2 1 0 Search for Galactic Cosmic Ray Sources with H.E.S.S. 2 n a Nukri Komin on behalf of the H.E.S.S. collaboration J LAPP, CNRS/IN2P3, 9 Chemin de Bellevue, 3 74941 Annecy-le-Vieux, France ] E Supernova remnants (SNRs) are the prime candidates for the acceleration of the Galactic H CosmicRays. TracersforinteractionsofCosmicRayswithambientmaterialaregammarays atTeVenergies,whichcanbeobservedwithgroundbasedCherenkovtelescopeslikeH.E.S.S. . h IntherecentyearsH.E.S.S.hasdetectedseveralSNRsandinteractionsofSNRswithmolecular p clouds. Herethecurrentresultsoftheseobservationsarepresentedandpossible leptonicand - hadronic scenarios are discussed. It is shown that it is likely that SNRs are the sources of o r Galactic Cosmic Rays. t s a [ 1 Introduction 1 v Since the discovery of the Cosmic Rays in 1912 their origin remains a mystery. Very promising 1 candidates for acceleration of at least the Galactic Cosmic Rays (up to ≈ 1015eV) are SNRs; it 4 1 6 has been shown that several percent of theenergy of every supernovaexplosion (of the order of 0 1051erg) can easily supply the entire power found in the Cosmic Rays. The theory of Diffusive . 1 Shock Acceleration (DSA)2 shows that particles can be accelerated up to relativistic energies 0 in shock fronts. Among others, these shock fronts can be found in the shells of SNRs. 2 1 As Cosmic Rays as charged particles are deflected in magnetic fields their direct observation : v does not carry any information on their origin. Observations of photons producedby relativistic i particles are much more promising messengers. The discovery of synchrotron emission from X SNRs shows that at least electrons and positrons are accelerated in the shells of SNRs. A tracer r a for acceleration of protons are gamma rays from hadronic interactions of relativistic protons with ambient material. The resulting photon spectrum is relatively flat and ranges from MeV to TeV energies. These gamma-rays can be observed by satellite experiments in the MeV and GeV energy range (e.g. by the Fermi/LAT) or with ground based telescopes above ≈ 100GeV. The High Energy Stereoscopic System (H.E.S.S.) is a Cherenkov telescope array located in Namibia for the observation of very heigh energy (VHE) gamma rays (≈ 100,GeV up to several 3 tens of TeV) . Due to its location in the southern hemisphere it can observe a large part of the Galactic plane. Stereoscopic observations allow a reconstruction of the photon direction with ◦ an angular resolution of better than 0.1 . The photon energies can be reconstructed with an accuracy of 25%. 2 Observations of Supernova Remnants Since the start of observations with H.E.S.S. 5 shell-type SNRs have been detected in VHE gamma rays. Four SNRs were known emitters of non-thermal X-rays (RXJ1713.7−3946, Table 1: Properties of the TeV emission of the shell-type supernova remnants (first part of the table) and SNR/Molecular Cloud interactions (second part of thetable) detected with H.E.S.S. name diam. spectral index diff. flux integral flux energy flux Φ(1TeV) [10−8 I (1−10TeV) F (1−10TeV) γ γ TeV−1m−2s−1] [m−2s−1] [ergcm−2s−1] RXJ1713.7−39465 1.2◦ 2.04±0.04a 21.3±0.5 2.63 10−7 7.5 10−11 RXJ0852−46226 2◦ 2.24±0.04 18.8±0.8 1.4 10−7 5.3 10−11 RCW867 0.9◦ 2.54±0.12 3.6±0.5 2.35 10−8 8.5 10−12 SN10068 0.48◦ 2.35±0.14 0.23±0.04 3.8 10−9 8.3 10−13 2.29±0.18 0.16±0.04 ( ) HESSJ1731−3479 0.54◦ 2.32±0.06 4.67±0.19b 1.9 10−8 6.9 10−12 HESSJ1745−30310 ≈ 0.4◦ 2.71±0.11 2.84±0.23 1.6 10−8 5.1 10−12 W2811 mult. 2.49±0.14 1.86±0.19 1.64 10−8 3.5 10−12 2.66±0.27 0.75±0.11 ( ) CTB37A12 ? 2.30±0.13 0.87±0.10 6.4 10−9 2.3 10−12 W51C13 ≈ 0.2◦ - - 0.62 10−8c - RXJ0852−4622, RCW86, SN1006; one SNR (HESS J1731−347) was discovered in the survey 4 of the Galactic plane conducted with HESS and was subsequently discovered in radio and X-rays. FortheSNRsRXJ1713.7−3946, RXJ0852−4622andHESSJ1731−347theTeVmorphology is clearly a shell-like structure, which is statistically preferred over a centre-filled morphology. The morphology of SN1006 are of two spots which are consistent with bright spots of non- thermal X-ray emission on the SNRs shell. For RCW86 a shell-like morphology could not yet been confirmed being statistically significant, further observations may clarify the morphology. The first part of Table 1 lists the SNRs detected with H.E.S.S. with their references and mor- ◦ ◦ phological andspectralparameters. Thediameters rangefrom0.5 upto2 . Theenergyspectra follow straight power laws, only for RXJ1713.7−3946 an exponential cut-off around 18TeV has beendetected. Thespectralindicesarebetween 2.0and2.5, whichis consistent withpredictions from DSA. The detection of gamma-rays from SNRs is evidence for particle acceleration in the SNR shells. Another proof of particle acceleration in SNRs are the detection of gamma rays from the interactions of SNRs with molecular clouds (MCs). Relativistic protons escape the SNR and interact with the material in a nearby molecular cloud. The high density in the cloud enhances the gamma-ray emission, and the gamma-ray morphology is in general correlated with the morphology of the MCs as traced by radio observations of CO emission lines. The second part of Table 1 summarisesthe objects detected with H.E.S.S.Themorphologies arerather irregular: the extension of CTB37A is not statistically significant, several clouds are seen around W28, and the diameters of the other sources are several tenth of a degree. The spectra follow straight power laws but are in general softer than those seen from shell-type SNRs. The spectral indices can go up to 2.7, similar to the index of the Cosmic Rays measured directly. aexponentialcut-off at 17.9±3.3TeV bat 0.783TeV ccurrently only theintegral flux is published Table 2: Leptonic and hadronicinterpretation of theemission of shell-typesupernovaremnants. name X-ray flux F magnetic field total energy in protons X (0.5−10keV) leptonic shock front hadronic scenario [ergcm−2s−1] B in fractions of E ≈ 1051erg lep SNR RXJ1713.7−3946 5.5 10−10d14 ≈10µG ≥ 65µG15 0.1...0.3 d 2 n −1 1kpc 1cm−3 RXJ0852−4622 9.9 10−1116 ≈ 6µG 270µG17 0.08 (cid:16) d (cid:17)2(cid:16) n (cid:17)−1 750pc 1cm−3 18 RCW86 2.1 10−10e7 15...30µG 24µG 0.2...0.4 (cid:16)d 2(cid:17) (cid:16) n (cid:17)−1 ≈100µG19 2.5kpc 0.7cm−3 SN1006 1.1 10−10f 8 ≈30µG ≥120µG19 0.2 (cid:16) d (cid:17)2 (cid:16) n (cid:17) −1 2.2kpc 0.085cm−3 HESSJ1731−347 3.7 10−11g9 25µG - 0(cid:16).2 d(cid:17) (cid:16)2 n (cid:17)−1 3.2kpc 1cm−3 (cid:16) (cid:17) (cid:16) (cid:17) 3 Interpretation The fact that the gamma-ray emission from MCs is correlated with the distribution of the target material for hadronic interaction and the absence of non-thermal X-ray emission from these sources is a good indication that the emission is indeed from relativistic protons. It can be shown that the Cosmic Ray density is enhanced with respect to the density in the solar neighbourhood. And the nearby SNR which is in interaction with the MC is a very good candidate for being the sources of these Cosmic Rays. Fortheshell-typeSNRsthesituationislessclearasthesynchrotronemissionhastobetaken into account. Synchrotron emission from the shells is evidence for a population of relativistic electrons. These electrons produce gamma-ray emission in inverse Compton (IC) scattering off the Cosmic Microwave Background (CMB) and the observed gamma-ray emission could be entirely due to IC emission. Assuming that synchrotron and IC emission are produced by the same electron population and that the gamma-ray emission is entirely dueto IC emission off the CMB the magnetic field in the SNR shell can be estimated from the gamma-ray energy flux F γ and X-ray energy flux F as B = 10FX µG. As shown in Table 2, the magnetic fields for X lep Fγ the shell-type SNRs estimated solelyqfrom the X-ray and gamma-ray fluxes are between 6 and 30 µG. A method to measure the magnetic field in a shock front is from the width of very thin 19 filaments observed in X-rays . The magnetic fields estimated from the shock front filaments are found to be of the order of several 100 µG, significantly larger than what was estimated for a purely leptonic scenario and consistent with expectations for magnetic field amplification as 19 a result of very efficient acceleration of nuclear Cosmic Rays . A higher magnetic field renders the synchrotron emission more efficient and the observed X-ray emission can be produced by a smaller electron population. The IC emission is therefore fainter and the gamma-ray radiation can be attributed to a possible hadronic scenario. Depending on the distance to the SNR and the density of the ambient material and assuming a certain spectral shape of the underlying proton population (typically a straight power law with index 2 between 1 GeV and 100 TeV) thetotal energy inprotonscan beestimated. Thisestimation for theSNRsdetected by H.E.S.S. is summarised in Table 2 for typical values of the distance d and the density n of the ambient material. It can beseen that the total energy in protons is of the order of several tens of percent d1−10keV e0.7−10keV f0.5−10keV gfor roughly 1/3 of theshell of an assumed supernova explosion energy of 1051erg. 4 Conclusion Observations with H.E.S.S.have revealed VHE gamma-ray emission from 5shell-typeSNRs and 4 interactions of SNRs with molecular clouds. In the case of the SNR/MC interactions the good correlation with the distribution of ambient material and the absence of synchrotron emission id a very good indication for hadronic interactions. The observed magnetic field amplification in the shells of supernova remnants seem to contradict a purely leptonic scenario and favour a hadronic scenario. SNRs are therefore very good candidates for acceleration of the Galactic Cosmic Rays. The extension of H.E.S.S. with a fifth telescope currently under construction will lower the energy threshold, allowing better studies of the energy spectra of the SNRs. Future instruments like the Cherenkov Telescope Array (CTA) will have a better angular resolution and a ten times better sensitivity, enabling detailed morphological studies and surveys for SNRs throughout the Galaxy. Acknowledgments The support of the Namibian authorities and of the University of Namibia in facilitating the construction and operation of H.E.S.S. is gratefully acknowledged, as is the support by the German Ministry for Education and Research (BMBF), the Max Planck Society, the French Ministry for Research, the CNRS-IN2P3 and the Astroparticle Interdisciplinary Programme of theCNRS,theU.K.ScienceandTechnology Facilities Council(STFC),theIPNPof theCharles University, the Polish Ministry of Science and Higher Education, the South African Department ofScienceandTechnologyandNationalResearchFoundation, andbytheUniversityofNamibia. We appreciate the excellent work of the technical support staff in Berlin, Durham, Hamburg, Heidelberg, Palaiseau, Paris, Saclay, and in Namibia in the construction and operation of the equipment. References 1. Ginzburg & Syrovatskii, The Origin of Cosmic Rays New York, Macmillan (1964). 2. Malkov & Drury, Rep. Prog. Phys. 64, 429 (2001). 3. HESS collaboration, A&A 457, 899 (2006). 4. HESS collaboration, A&A 477, 353 (2008). 5. HESS collaboration, A&A 464, 235 (2007). 6. HESS collaboration, ApJ 661, 236 (2007). 7. HESS collaboration, ApJ 692, 1500 (2009). 8. HESS collaboration, A&A 516, A62 (2010). 9. HESS collaboration, A&A 531, A81 (2011). 10. HESS collaboration, A&A 483, 509 (2008). 11. HESS collaboration, A&A 481, 401 (2008). 12. HESS collaboration, A&A 490, 685 (2008). 13. Fiasson for the HESS collaboration, ICRC , (2009). 14. Acero et al, A&A 505, 157 (2009). 15. Berezhko&V¨olk, A&A 451, 981 (2006). 16. Slane et al, ApJ 548, 814 (2001). 17. Katsuda et al, ApJ 678, L35 (2008). 18. Vink et al, ApJ 648, L33 (2006). 19. V¨olk et al, A&A 433, 229 (2005).