CONSTRAINTS ON THE COSMIC-RAY DENSITY GRADIENT BEYOND THE SOLAR CIRCLE FROM FERMI {gamma}-RAY OBSERVATIONS OF THE THIRD GALACTIC QUADRANT

Ackermann, M; Ajello, M; Bechtol, K; Berenji, B; Bloom, E D; Borgland, A W; Buehler, R; Baldini, L; Bellazzini, R; Bregeon, J; Brez, A; Ballet, J; Barbiellini, G; Bastieri, D; Buson, S; Bonamente, E; Brandt, T J; Brigida, M; Bruel, P

doi:10.1088/0004-637X/726/2/81

Title: CONSTRAINTS ON THE COSMIC-RAY DENSITY GRADIENT BEYOND THE SOLAR CIRCLE FROM FERMI {gamma}-RAY OBSERVATIONS OF THE THIRD GALACTIC QUADRANT

Journal Article · Mon Jan 10 00:00:00 EST 2011 · Astrophysical Journal

DOI:https://doi.org/10.1088/0004-637X/726/2/81· OSTI ID:21567659

Ackermann, M; Ajello, M; Bechtol, K; Berenji, B; Bloom, E D; Borgland, A W; Buehler, R ^[1]; Baldini, L; Bellazzini, R; Bregeon, J; Brez, A ^[2]; Ballet, J ^[3]; Barbiellini, G ^[4]; Bastieri, D; Buson, S ^[5]; Bonamente, E ^[6]; Brandt, T J ^[7]; Brigida, M ^[8]; Bruel, P ^[9]

W. W. Hansen Experimental Physics Laboratory, Kavli Institute for Particle Astrophysics and Cosmology, Department of Physics and SLAC National Accelerator Laboratory, Stanford University, Stanford, CA 94305 (United States)
Istituto Nazionale di Fisica Nucleare, Sezione di Pisa, I-56127 Pisa (Italy)
Laboratoire AIM, CEA-IRFU/CNRS/Universite Paris Diderot, Service d'Astrophysique, CEA Saclay, 91191 Gif sur Yvette (France)
Istituto Nazionale di Fisica Nucleare, Sezione di Trieste, I-34127 Trieste (Italy)
Istituto Nazionale di Fisica Nucleare, Sezione di Padova, I-35131 Padova (Italy)
Istituto Nazionale di Fisica Nucleare, Sezione di Perugia, I-06123 Perugia (Italy)
Centre d'Etude Spatiale des Rayonnements, CNRS/UPS, BP 44346, F-30128 Toulouse Cedex 4 (France)
Dipartimento di Fisica 'M. Merlin' dell'Universita e del Politecnico di Bari, I-70126 Bari (Italy)
Laboratoire Leprince-Ringuet, Ecole polytechnique, CNRS/IN2P3, Palaiseau (France)

We report an analysis of the interstellar {gamma}-ray emission in the third Galactic quadrant measured by the Fermi Large Area Telescope. The window encompassing the Galactic plane from longitude 210{sup 0} to 250{sup 0} has kinematically well-defined segments of the Local and the Perseus arms, suitable to study the cosmic-ray (CR) densities across the outer Galaxy. We measure no large gradient with Galactocentric distance of the {gamma}-ray emissivities per interstellar H atom over the regions sampled in this study. The gradient depends, however, on the optical depth correction applied to derive the H I column densities. No significant variations are found in the interstellar spectra in the outer Galaxy, indicating similar shapes of the CR spectrum up to the Perseus arm for particles with GeV to tens of GeV energies. The emissivity as a function of Galactocentric radius does not show a large enhancement in the spiral arms with respect to the interarm region. The measured emissivity gradient is flatter than expectations based on a CR propagation model using the radial distribution of supernova remnants and uniform diffusion properties. In this context, observations require a larger halo size and/or a flatter CR source distribution than usually assumed. The molecular mass calibrating ratio, X{sub CO} = N(H{sub 2})/W{sub CO}, is found to be (2.08 {+-} 0.11) x 10{sup 20} cm{sup -2}(K km s{sup -1}){sup -1} in the Local arm clouds and is not significantly sensitive to the choice of H I spin temperature. No significant variations are found for clouds in the interarm region.

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OSTI ID:: 21567659

Journal Information:: Astrophysical Journal, Vol. 726, Issue 2; Other Information: DOI: 10.1088/0004-637X/726/2/81; ISSN 0004-637X

Country of Publication:: United States

Language:: English

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Title: CONSTRAINTS ON THE COSMIC-RAY DENSITY GRADIENT BEYOND THE SOLAR CIRCLE FROM FERMI {gamma}-RAY OBSERVATIONS OF THE THIRD GALACTIC QUADRANT

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