Speaker
Ruizhi Yang
(Purple Mountain Observatory)
Description
We report the results of our study on the energy spectra and absolute fluxes of cosmic rays (CRs) in the Local Galaxy based on three-year $\gamma$-ray observations of ten nearby giant molecular clouds (GMCs) belongs to the Gould Belt, with the Fermi Large Area Telescope (LAT). The $\gamma$-ray signals obtained with high statistical significance allow the determination of $\gamma$-ray spectra above 300 MeV with adequate precision for extraction of the energy distributions of CRs in these clouds. Remarkably, both the derived spectral indices and the absolute fluxes of CR protons in the energy interval 10-100 GeV are in good agreement with the recent direct measurements of local CRs by the PAMELA experiment. This is a strong evidence for a quite homogeneous distribution of CRs, at least within several hundred parsecs of the Local Galaxy. Combined with the well established energy-dependent time of escape of CRs from the Galaxy, $\tau(E) ~ E^{-\delta}$ with $\delta \simeq 0.5 - 0.6$, the measured spectrum implies a CRs spectral index of the (acceleration) source of $E^{-2.3}$. At low energies, the spectra of $\gamma$-rays appear to vary from one cloud to another. This implies spatial variations of the energy spectra of CR below 10 GeV which at such low energies could be naturally explained by the impact of the propagation effects as well as by the contribution of CR locally accelerated inside the clouds.
Author
Ruizhi Yang
(Purple Mountain Observatory)