Auteur(s): Ackermann M., Ajello M., Allafort A., Antolini E., Baldini L., Ballet J., Barbiellini G., Bastieri D., Bechtol K., Bellazzini R., Berenji B., D. Blandford R., D. Bloom E., Bonamente E., W. Borgland A., Bottacini E., J. Brandt T., Bruel P., Cohen-Tanugi J., J. Fegan S., Horan D., Hou X., Kn¨odlseder J., Lande J., Mehault J., Nuss E., Pelassa V., Pierbattista M., Piron F., Vasileiou V.

(Article) Publié: The Astrophysical Journal / The Astrophysical Journal Letters, vol. 756 p.4 (2012)

Ref HAL: in2p3-00747339_v1
DOI: 10.1088/0004-637X/756/1/4
Exporter : BibTex | endNote
23 citations

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.