Probing local cosmic rays using Fermi-LAT observations of a mid-latitude region in the third Galactic quadrant

摘要

The γ-ray observation of interstellar gas provides a unique way to probe the cosmic rays (CRs) outside the solar system. In this work, we use an updated version of Fermi-LAT data and recent multi-wavelength tracers of interstellar gas to reanalyze a mid-latitude region in the third Galactic quadrant and estimate the local CR proton spectrum. Two γ-ray production cross section models for pp interaction, the commonly used one from Kamae et al. (2006) and the up-to-date one from Kafexhiu et al. (2014), are adopted separately in the analysis. Both of them can well fit the emissivity and the derived proton spectra roughly resemble the direct measurements from AMS-02 and Voyager 1, but rather different spectral parameters are indicated. A break at 4 ± 1 GeV c~(-1) is shown if the cross section model by Kamae et al. (2006) is adopted. The resulting spectrum is ?20% larger than the AMS-02 observation above 15 GeV and consistent with the demodulated spectrum within 2%. The proton spectrum based on the cross section model of Kafexhiu et al. (2014) is about 1.4-1.8 times that of AMS-02 at 2-100 GeV, however the difference decreases to 20% below 10 GeV with respect to the de-modulated spectrum. A spectral break at 20 ± 11 GeV c~(-1) is required in this model. An extrapolation down to 300 MeVis performed to compare with the observation of Voyager 1, and we find a deviation of ?2.5σ for both the models. In general, an approximately consistent CR spectrum can be obtained using γ-ray observation nowadays, but we still need a better γ-ray production cross section model to derive the parameters accurately.