Fermi-LAT Study of Cosmic-Ray Gradient in the Outer Galaxy --- Fermi-LAT view of the 3 rd 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 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(H 2 )/W co ), compatible with that by the relevant LAT study in the 2 nd quadrant, was also obtained. Summary: We measured the cosmic-ray (CR) gradient and Xco evolution toward the outer Galaxy in the 3 rd quadrant. The CR gradient is found to be flatter than that predicted by a conventional model. 1. Modivation: Diffuse Gammas probe CRs and the ISM HE gamma-rays are produced via interactions btw. Galactic cosmic-rays (CRs) and the interstellar medium (or the interstellar radiation field) e + - X,γ ISM π0 synchrotron bremss HESS SNR RX J B Pulsar,  -QSO PHeCNO IC ACTs, Fermi gas ISRF e + - π + - (CR Accelerator) (Interstellar space) (Observer) Energy density in the interstellar space E CR ~ 0.6 MeV/m 3 E B ~0.3 MeV/m 3 E starlight ~ 0.3 MeV/m 3 Main part of CRs is accessible by Fermi-LAT through  -ray observations Fermi-LAT (E  ~ GeV) 2. Observation and Analysis: Fermi-LAT view of the 3rd Quadrant HI, Local armCO, Local arm HI, Perseus arm Gal. Center Inner Galaxy Outer Galaxy Excess E(B-V) Vela Count Map (E>100 MeV) Crab Orion A/B Geminga Preliminary One of the best studied regions in  -rays Green square (our ROI) is ideal to study diffuse  -rays  Small point source contamination, kinematically well- separated arms Fit gamma-ray data with a linear combination of model maps  I(E, l, b) =  A(E)*HI(l,b) +  B(E)*Wco(l,b) + C(E)*E(B-V) excess (l,b) + IC(l,b) + Iso(E)+  point_sources Coefficients give the gamma-ray (CR) spectral distribution and Xco evolution in the outer Galaxy local arm Perseus arm Diffuse Gamma-ray Emission is a powerful probe to study CRs in distant locations 3. Results: Emissivity, Xco and Comparison with the EGRET Study Best quality emissivity spectra in this region Spectral shape agree well with the model for LIS No significant spectral variation in the outer Galaxy Similar gamma-ray spectra to those from HI gas  Galactic CRs penetrate to the dense core of molecular clouds Xco = 1.3 x cm -2 (K km s -1 ) -1 and 2.0 x cm -2 (K km s -1 ) -1 for the local arm and the interarm, respectively  Xco evolution similar to that found by the 2 nd quadrant analysis (P4-136) Effect of the point source thres. is <=5% EGRET result is compatible with ours in their 2  statistical error arm-interarm contrast suggested by the EGRET study is not observed 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 3 rd 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 CR spectrum in Tp~1-100 GeV do not vary significantly in the region studied 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 z h >=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 5. Summary Reference Digel et al (ApJ 555, 12) Tibaldo & Grenier 2009 (P4-136) Abdo et al (ApJ 703, 1249) Strong and Moskalenko 1998 (ApJ 509, 212) Case & Bhattacharya 1998 (ApJ 504, 761) Strong et al (A&A 422, L47) diffusion energy losses energy losses reacceleration reacceleration convection convection etc. etc. Preliminary Preliminary Preliminary Preliminary Preliminary PreliminaryPreliminary SNR distirubion (solid line) Z h =1, 2, 4, 10, 15 and 20 kpc flat source distribution in R>=R bk (dotted line)