gamma-ray emission from molecular clouds: a probe of cosmic-ray origin and propagation

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  • Progress in Particle and Nuclear Physics 66 (2011) 681685

    Contents lists available at ScienceDirect

    Progress in Particle and Nuclear Physics

    journal homepage: www.elsevier.com/locate/ppnp

    Review

    Gamma-ray emission from molecular clouds: A probe of cosmic-rayorigin and propagationSabrina Casanova Max Planck Institut fr Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, GermanyRuhr Universitt Bochum, Universittsstrasse 150, 44801 Bochum, Germany

    a r t i c l e i n f o

    Keywords:Cosmic raysGamma-ray emissionCosmic-ray flux

    a b s t r a c t

    Cosmic rays up to at least 1015 eV (PeV) are believed to be emitted by Galactic sources,such as supernova remnants. However, no conclusive evidence of their acceleration hasbeen found yet. A trace of ongoing cosmic-ray acceleration is the gamma-ray emissionproduced by these highly energetic particles when they scatter off the interstellar mediumgas, mainly atomic and molecular hydrogen. Whereas the atomic hydrogen is uniformlydistributed in the Galaxy, the molecular hydrogen is usually aggregated in dense clouds,and the gamma-ray emission fromsuch clouds is particularly intense and localised. Amulti-frequency approach, which combines the data from the upcoming and future gamma-rayemissions with the data from the submillimeter and millimeter surveys of the molecularhydrogen, is therefore crucial to probe the Galactic cosmic-ray flux. In order to fully exploitthis multi-frequency approach, one needs to develop predictions of the expected emission.Here we will discuss the GeV to TeV emission from runaway CRs penetrating molecularclouds close to the young supernova remnant RX J1713-3946 and in molecular cloudsilluminated by the background cosmic-ray flux.

    2011 Published by Elsevier B.V.

    1. The standard model of cosmic rays

    Cosmic rays (CRs) are the highly energetic protons and nuclei which fill the Galaxy and carry, at least in the vicinity of theSun, as much energy per unit volume as the energy density of starlight or of the interstellar magnetic fields or the kineticenergy density of the interstellar gas. One hundred years after their discovery by the Austrian physicist Victor Hess, theorigin of cosmic rays is still unclear. Diffusive shock acceleration in supernova remnants (SNRs) is the most widely invokedparadigm to explain the Galactic cosmic-ray spectrum. Galactic SNRs provide, in fact, the necessary power to sustain theGalactic cosmic-ray population. One expects about one supernova event every 30 years, and, in order to account for theenergy density of cosmic rays (about 1 eV/cm3) and the cosmic-ray confinement time deduced from spallation, the typicalnon-thermal energy release per supernova has to be about 1050 ergs, which is about ten percent of the total energy releasedin supernova explosion [1]. This is in good agreementwith the typical amount of energy predicted to be produced during theacceleration of relativistic particles in SNR shocks. If the bulk of Galactic CRs up to at least PeV energies are indeed acceleratedin SNRs, then TeV -rays are expected to be emitted during the acceleration process CRs undergo within SNRs [2]. IndeedTeV -rays have been detected from the shells of SNRs, such as RX J1713.7-3946 [3]. However, such observations do notconstitute a definitive proof that CRs are accelerated in SNRs, since the observed emission could be produced by energeticelectrons up scattering low energy photon fields.

    The direct observation of cosmic rays from the candidate injection sites such as supernova remnants is not possible sinceCRs escape the acceleration sites and eventually propagate into the Galactic magnetic fields. CR secondary data suggest that

    Corresponding address: Max Planck Institut fr Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany.E-mail address: Sabrina.Casanova@mpi-hd.mpg.de.

    0146-6410/$ see front matter 2011 Published by Elsevier B.V.doi:10.1016/j.ppnp.2011.01.029

  • 682 S. Casanova / Progress in Particle and Nuclear Physics 66 (2011) 681685

    cosmic protons and nuclei diffuse in the magnetic fields for timescales of the order of about tescape 107(

  • S. Casanova / Progress in Particle and Nuclear Physics 66 (2011) 681685 683

    1e-16

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    Fig. 1. The -ray energy flux in four different regions of 0.2 0.2 around the positions a = (346.8,0.4), b = (346.9,1.4), c = (347.1,3.0)and d = (346.2, 0.2). The emission produced along the line of sight distance between 900 and 1100 parsecs is plotted in dashed lines, while the emissionobtained by summing the radiation contributions over the whole line of sight distance, from 50 parsecs to 30 000 parsecs, is shown in solid lines. Theemission in the panels a, b, c and d, corresponding to the different locations, is plotted for different diffusion coefficientsD0 . The emission from backgroundCRs is also shown in each panel for comparison. The contribution to the emission from inverse Compton scattering of background electrons is indicatedwith a dashed light blue line. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

    Fig. 2. Ratio of the emission due to the sum of background CRs and runaway CRs and background CRs only. The SNR is supposed to have exploded at 1kpc distance from the Sun at 347.3 longitude and0.5 latitude 1600 years ago and to have started injecting the most energetic protons 100 years afterthe explosion. The diffusion coefficient assumed within the region 340 < l < 350 and5 < b < 5 is 1026 cm2/s in the left panel, 1027 cm2/s in themiddle panel and 1028cm2/s in the right panel. In the three panels the ratio of the emission is shown for different energies from 1 GeV to 10 TeV.

    dominated by the contribution from IC scattering of background electrons, almost at all energies. In regions closer to theGalactic Plane the emission from inverse Compton scattering of background electrons is subdominant at TeV energies, whererunaway cosmic rays produce the enhanced emission. Therefore the regions where to look for the emission from runawayparticles are low latitude regions of higher gas density. The -ray spectra show a peculiar concave shape, being soft at lowenergies and hard at high energies, which, as discussed in [9],might be important for the studies of the spectral compatibilityof GeV and TeV gamma-ray sources. The peculiar spectral and morphological features of the -ray due to runaway CRs canbe therefore revealed by combining the spectra and -ray images provided by the Fermi and Agile telescopes at GeV and bypresent and future ground based detectors at TeV energies. As shown by the surveys of the Galaxy, published by Fermi atMeVGeV energies, by HESS at TeV energies and at very high energies by the Milagro Collaboration, the various extendedGalactic sources differ in spectra, flux and morphology. However, there is growing evidence for the correlation of GeV andTeV energy sources. These sources appear often spectrally and morphologically different at different energies, possibly duenot only to the better angular resolution obtained by the instruments at TeV energies, but also to the energy dependence ofphysical processes, such as CR injection and CR diffusion. For this reason it is important to properly model what we expectto observe at different energies by conveying in a quantitative way all information by recognizing that the environment,the source age, the acceleration rate and history, all play a role in the physical process of injection and all have to be takeninto account for the predictions. Fig. 2 shows the ratio of the hadronic gamma-ray emission due to total CR spectrum tothat of the background CRs for the entire region under consideration. In our modeling only CRs with energies above about100 TeV have left the acceleration site and the morphology of the emission depends upon the energy at which one observesthe hadronic gamma-ray emission. The different spatial distribution of the emission is also due to the different energy-dependent diffusion coefficients, assumed in the three different panels, making it into a useful tool to investigate the highlyunknown CR diffusion coefficient [20,22].

  • 684 S. Casanova / Progress in Particle and Nuclear Physics 66 (2011) 681685

    3. On the level of the CR background

    The cosmic-ray flux measured close to the Earth, with its characteristic soft 2.7 spectrum, is usually assumed to berepresentative of the average cosmic-ray flux throughout the Galaxy. By combining the locally observed spectrum and thepropagation effects, one would conclude that the injection spectra from individual sources have a power-law slope close to2, in accordancewith the predictions of the diffusive shock acceleration theory. However, on the basis of the state of the artknowledge of CR propagation processes, a somehow steeper source spectrum up to2.4 cannot be excluded assuming thatCRs might re-accelerate in the Galaxy [2]. Also, for protons and nuclei a good working hypothesis to