Fermi-LAT Study of Cosmic-Ray Gradient in the Outer Galaxy --- Fermi-LAT view of the 3 rd Quadrant ---

1. SNR RX J1713-3946 B HESS π 0 e e π gas + + + - - - gas Fermi-LAT Study of Cosmic-Ray Gradient in the Outer Galaxy --- Fermi-LAT view of the 3rd Quadrant --- Tsunefumi Mizuno (Hiroshima Univ.), Luigi Tibaldo (INFN Sez. Di Padova & Univ. di Padova), Isabelle Grenier (CEA Saclay) on behalf of the Fermi Large Area Telescope Collaboration local arm Summary: We measured the cosmic-ray (CR) gradient and Xco evolution toward the outer Galaxy in the 3rd quadrant. The CR gradient is found to be flatter than that predicted by a conventional model. Perseus arm Abstract The distribution of cosmic-rays within our Galaxy is a key information to understand their origin and propagation. High energy cosmic-rays interact with the interstellar medium or the interstellar radiation field and produce the diffuse gamma-ray emission (via pion production, electron bremsstrahlung and inverse Compton scattering). This fact enables us to study the Galactic cosmic-rays using high-energy gamma-ray observations. Here we report an analysis of the diffuse gamma-rays in the third quadrant with Fermi-LAT. The region has kinematically well-defined segments of the local arm and the Perseus arm, and thus is one of the best regions for the study of cosmic-ray density distribution across the outer Galaxy. A larger halo size and/or a flatter cosmic-ray source distribution than those of a conventional model are required to reproduce the LAT data. Evolution of Xco (=N(H2)/Wco) , compatible with that by the relevant LAT study in the 2nd quadrant, was also obtained. 1. Modivation:Diffuse Gammas probe CRs and the ISM 2. Observation and Analysis: Fermi-LAT view of the 3rd Quadrant Count Map (E>100 MeV) Preliminary • HE gamma-rays are produced via interactions btw. Galactic cosmic-rays (CRs) and the interstellar medium (or the interstellar radiation field) • Energy density in the interstellar space • ECR ~ 0.6 MeV/m3 • EB~0.3 MeV/m3 • Estarlight ~ 0.3 MeV/m3 • Main part of CRs is accessible by Fermi-LAT through g-ray observations Geminga • One of the best studied regions in g-rays • Green square (our ROI) is ideal to study diffuse g-rays • Small point source contamination, kinematically well-separated arms • Fit gamma-ray data with a linear combination of model maps • I(E, l, b) = SA(E)*HI(l,b) + SB(E)*Wco(l,b) + • C(E)*E(B-V)excess(l,b) + IC(l,b) + Iso(E)+S point_sources • Coefficients give the gamma-ray (CR) spectral distribution and Xco evolution in the outer Galaxy Vela (CR Accelerator) (Interstellar space) (Observer) Crab ISM X,γ Orion A/B synchrotron IC ISRF P He CNO bremss diffusion energy losses reacceleration convection etc. Pulsar, m-QSO Fermi-LAT (Eg ~ 0.1-10 GeV) ACTs, Fermi Gal. Center Diffuse Gamma-ray Emission is a powerful probe to study CRs in distant locations Inner Galaxy Outer Galaxy HI, Local arm HI, Perseus arm CO, Local arm Excess E(B-V) 3. Results:Emissivity, Xco and Comparison with the EGRET Study • Effect of the point source thres. is <=5% • EGRET result is compatible with ours in their 2s statistical error • arm-interarm contrast suggested by the EGRET study is not observed • Best quality emissivity spectra in this region • Spectral shape agree well with the model for LIS • No significant spectral variation in the outer Galaxy Preliminary CR spectrum in Tp~1-100 GeV do not vary significantly in the region studied Preliminary • Similar gamma-ray spectra to those from HI gas • Galactic CRs penetrate to the dense core of molecular clouds • Xco = 1.3 x 1020 cm-2 (K km s-1)-1 and 2.0 x 1020 cm-2 (K km s-1)-1 for the local arm and the interarm, respectively • Xco evolution similar to that found by the 2nd quadrant analysis (P4-136) Preliminary Preliminary Preliminary 5. Summary 4. Discussion:CR Gradient in the Outer Galaxy • Diffuse gamma-ray emission is a powerful probe to study CR and ISM distribution in the Galaxy • We present the analysis of the 3rd quadrant, one of best regions for such a study • CR spectral shape is found not to vary significantly in the region studied. • Evolution of Xco compatible with that of the relevant Fermi-LAT study is obtained • A large halo size and/or a flat CR source distribution are indicated from the LAT data • We ran GALPROP with the SNR density distribution by Case and Bhattacharya (1998) • Prediction by a conventional model is shown by a solid line in the middle panel • Halo size of zh>=15 kpc is required to reproduce the LAT data • Or, a flat source distribution in R>10-12 kpc is required A larger halo size and/or a flatter CR source distribution than those by a conventional models are required SNR distirubion (solid line) Zh=1, 2, 4, 10, 15 and 20 kpc flat source distribution in R>=Rbk (dotted line) Reference Preliminary Preliminary Digel et al. 2001 (ApJ 555, 12) Tibaldo & Grenier 2009 (P4-136) Abdo et al. 2009 (ApJ 703, 1249) Strong and Moskalenko 1998 (ApJ 509, 212) Case & Bhattacharya 1998 (ApJ 504, 761) Strong et al. 2004 (A&A 422, L47)