajor components of radio and microwave band radia-tion. This emission depends on the CR electron spec-trum and distribution in the Galaxy, and on the B-eld(see eg. ).
Observations of gamma-ray and radio emission fromour Galaxy are an important tool for studying CR, theISRF, interstellar gas and the B-eld. Using the combi-nation of these energy bands and direct measurementsof local CRs, we can gain information on CR propaga-tion models, CR source distribution, and CR spectra in
interstellar space. The importance of parallel studies ofradio and gamma-ray emission was recognized alreadyin the 80s, eg. in , ,  and  and more re-cently in , . This approach has advantages overstudying these data separately, since the CR electronsare traced by both gamma-ray and radio emissions andso degeneracy is removed. CR electrons are measuredin direct observations, but these are aected by solarmodulation at lower energies.
We present here an overview of the latest results withFermi-LAT on the gamma-ray diuse emission inducedby CR nuclei and electrons. Then we focus on thecomplementary studies of the synchrotron emission inthe light of the latest gamma-ray results. These stud-ies made use of the numerical model of CR propagationand interactions in the Galaxy, GALPROP [14, 24], de-scribed in Section 3.
2. Cosmic rays throughout the Galaxy
CRs are believed to originate in supernovae rem-nants (SNRs). During propagation in the Galaxy, theCR spectra are steepened by energy losses and energy-dependent diusion, but can also be aected at low en-ergies by diusive reacceleration. The spectrum orig-inating in the sources is called the injected spectrum,
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the propagated spectrum pertains to interstellar space,while that measured in the Solar System is the solar-modulated spectrum. Inverse-Compton scattering, syn-chrotron radiation and ionization are responsible for en-ergy losses of electrons and positrons. Direct mea-surements of CRs by balloon experiments and satellitesconstrain models of propagation and local CR spec-tra. However solar modulation complicates the inter-pretation of the observations at energies below about 10GeV/n. An important point is that gamma-ray and syn-chrotron emission probe the interstellar CR spectra freefrom the eects of modulation.
3. Modelling interstellar emission with GALPROP
GALPROP is a software package for modelling thepropagation of CR in the Galaxy and their interactionsin the ISM. Descriptions of the GALPROP software canbe found in , ,  and  and referencestherein. This project started in the late 1990s [14, 24].Since then, it has been continuously improved and up-dated. It enables simultaneous predictions of obser-vations of CRs, gamma rays and recently synchrotronradiation [16, 19, 29]. GALPROP models were usedto analyse the diuse gamma rays detected by CGROEGRET and COMPTEL, INTEGRAL , and morerecently radio surveys and WMAP data [19, 29]. It isused in the Fermi-LAT collaboration for interpreting thephysics of the Galactic emission . For details we re-fer the reader to the relevant papers [6, 29] and the ded-icated website1. GALPROP solves the transport equa-tion for a given CR source distribution and boundaryconditions, for all required CR species. It takes into ac-count diusion, convection, energy losses, and diusivereacceleration processes. Secondary CRs produced bycollisions in the ISM and decay of radiative isotopes areincluded. The propagation equation is solved numeri-cally on a user-dened spatial grid in 2D or in 3D, andenergy grid. The solution proceeds until a steady-statesolution is obtained, starting with the heaviest primarynucleus, and then electrons, positrons, and antiprotonsare computed. GALPROP models gamma-ray emissionof the three components, pion decay, bremsstrahlungand Inverse Compton for a user-dened CR source dis-tribution and CR spectra. Gas maps and ISRF areprovided and updated to the most recent observations. More recently, GALPROP calculation of interstel-lar synchrotron emission has been improved  and it
IR/optical Cosmic rays
Gamma rays synchrotron
Figure 1: Global luminosity spectra of the Galaxy. Line styles: inter-stellar radiation eld, including optical and infrared (magenta solid),protons (red dotted line), helium (blue dotted line), primary electrons(green dotted line); CR-induced diuse emissions (solid lines): IC(green), bremsstrahlung (cyan), 0 decay (red), synchrotron (black),total (blue). CR particle luminosities are also shown as labelled.
has been extended to include also synchrotron polariza-tion, and free-free emission and absorption .
In  we used the GALPROP code to derive thebroad-band CR-induced luminosity spectrum of theGalaxy. Figure 1 shows an example of the modelledluminosity of the Galaxy. The total luminosity in syn-chrotron and inverse Compton emission are similar inmagnitude. We found the Galaxy to be a fairly good lep-ton calorimeter, when both inverse Compton and syn-chrotron losses are accounted for.
4. Interstellar gamma-ray emission
Interstellar gamma-ray emission accounts for at least80% of the gamma-ray photons detected by Fermi-LAT. An early result from Fermi-LAT  was a studyof this gamma-ray emission. This study was car-ried out using data from intermediate Galactic lati-tudes, where the interstellar emission comes mainlyfrom local CR interactions with the ISM. The spec-trum was found to be well described by pion decay,IC and bremsstrahlung, plus the contribution from pointsources and an isotropic component, which included theextragalactic background. This result was conrmed in and , where the local HI gamma-ray emissivitywas derived for the 2nd and 3rd Galactic quadrants. Thelatter studies also showed an increase in the H2-to-CO
E. Orlando, A. Strong / Nuclear Physics B (Proc. Suppl.) 239240 (2013) 6469 65
factor (XCO) with Galactocentric radius and larger in-tensities in the outer Galaxy with respect to the a priorimodel based on local CR measurements. A atter dis-tribution of CR sources in the outer Galaxy or a largeramount of gas could explain this eect. Models used forthese papers, generated with GALPROP, included diu-sive reacceleration with a 4 kpc halo height, the locally-measured CR proton and Helium spectra, and the elec-tron spectrum as measured by Fermi  and . Modelsused Alfven velocity in the range 30-39 km s1 (i.e. see, , , , ). CR sources were assumedto follow supernova remnants as traced by pulsars. Apreliminary study including the Galactic plane was pre-sented in , where the spectrum of the inner Galaxy,longitude and latitude proles were shown. The modelreproduced the Fermi data over the sky within 20%; theinner Galaxy revealed an excess at GeV energies thatcan plausibly be explained by unresolved sources. Itwas shown that latitude proles improved with a largerhalo height of about 10 kpc, consistent with the pre-vious works mentioned above. More recently  in-vestigated models of the gamma-ray diuse emission,taking into account uncertainties associated with the as-trophysical parameters. The distribution of cosmic-raysources, the size of the cosmic-ray halo, and the inter-stellar gas distribution were varied within observationallimits. They produced a grid of GALPROP models andcompared them with Fermi-LAT data for the rst 21months of observations. They found a general agree-ment between models and Fermi-LAT data, showingthat the physics of the diuse emission is broadly un-derstood. The gamma-ray ts improved using dust as atracer of gas, conrming the presence of molecular hy-drogen that is not traced by CO: the so-called dark gasas found by  and recently conrmed by . Nosingle model gave a best t over all sky regions. In gen-eral, the models were consistent with the data at highand intermediate latitudes. However, all models under-predicted the data in the inner Galaxy for energies abovea few GeV. In the outer Ga