Cosmic rays: interstellar gamma-ray and radio emission

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<ul><li><p>Cosmic rays: interstellar gamma-ray and radio emission</p><p>Elena Orlandoa, Andrew Strongb</p><p>aW.W. Hansen Experimental Physics Laboratory, Kavli Institute for Particle Astrophysics and Cosmology, Stanford University, Stanford, CA,94305, USA</p><p>bMax-Planck-Institut fuer extraterrestrische Physik, Postfach 1312, 85741, Garching, Germany</p><p>Abstract</p><p>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.</p><p>Keywords:Interstellar gamma rays; interstellar radio emission - Cosmic rays</p><p>1. Introduction</p><p>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. [1]). 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. [29]).</p><p>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</p><p>interstellar space. The importance of parallel studies ofradio and gamma-ray emission was recognized alreadyin the 80s, eg. in [11], [12], [23] and [25] and more re-cently in [18], [29]. 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.</p><p>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.</p><p>2. Cosmic rays throughout the Galaxy</p><p>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,</p><p>Available online at</p><p>Nuclear Physics B (Proc. Suppl.) 239240 (2013) 6469</p><p>0920-5632/$ see front matter 2013 Elsevier B.V. All rights reserved.</p><p></p><p></p></li><li><p>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.</p><p>3. Modelling interstellar emission with GALPROP</p><p>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 [15], [26], [27] and [34] 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 [21], and morerecently radio surveys and WMAP data [19, 29]. It isused in the Fermi-LAT collaboration for interpreting thephysics of the Galactic emission [6]. 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[6]. More recently, GALPROP calculation of interstel-lar synchrotron emission has been improved [29] and it</p><p>1</p><p>IR/optical Cosmic rays </p><p>Gamma rays synchrotron </p><p>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.</p><p>has been extended to include also synchrotron polariza-tion, and free-free emission and absorption [19].</p><p>In [28] 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.</p><p>4. Interstellar gamma-ray emission</p><p>Interstellar gamma-ray emission accounts for at least80% of the gamma-ray photons detected by Fermi-LAT. An early result from Fermi-LAT [1] 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[2] and [5], 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</p><p>E. Orlando, A. Strong / Nuclear Physics B (Proc. Suppl.) 239240 (2013) 6469 65</p></li><li><p>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 [3] and [4]. Modelsused Alfven velocity in the range 30-39 km s1 (i.e. see[6], [27], [28], [33], [34]). CR sources were assumedto follow supernova remnants as traced by pulsars. Apreliminary study including the Galactic plane was pre-sented in [32], 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 [6] 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 [10] and recently conrmed by [20]. 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 Galaxy, they found that the dataprefer models with a atter distribution of cosmic-raysources or greater gas density. In general, all modelswere within 15% of data, except for some regions suchas Loop I, and the structures coincident with the bub-bles found by [30], and the above-mentioned excess inthe outer Galaxy.</p><p>5. Interstellar radio emission</p><p>Diuse synchrotron radiation is the major contribu-tor to the Galactic radio emission from tens of MHz</p><p>to a few GHz. At higher frequencies and in the mi-crowave bands, free-free and dust emission tend to dom-inate. Synchrotron emission is also a foreground com-ponent for CMB studies. Recent studies of the Galacticsynchrotron emission using CR electron measurementswere published in [17, 29]. Below a few GeV the localinterstellar electron spectrum cannot be directly mea-sured, because CR electrons with energy lower than afew GeV are aected by solar modulation. The syn-chrotron spectral index depends on the CR electron andpositron spectral index, while synchrotron intensity de-pends on the B-eld intensity and electron ux. In par-ticular CR electrons from 500 MeV to 20 GeV producesynchrotron emission from tens of MHz to hundreds ofGHz for a B-eld of few G, and hence this can beused in conjunction with direct measurements to con-struct the full interstellar electron spectrum from GeVto TeV.</p><p>Using a collection of radio surveys and WMAP data,out of the Galactic plane, [29] found that the propagatedlocal interstellar electron spectrum turns over belowa few GeV, with an ambient interstellar index around2, independent of modulation. We tested propagationmodels, generated with GALPROP, in order to constrainthe injected CR electron spectrum, before propagation.We found the need for an injection electron spectral in-dex between 1.3 and 1.6 below 4 GeV (see Fig 2). Oneresult was the recognition of the importance of includ-ing secondary positrons and electrons in synchrotronmodels. While plain diusion models tted the datawell, standard reacceleration models often used to ex-plain CR secondary-to-primary ratios were not consis-tent with the observed synchrotron spectrum, since thetotal intensity from primary and secondary leptons ex-ceeded the measured synchrotron emission at low fre-quencies, as shown in Fig 2. We conrm here that plaindiusion models or diusive reacceleration models withsmall Alfven velocity are preferred to existing reaccel-eration models, as found previously in [29]. Spectraof the total synchrotron emission for high latitudes re-gions and for the case of lower Alfven velocity thanusually assumed are shown in Fig 2. Reaccelerationmodels with Alfven velocity around 20 km s1 also tthe synchrotron data. A detailed description of our re-cent improvements in the radio modelling is given in[19]. An example of our modelling of the radio emis-sion is shown on Figure 3. Here spectra of total (I) andpolarized (P) emission from radio surveys and WMAPdata are compared with our model in the inner Galaxy,showing reasonable agreement. Polarized synchrotronemission allows the regular component of the B-eld tobe studied, complementing the total synchrotron which</p><p>E. Orlando, A. Strong / Nuclear Physics B (Proc. Suppl.) 239240 (2013) 646966</p></li><li><p>frequency, MHz 1 10 210 310 410 510</p><p>-1</p><p> H</p><p>z-1</p><p> s</p><p>-1</p><p> sr</p><p>-2</p><p>inte</p><p>nsi</p><p>ty, erg</p><p> cm</p><p>-2110</p><p>-2010</p><p>-1910</p><p>-1810</p><p>-1710</p><p>-1610 galdef ID 55_z04LMPD_SUNfitE</p><p> 40.00</p></li><li><p>frequency, MHz 1 10 210 310 410 510</p><p>-1</p><p> H</p><p>z-1</p><p> s</p><p>-1</p><p> sr</p><p>-2</p><p>inte</p><p>nsi</p><p>ty, erg</p><p> cm</p><p>-2210</p><p>-2110</p><p>-2010</p><p>-1910</p><p>-1810</p><p>-1710</p><p>-1610 galdef ID 55_z04LMPD_SUNfitE</p><p> 0.25</p></li><li><p>Figure 5: An example of the modelled synchrotron spectral indexfrom 408 MHz to 23 GHz for the full sky. The map is in Galacticcoordinates.</p><p>Acknowledgments: This work makes use ofHEALPix2 described in [9]. E.O. acknowledgessupport via NASA Grant No. NNX13AH72G and NNX09AC15G.</p><p>References</p><p>[1] Abdo, A. A., Ackermann, M., Ajello, M., et al. 2009, Phys. 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