Gamma-Ray Observations of the Orion Molecular Clouds with the Fermi Large Area Telescope

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 (W <SUB>CO</SUB>) at a 1 1 pixel level. The correlation is found to be linear over a W <SUB>CO</SUB> range of ~10-fold when divided in three regions, suggesting penetration of nuclear CRs to most of the cloud volumes. The W <SUB>CO</SUB>-to-mass conversion factor, X <SUB>CO</SUB>, is found to be ~2.3 10<SUP>20</SUP> cm<SUP>-2</SUP>(K km s<SUP>-1</SUP>)<SUP>-1</SUP> for the high-longitude part of Orion A (l > 212), ~1.7 times higher than ~1.3 10<SUP>20</SUP> found for the rest of Orion A and B. We interpret the apparent high X <SUB>CO</SUB> in the high-longitude region of Orion A in the light of recent works proposing a nonlinear relation between H<SUB>2</SUB> and CO densities in the diffuse molecular gas. W <SUB>CO</SUB> decreases faster than the H<SUB>2</SUB> column density in the region making the gas "darker" to W <SUB>CO</SUB>.