Evidence of Cosmic-Ray Excess from Local Giant Molecular Clouds

TL;DR

This study analyzes gamma-ray data from nearby giant molecular clouds to investigate cosmic-ray distributions, revealing potential local acceleration sources and deviations from expected spectra, indicating complex cosmic-ray interactions.

Contribution

It provides the first detailed gamma-ray spectral analysis of multiple nearby molecular clouds, identifying deviations from standard cosmic-ray models and suggesting local acceleration mechanisms.

Findings

A subset of clouds shows excess gamma-ray emission inconsistent with the AMS-02 cosmic-ray spectrum.

Deviations suggest additional cosmic-ray acceleration within the clouds.

Potential acceleration sources include OB star winds, T-Tauri stars, and supernova remnants.

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.