Submitted to ApJL. V6.0 draft (January 16, 2010) Fermi-LAT discovery of GeV gamma-ray emission from the young supernova remnant Cassiopeia A 0 1 0 A. A. Abdo2,3, M. Ackermann4, M. Ajello4, A. Allafort4, L. Baldini5, J. Ballet6, 2 G. Barbiellini7,8, M. G. Baring9, D. Bastieri10,11, B. M. Baughman12, K. Bechtol4, n R. Bellazzini5, B. Berenji4, R. D. Blandford4, E. D. Bloom4, E. Bonamente13,14, a J A. W. Borgland4, J. Bregeon5, A. Brez5, M. Brigida15,16, P. Bruel17, R. Buehler4, 6 T. H. Burnett18, G. Busetto10,11, G. A. Caliandro19, R. A. Cameron4, P. A. Caraveo20, 1 J. M. Casandjian6, C. Cecchi13,14, O¨. C¸elik21,22,23, E. Charles4, S. Chaty6, A. Chekhtman2,24, ] E C. C. Cheung2,3, J. Chiang4, A. N. Cillis21, S. Ciprini14, R. Claus4, J. Cohen-Tanugi25, H J. Conrad26,27,28, S. Corbel6, F. de Palma15,16, S. W. Digel4, M. Dormody29, . h E. do Couto e Silva4, P. S. Drell4, R. Dubois4, D. Dumora30,31, Y. Edmonds4, C. Farnier25, p - C. Favuzzi15,16, S. J. Fegan17, E. C. Ferrara21, W. B. Focke4, P. Fortin17, M. Frailis32, o r Y. Fukazawa33, S. Funk1,4, P. Fusco15,16, F. Gargano16, D. Gasparrini34, N. Gehrels21,35,36, t s S. Germani13,14, G. Giavitto7,8, N. Giglietto15,16, F. Giordano15,16, T. Glanzman4, a [ G. Godfrey4, I. A. Grenier6, M.-H. Grondin30,31, J. E. Grove2, L. Guillemot37, S. Guiriec38, 2 Y. Hanabata33, E. Hays21, A. K. Harding21, M. Hayashida4, D. Horan17, R. E. Hughes12, v M. S. Jackson27,39, A. S. Johnson4, T. J. Johnson21,36, W. N. Johnson2, T. Kamae4, 9 1 H. Katagiri33, J. Kataoka40, N. Kawai41,42, M. Kerr18, J. Kno¨dlseder43, M. Kuss5, 4 J. Lande4, L. Latronico5, M. Lemoine-Goumard30,31, F. Longo7,8, F. Loparco15,16, 1 1. B. Lott30,31, M. N. Lovellette2, P. Lubrano13,14, A. Makeev2,24, M. N. Mazziotta16, 0 C. Meurer26,27, P. F. Michelson4, W. Mitthumsiri4, T. Mizuno33, C. Monte15,16, 0 1 M. E. Monzani4, A. Morselli44, I. V. Moskalenko4, S. Murgia4, T. Nakamori41, P. L. Nolan4, : v J. P. Norris45, E. Nuss25, T. Ohsugi33, A. Okumura46, N. Omodei5, E. Orlando47, i X J. F. Ormes45, D. Paneque4, J. H. Panetta4, V. Pelassa25, M. Pepe13,14, M. Pesce-Rollins5, r F. Piron25, M. Pohl48, T. A. Porter29, S. Raino`15,16, R. Rando10,11, A. Reimer49,4, a O. Reimer49,4, T. Reposeur30,31, S. Ritz29,29, A. Y. Rodriguez19, R. W. Romani4, M. Roth18, H. F.-W. Sadrozinski29, A. Sander12, P. M. Saz Parkinson29, J. D. Scargle50, C. Sgro`5, E. J. Siskind51, D. A. Smith30,31, P. D. Smith12, P. Spinelli15,16, M. S. Strickman2, D. J. Suson52, H. Tajima4, T. Takahashi53, T. Tanaka4, J. B. Thayer4, J. G. Thayer4, D. J. Thompson21, S. E. Thorsett29, L. Tibaldo10,11,6, O. Tibolla54, D. F. Torres55,19, G. Tosti13,14, A. Tramacere4,56, Y. Uchiyama1,4, T. L. Usher4, A. Van Etten4, V. Vasileiou22,23, C. Venter21,57, N. Vilchez43, V. Vitale44,58, A. P. Waite4, P. Wang4, B. L. Winer12, K. S. Wood2, R. Yamazaki33, T. Ylinen39,59,27, M. Ziegler29 – 2 – 1Corresponding authors: Stefan Funk ([email protected]); Yasunobu Uchiyama ([email protected]) 2Space Science Division, Naval Research Laboratory,Washington, DC 20375, USA 3 National Research Council Research Associate, National Academy of Sciences, Washington, DC 20001, USA 4W. W. Hansen ExperimentalPhysicsLaboratory,KavliInstitute for ParticleAstrophysics andCosmol- ogy, Department of Physics and SLAC National Accelerator Laboratory,Stanford University, Stanford, CA 94305,USA 5Istituto Nazionale di Fisica Nucleare, Sezione di Pisa, I-56127 Pisa, Italy 6Laboratoire AIM, CEA-IRFU/CNRS/Universit´e Paris Diderot, Service d’Astrophysique, CEA Saclay, 91191 Gif sur Yvette, France 7Istituto Nazionale di Fisica Nucleare, Sezione di Trieste, I-34127 Trieste, Italy 8Dipartimento di Fisica, Universit`a di Trieste, I-34127Trieste, Italy 9Rice University, Department of Physics and Astronomy, MS-108, P. O. Box 1892, Houston, TX 77251, USA 10Istituto Nazionale di Fisica Nucleare, Sezione di Padova,I-35131 Padova, Italy 11Dipartimento di Fisica “G. Galilei”, Universit`a di Padova,I-35131 Padova, Italy 12 Department of Physics, Center for Cosmology and Astro-Particle Physics, The Ohio State University, Columbus, OH 43210,USA 13Istituto Nazionale di Fisica Nucleare, Sezione di Perugia,I-06123 Perugia, Italy 14Dipartimento di Fisica, Universit`a degli Studi di Perugia, I-06123 Perugia,Italy 15Dipartimento di Fisica “M. Merlin” dell’Universit`a e del Politecnico di Bari, I-70126 Bari, Italy 16Istituto Nazionale di Fisica Nucleare, Sezione di Bari, 70126 Bari, Italy 17Laboratoire Leprince-Ringuet, E´cole polytechnique, CNRS/IN2P3, Palaiseau, France 18Department of Physics, University of Washington, Seattle, WA 98195-1560,USA 19Institut de Ciencies de l’Espai (IEEC-CSIC), Campus UAB, 08193 Barcelona, Spain 20 INAF-Istituto di Astrofisica Spaziale e Fisica Cosmica, I-20133 Milano, Italy 21NASA Goddard Space Flight Center, Greenbelt, MD 20771,USA 22Center for Research and Exploration in Space Science and Technology (CRESST) and NASA Goddard Space Flight Center, Greenbelt, MD 20771,USA 23DepartmentofPhysicsandCenterforSpaceSciencesandTechnology,UniversityofMarylandBaltimore County, Baltimore, MD 21250,USA 24George Mason University, Fairfax, VA 22030,USA – 3 – 25Laboratoire de Physique Th´eorique et Astroparticules, Universit´e Montpellier 2, CNRS/IN2P3, Mont- pellier, France 26Department of Physics, Stockholm University, AlbaNova, SE-106 91 Stockholm, Sweden 27 The Oskar Klein Centre for Cosmoparticle Physics, AlbaNova, SE-106 91 Stockholm, Sweden 28Royal Swedish Academy of Sciences Research Fellow, funded by a grant from the K. A. Wallenberg Foundation 29Santa Cruz Institute for Particle Physics, Department of Physics and Department of Astronomy and Astrophysics, University of California at Santa Cruz, Santa Cruz, CA 95064,USA 30Universit´edeBordeaux,Centred’E´tudesNucl´eairesBordeauxGradignan,UMR5797,Gradignan,33175, France 31CNRS/IN2P3, Centre d’E´tudes Nucl´eaires Bordeaux Gradignan, UMR 5797,Gradignan, 33175,France 32DipartimentodiFisica,Universit`adiUdineandIstitutoNazionalediFisicaNucleare,SezionediTrieste, Gruppo Collegato di Udine, I-33100Udine, Italy 33Department of Physical Sciences, Hiroshima University, Higashi-Hiroshima,Hiroshima 739-8526,Japan 34Agenzia Spaziale Italiana (ASI) Science Data Center, I-00044 Frascati (Roma), Italy 35DepartmentofAstronomyandAstrophysics,PennsylvaniaStateUniversity,UniversityPark,PA16802, USA 36 DepartmentofPhysicsandDepartmentofAstronomy,UniversityofMaryland,CollegePark,MD20742, USA 37Max-Planck-Institut fu¨r Radioastronomie,Auf dem Hu¨gel 69, 53121 Bonn, Germany 38Center for Space Plasma and Aeronomic Research (CSPAR), University of Alabama in Huntsville, Huntsville, AL 35899,USA 39DepartmentofPhysics,RoyalInstitute ofTechnology(KTH),AlbaNova,SE-10691Stockholm,Sweden 40Waseda University, 1-104 Totsukamachi, Shinjuku-ku, Tokyo, 169-8050,Japan 41 Department of Physics, Tokyo Institute of Technology, Meguro City, Tokyo 152-8551,Japan 42Cosmic Radiation Laboratory, Institute of Physical and Chemical Research (RIKEN), Wako, Saitama 351-0198,Japan 43Centre d’E´tude Spatiale des Rayonnements,CNRS/UPS, BP 44346,F-30128Toulouse Cedex 4, France 44Istituto Nazionale di Fisica Nucleare, Sezione di Roma “Tor Vergata”, I-00133 Roma, Italy 45Department of Physics and Astronomy, University of Denver, Denver, CO 80208,USA 46DepartmentofPhysics,GraduateSchoolofScience,UniversityofTokyo,7-3-1Hongo,Bunkyo-ku,Tokyo 113-0033,Japan 47Max-Planck Institut fu¨r extraterrestrische Physik, 85748 Garching, Germany – 4 – ABSTRACT We report on the first detection of GeV high-energy gamma-ray emission from a young supernova remnant with the Large Area Telescope aboard the Fermi Gamma-ray Space Telescope. These observations reveal a source with no discernible spatial extension detected at a significance level of 12.2σ above 500 MeV at a location that is consistent with the position of the remnant of the supernova explosion that occurred around 1680 in the Cassiopeia constellation – Cassiopeia A.The gamma-rayfluxandspectral shapeof thesourceareconsistent with a scenario in which the gamma-ray emission originates from relativistic particles accelerated in the shell of this remnant. The total content of cosmic rays (electrons and protons) accelerated in Cas A can be estimated as W ≃ CR (1–4)×1049 erg thanks to the well-known density in the remnant assuming that the observed gamma-ray originates in the SNR shell(s). The magnetic field in the radio-emitting plasma can be robustly constrained as B ≥ 0.1 mG, providing new evidence of the magnetic field amplification at the forward shock and the strong field in the shocked ejecta. Subject headings: acceleration of particles — ISM: individual(Cassiopeia A) — radiation mechanisms: non-thermal 48Department of Physics and Astronomy, Iowa State University, Ames, IA 50011-3160,USA 49Institut fu¨r Astro- und Teilchenphysik and Institut fu¨r Theoretische Physik, Leopold-Franzens- Universit¨at Innsbruck, A-6020 Innsbruck, Austria 50Space Sciences Division, NASA Ames Research Center, Moffett Field, CA 94035-1000,USA 51NYCB Real-Time Computing Inc., Lattingtown, NY 11560-1025,USA 52Department of Chemistry and Physics, Purdue University Calumet, Hammond, IN 46323-2094,USA 53 InstituteofSpaceandAstronauticalScience,JAXA,3-1-1Yoshinodai,Sagamihara,Kanagawa229-8510, Japan 54Max-Planck-Institut fu¨r Kernphysik, D-69029 Heidelberg, Germany 55Institucio´ Catalana de Recerca i Estudis Avan¸cats (ICREA), Barcelona,Spain 56Consorzio Interuniversitario per la Fisica Spaziale (CIFS), I-10133 Torino, Italy 57North-West University, PotchefstroomCampus, Potchefstroom2520, South Africa 58Dipartimento di Fisica, Universit`a di Roma “Tor Vergata”,I-00133 Roma, Italy 59School of Pure and Applied Natural Sciences, University of Kalmar, SE-391 82 Kalmar, Sweden – 5 – 1. Introduction Supernovaremnants(SNRs)havelongbeenconsideredastheprimarycandidatesforthe origin of Galactic Cosmic Rays (CRs). Specifically, diffusive shock acceleration (Bell 1978; Blandford & Ostriker 1978; Jones & Ellison 1991; Malkov & O’C Drury 2001) is widely ac- cepted as the mechanism by which charged particles can be accelerated to very high energies at collisionless shocks driven by supernova explosions. To maintain the energy density of the Galactic CRs, the kinetic energy released in supernova explosions has to be efficiently transferred to CRs with a conversion efficiency of ∼ 10% (Ginzburg & Syrovatskii 1964). However, it has not yet been confirmed whether strong shock waves in SNRs are indeed ca- pable of transferring this amount of energy into the acceleration of CR ions, and in doing so generating a CR energy density comparable to the energy density contained in the expansion ram pressure of the supernova. Shock energy can be converted to another form of energy: turbulent magnetic fields. Recent theoretical work indicates that the magnetic field can be largely amplified by stream- ing of CRs themselves, as an integral part of efficient CR acceleration at collisionless shocks (Bell & Lucek 2001; Bell 2004). Observations of synchrotron X-ray emission in young SNRs have shown that the magnetic field at supernova shocks can be amplified way beyond the factor of ∼ 4 that is expected for standard compression of interstellar magnetic fields in the absence of CR modification of the shock’s hydrodynamic structure (Uchiyama et al. 2007; V¨olk et al. 2005; Bamba et al. 2005; Vink & Laming 2003). The amplified magnetic field would allow acceleration of protons up to the knee in the CR spectrum at ∼ 1015 eV, the presumed endpoint of the Galactic component (Bell & Lucek 2001). Cassiopeia A (Cas A) is the remnant of one of a handful of historical supernovae; the explosion around AD 1680 that gave rise to Cas A has probably been the last Galactic supernova witnessed by humans (Ashworth 1980). It is the brightest radio source in our Galaxy (Baars et al. 1977) and its overall brightness across the electromagnetic spectrum makes it a unique laboratory for studying high-energy phenomena in SNRs. Cas A has an angular size of 2.5′ in radius corresponding to a physical size of 2.34 pc at a distance of 3.4+0.3 kpc (Reed et al. 1995). Cas A was the first SNR detected in TeV gamma rays, first −0.1 by HEGRA (Aharonian et al. 2001) and later confirmed by MAGIC (Albert et al. 2007) and VERITAS (Humensky 2008), firmly establishing the existence of multi-TeV particles. These higher-energy gamma rays are generally attributed to particles accelerated in the shock waves of the SNR, even though the TeV gamma-ray source is not resolved. The emission mechanism(s) responsible for the TeV gamma-ray emission remained unsettled. EGRET reported only upper limits on the gamma-ray emission in the GeV band (Esposito et al. 1996). – 6 – The advent of the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope for the first time makes it possible to detect GeV gamma rays from the shell of Cas A. The relativistic bremsstrahlung of accelerated electrons has been predicted to lie above the sensitivity of the LAT (Atoyan et al. 2000). The prediction of the bremsstrahlung flux is controlled by the strength of the magnetic field and therefore the gamma-ray flux provides a measure of the amplified magnetic field in the supernova shell (Cowsik & Sarkar 1980). Inverse Compton (IC) scattering is another important mechanism of gamma-ray production by high-energy electrons; seed photons that can be upscattered to gamma-rays include the interstellar radiation field, the cosmic microwave background (CMB), and the far-infrared (FIR) radiation by Cas A itself. In addition to these leptonic emissions, the decay of π0-mesons produced in inelastic collisions between high-energy protons (and nuclei) and background gas may contribute to the GeV gamma-ray flux. The importance of the GeV observations of young SNRs is emphasized by the fact that the energy density of the accelerated particles is measurable thanks to the well-constrained gas and radiation density in Cas A in addition to that of the magnetic field. In this paper, we report the discovery of GeV gamma-ray emission coincident with Cas A based on the first year observations with the Fermi LAT. 2. Observations The LAT onboard Fermi is a pair-conversion gamma-ray detector operating between 20 MeV and 300 GeV. The LAT has a wide field of view of ∼ 2.4 sr at 1 GeV, and observes the entire sky every 2 orbits (∼ 3 hr for Fermi’s orbit at an altitude of ∼ 565 km). The full details of the instruments are given in Atwood et al. (2009). A total exposure of 3 × 1010 cm2 s (at 1 GeV) has been obtained for Cas A during the period between August 4th 2008 and September 4th 2009, corresponding to 394 days of observations. ThedataanalysiswasperformedusingtheLATScienceToolspackagewiththe P6 V3 post-launch instrument response function (Rando et al. 2009). The standard event selection for source analysis, resulting in the strongest background-rejection power (diffuse event class) was applied. In addition photons coming from zenith angles larger than 105◦ were rejected to reduce the background from gamma rays produced in the atmosphere of the Earth. The analysis was further restricted to the energy range above 200 MeV where the uncertainties in the effective area become smaller than 10%. – 7 – 3. Analysis and Results The analysis of the gamma-ray event data was performed using gtlike, which is avail- able as part of Fermi Science Tools1. The tool gtlike employs a maximum likelihood technique to assess the statistical significance of sources and to estimate spectral param- eters (Mattox et al. 1996). The background gamma-ray model includes background sources from the 11-month catalog of Fermi-LAT sources at fixed coordinates, the galactic diffuse emission (using gll iem v02.fit) and an isotropic component (using isotropic iem v02.txt). The maximum likelihood analysis is performed inside a region-of-interest (ROI) of 10◦ ra- dius centered on Cas A. The independent tool sourcelike was employed to determine the source position and constrain the angular extent of the source. Sourcelike performs a max- imum likelihood fit to simultaneously optimize position and extent of the source (given an assumption about the shape of the source) in independent energy bands. Sourcelike can also be used to assess the test-statistic (TS) value and to compute the spectra of both extended and point-sources. The position determined with sourcelike is consistent with what was found by the automated LAT catalog tool. The spectral parameters between sourcelike and gtlike for this best-fit position are found to be consistent, yielding an independent check of the main results of the paper. The analysis clearly shows a source above the background coincident with the SNR Cas A (see Fig. 1). The source is detected at a significance level of 12.2 σ (or a TS value of 148)abovethebackgroundatabestfitpositionofα = 23h23m17.5s,δ = 58h49m43.1s 2000 2000 with a statistical uncertainty on the location of 1.0′ (68% confidence level). The systematic error in the position due to alignment of the telescope system and inaccurate description of the point-spread function of the instrument is estimated to be 0.3′. The position is spatially coincident with Cas A and with the MAGIC and VERITAS-detected TeV gamma- raysource(Albert et al.2007;Humensky 2008)asshowninFig.2. Anupperlimitonthesize of the gamma-ray emission has been obtained in a maximum likelihood fit by investigating thedecreaseofthelikelihoodwithincreasingsourcesize. UndertheassumptionofaGaussian shape this 68% upper limit amounts to 3.5′ (1-σ radius). This value is larger than the radius of the SNR (2.5′), which thus currently cannot be resolved by the Fermi-LAT. A check for variability in this data set with one-month time bins shows no sign for any change in the gamma-ray flux with time, indicating a steady source of emission. A spectral analysis of the gamma-ray emission has been performed in an energy range of 200 MeV–50 GeV. The source is detected only above 500 MeV. The spectrum shown in 1Software and documentation of the Fermi Science Tools are distributed by the Fermi Science Support Center at http://fermi.gsfc.nasa.gov/ssc – 8 – Fig. 3 reveals a relatively flat energy distribution and can be fitted by a power law with spectral index of Γ = 2.0 ± 0.1 between 0.5 and 50 GeV. The systematic uncertainty in the spectral index determination from the uncertainty in the normalisation of the Galactic diffuse emission (conservatively estimated to be 10%) amounts to ∼ 0.1. The integral photon flux above 500 MeV amounts to (8.7 ± 1.3) × 10−9 photons cm−2 s−1. A likelihood-ratio test was performed to check the presence of a spectral cutoff using a spectral function of dN/dE = KE−Γexp(−E/E ). The presence of the exponential cutoff is not statistically cut significant, given the resultant likelihood ratio of −2ln(L /L ) = 2.6. We formally PL cutoff report the best-fit parameters of the cutoff power-law model to make a comparison with LAT pulsars: Γ = 1.7±0.2 and E = 16±9 GeV. cut 4. Discussion The detection of gamma-ray emission from the direction of Cas A raises the following questions: “Is there a compact source such as a pulsar dominating the emission?” and “What is the radiating particle population responsible for the emission?”. Two emission scenarios seem plausible: emission from the central compact object (CCO) in Cas A or emission generated in the SNR. Since the GeV gamma-ray source coinciding with Cas A is a point-source in the LAT, these two scenarios cannot be distinguished on positional grounds therefore circumstantial evidence has to be considered. The point-like central X-ray source (Tananbaum 1999) is generally thought to be the left-over of the explosion of the massive progenitor star (Pavlov et al. 2000) and known as a CCO given that it is radio quiet, un-pulsed in X-rays, and has an X-ray spectrum de- scribed by blackbody with characteristic temperatures of about 0.4 keV without indication of a non-thermal component. A recent Chandra observation resulted in a 3σ limit on the pulsed fraction of 16% for periods larger than 0.68 s (Pavlov & Luna 2009). The fact that no pulsation has been detected at any waveband for the CCO does not rule out, that the neutron star is emitting gamma rays. The Fermi-LAT has detected several neutron stars as gamma-ray-only pulsars, pulsars not previously known from observations in other wave- bands (Abdo et al. 2008, 2009a). We searched for gamma-ray pulsations from the source coincident with Cas A using the full data set. We applied the standard time-differencing technique (Atwood et al. 2006), using a maximum frequency of 64 Hz, and a long time-difference window of ∼12 days. We found no evidence for pulsations which makes the neutron star scenario less likely. A comparison of the blind search pulsars discovered so far (Abdo et al. 2009a) and the known radio pulsars detected by the LAT, suggests that the blind search is approximately 2–3 times – 9 – less sensitive than a standard pulsation search using the known timing solution. This results in a 5σ limit on the pulsed flux of ∼ 2 × 10−7 photons cm−2 s−1 (Abdo et al. 2009b). In addition, there is no GeV gamma ray source in the 1st Fermi-LAT source catalog that is associated with a known CCO. Furthermore, the spectrum does not support a pulsar hypothesis. The energy spectra of pulsars are usually flat at energies below 1 GeV and show exponential cutoffs in the energy range between 1 GeV and 8 GeV (Abdo et al. 2009b). These characteristics do not mirror the LAT spectrum which is best described by a power-law shape with an index of Γ = 2.0±0.1 up to 50 GeV with no significant sign of a high-energy cutoff. A formal fit with an exponential cutoff model yields E = 16±9 GeV. This is rather high for a gamma-ray cut pulsar; no LAT pulsars show E > 8 GeV so far (Abdo et al. 2009a). cut The scenario in which the GeV gamma rays are emitted in the shell of the SNR is thereforefavored. Thegamma-rayemissioncouldbeproducedbyelectronsacceleratedatthe forward shock through relativistic bremsstrahlung or IC. Alternatively, the GeV gamma-ray emission could be predominantly produced by accelerated hadrons through interaction with the background gas and subsequent π0-decay. Recent studies showed electron acceleration to multi-TeV energies is likely to take place also at the reverse shock in the supernova ejecta (Uchiyama & Aharonian 2008; Helder & Vink 2008). First, we consider the shocked circumstellar region between the contact discontinuity and the forward shock (see Gotthelf et al. 2001), assumed to have constant magnetic field of B (a free parameter) and shocked circumstellar medium with a constant density of n = H 10 cm−3 (Laming & Hwang 2003). Electrons are accelerated to multi-TeV energies at the forwardshockastracedbysynchrotronX-rayouterfilaments(Hughes et al.2000). Weadopt here an electron acceleration spectrum Q (E) ∝ E−2.34exp(−E/E ) to match the radio- e m IR spectral index of α = 0.67 (Rho et al. 2003), since both the GeV gamma-ray emission and the radio synchrotron emission sample similar electron energies. About half of the total radio flux is attributable to this region (so-called plateau), while another half is to the reverse shock region (so-called bright ring; see below). Given the radio flux and the effective density (n = Σn Z (Z +1) ≃ 26 cm−3), the flux of bremsstrahlung is controlled eff i i i by B. In Figure 3, we show a leptonic model with B = 0.12 mG (red curves), which can broadly explain the observed GeV flux. The maximum energy is set to be E = 40 TeV m (Vink & Laming 2003). Shown are contributions from bremsstrahlung (dashed) and from IC scattering (dotted) produced by accelerated electrons that suffer synchrotron cooling at high energies. The bremsstrahlung spectrum consisting of electron-ion and electron-electron components is computed as in Baring et al. (1999). The radiation field for the IC component isdominatedbyFIRemissionfromtheCasAejecta, characterizedbyatemperatureof100K – 10 – andanenergydensityof∼ 2eVcm−3(Mezger et al.1986),afactorof8largerthantheenergy density in the CMB. The IC/FIR emission exceeds IC/CMB by a factor of 2.7 at a gamma- ray energy of 10 GeV. The value of B = 0.12 mG is consistent with B = 0.08–0.16 mG at the forward shock estimated by Vink & Laming (2003) based on the width of a synchrotron X-ray filament. Note however that a somewhat higher value of B ≃ 0.3 mG was obtained by Parizot et al. (2006) using the same filament width but including projection effects. The total amount of electrons in this case is W (> 10 MeV) ≃ 1×1049 erg. Also shown in Fig. 3 e is the case of B = 0.3 mG (blue curves), which predicts a lower gamma-ray flux than the observed one. Inascenario inwhich thegamma-raysaregeneratedby π0-decayofaccelerated hadrons, the gamma-ray spectrum can be well matched with either the proton acceleration spec- trum Q (p) ∝ p−2.3 (a red curve in Fig. 4), or a harder proton spectrum of Q (p) ∝ p−2.1 p p with an exponential cutoff at 10 TeV that is arbitrary introduced (blue curve). Here p de- notes momentum of accelerated protons. The gamma-ray spectrum is calculated following Kamae et al. (2006) with a scaling factor of 1.85 for helium and heavy nuclei (Mori 2009). The total proton content amounts to W (> 10 MeVc−1) ≃ 3.8×1049 erg in the case of the p softer spectrum and to W (> 10 MeVc−1) ≃ 3.2×1049 erg in the case of the harder proton p spectrum withthecutoff. Inbothcases theenergy content corresponds toless than2%of the estimated explosion kinetic energy of E = 2×1051 erg. Therefore, the cosmic-ray pressure sn would not be large enough to change the hydrodynamics of Cas A. This is consistent with Chandra X-ray measurements of the remnant’s spatial structure (e.g., Gotthelf et al. 2001); the ratio of the radii of forward and reverse shocks can be reproduced by hydrodynamical models that do not include cosmic-ray acceleration (Laming & Hwang 2003). Comparing the leptonic and hadronic models, it seems clear that the hadronic scenario can better fit the data due to the turnover at low energies that is not well reproduced in the leptonic scenario. Given the uncertainties in the diffuse model that have stronger effects at the low-energy end, we refrain, however, from strong claims about the radiating particle population at this point. Changing the diffuse model normalisation by ±10% (a conservative assumption on the uncertainty) largely affects the energy points at and below 1 GeV. The resulting effect on the flux point at 1 GeV is a 25% upward and 65% downward shift. A more detailed investigation of the lower energy end of the LAT spectrum will be possible with future LAT data. The shocked ejecta gas that emits strong radio and infrared synchrotron light (known as the bright ring) is another potential gamma-ray emitting region. Using M = 2M ejecta ⊙ (Willingale et al. 2002; Laming & Hwang 2003) comprised of only oxygen, we obtain n = eff n Z (Z + 1) ≃ 32 cm−3. This happens to be similar to the value of n in the forward O O O eff