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Abstract

We report the analysis of the Fermi-Large Area Telescope data from six nearby
giant molecular clouds (MCs) belonging to the Gould Belt and the Aquila Rift
regions. The high statistical {\gamma}-ray spectra above 3 GeV well described
by power laws make it possible to derive precise estimates of the cosmic-ray
(CR) distribution in the MCs. The comparison of {\gamma}-ray spectra of Taurus,
Orion A, and Orion B clouds with the model expected from Alpha Magnetic
Spectrometer (AMS-02) CR measurements confirms these clouds as passive clouds,
immersed in an AMS-02-like CR spectrum. A similar comparison of Aquila Rift,
Rho Oph, and Cepheus spectra yields significant deviation in both spectral
indices and absolute fluxes, which can imply an additional acceleration of CRs
throughout the entire clouds. Besides, the theoretical modeling of the excess
{\gamma}-ray spectrum of these clouds, assuming {\pi}0-decay interaction of CRs
in the cloud, gives a considerable amount of an enhanced CR energy density and
it shows a significant deviation in spectral shapes compared to the average
AMS-02 CR spectrum between 30 GeV and 10 TeV. We suggest that this variation in
the CR spectrum of Cepheus could be accounted for by an efficient acceleration
in the shocks of winds of OB associations, while in Rho Oph, similar
acceleration can be provided by multiple T-Tauri stars populated in the whole
cloud. In the case of Aquila Rift, the excess in absolute CR flux could be
related to an additional acceleration of CRs by supernova remnants or
propagation effects in the cloud.