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The Astrophysical Journal 756 (2012) 4
M. Ackermann, M. Ajello, A. Allafort, E. Antolini, L. Baldini, J. Ballet1, G. Barbiellini, D. Bastieri, K. Bechtol, R. Bellazzini, B. Berenji, R. D. Blandford, E. D. Bloom, E. Bonamente, A. W. Borgland, E. Bottacini, T. J. Brandt2, J. Bregeon, M. Brigida, P. Bruel3, R. Buehler, S. Buson, G. A. Caliandro, R. A. Cameron, P. A. Caraveo, C. Cecchi, A. Chekhtman, J. Chiang, S. Ciprini, R. Claus, J. Cohen-Tanugi4, J. Conrad, F. D'ammando, A. De Angelis, F. De Palma, C. D. Dermer, E. Do Couto E Silva, P. S. Drell, A. Drlica-Wagner, T. Enoto, L. Falletti, C. Favuzzi, S. J. Fegan3, E. C. Ferrara, W. B. Focke, Y. Fukazawa, Y. Fukui, P. Fusco, F. Gargano, D. Gasparrini, S. Germani, N. Giglietto, F. Giordano, M. Giroletti, T. Glanzman, G. Godfrey, S. Guiriec, D. Hadasch, Y. Hanabata, A. K. Harding, M. Hayashida, K. Hayashi, D. Horan3, X. Hou5, R. E. Hughes, M. S. Jackson, G. J 'ohannesson, A. S. Johnson, T. Kamae, H. Katagiri, J. Kataoka, M. Kerr, J. Kn¨odlseder2, M. Kuss, J. Lande2, S. Larsson, S.-H. Lee, F. Longo, F. Loparco, M. N. Lovellette, P. Lubrano, K. Makishima, M. N. Mazziotta, J. Mehault4, W. Mitthumsiri, A. A. Moiseev, C. Monte, M. E. Monzani, A. Morselli, I. V. Moskalenko, S. Murgia, T. Nakamori, M. Naumann-Godo, S. Nishino, J. P. Norris, E. Nuss4, M. Ohno, T. Ohsugi, A. Okumura, M. Orienti, E. Orlando, J. F. Ormes, M. Ozaki, D. Paneque, J. H. Panetta, D. Parent, V. Pelassa4, M. Pesce-Rollins, M. Pierbattista1, F. Piron4, G. Pivato, T. A. Porter, S. Rain'o, M. Razzano, A. Reimer, O. Reimer, M. Roth, H. F.-W. Sadrozinski, C. Sgr'o, E. J. Siskind, G. Spandre, P. Spinelli, A. W. Strong, H. Takahashi, T. Takahashi, T. Tanaka, J. G. Thayer, J. B. Thayer, O. Tibolla, M. Tinivella, D. F. Torres, A. Tramacere, E. Troja, Y. Uchiyama, T. L. Usher, J. Vandenbroucke, V. Vasileiou4, G. Vianello, V. Vitale, A. P. Waite, P. Wang, B. L. Winer, K. S. Wood, Z. Yang, S. Zimmer

We report on the gamma-ray observations of giant molecular clouds Orion A and B with the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope. The gamma-ray emission in the energy band between ∼100 MeV and ∼100 GeV is predicted to trace the gas mass distribution in the clouds through nuclear interactions between the Galactic cosmic rays (CRs) and interstellar gas. The gamma-ray production cross-section for the nuclear interaction is known to ∼10% precision which makes the LAT a powerful tool to measure the gas mass column density distribution of molecular clouds for a known CR intensity. We present here such distributions for Orion A and B, and correlate them with those of the velocity-integrated CO intensity (WCO) at a 1◦ × 1◦ pixel level. The correlation is found to be linear over a WCO range of ∼10-fold when divided in three regions, suggesting penetration of nuclear CRs to most of the cloud volumes. The WCO-to-mass conversion factor, XCO, is found to be ∼2.3 × 1020 cm−2 (K km s−1)−1 for the high-longitude part of Orion A (l > 212◦), ∼1.7 times higher than ∼1.3 × 1020 found for the rest of Orion A and B. We interpret the apparent high XCO in the high-longitude region of Orion A in the light of recent works proposing a nonlinear relation between H2 and CO densities in the diffuse molecular gas. WCO decreases faster than the H2 column density in the region making the gas "darker" to WCO.
1:  AIM - UMR 7158 - Astrophysique Interactions Multi-échelles
2:  IRAP - Institut de recherche en astrophysique et planétologie
3:  LLR - Laboratoire Leprince-Ringuet
4:  LUPM - Laboratoire Univers et Particules de Montpellier
5:  CENBG - Centre d'Etudes Nucléaires de Bordeaux Gradignan
Physics/Astrophysics/High Energy Astrophysical Phenomena

Sciences of the Universe/Astrophysics/High Energy Astrophysical Phenomena
gamma rays: ISM - ISM: clouds - ISM: general - ISM: individual objects (Orion A and Orion B) Online-only material: color figures