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Gas and Cosmic-Ray Properties in the MBM 53, 54, and 55 Molecular Clouds and the Pegasus Loop as Revealed by HI Line Profiles, Dust, and Gamma-Ray Data

In studying the interstellar medium (ISM) and Galactic cosmic rays (CRs), uncertainty of the interstellar gas density has always been an issue. To overcome this difficulty, we used a component decomposition of the 21-cm HI line emission and used the resulting gas maps in an analysis of $γ$-ray data obtained by the Fermi Large Area Telescope (LAT) for the MBM~53, 54, and 55 molecular clouds and the Pegasus loop. We decomposed the ISM gas into intermediate-velocity clouds, narrow-line and optically thick HI, broad-line and optically thin HI, CO-bright H2, and CO-dark H2 using detailed correlations with the HI line profiles from the HI4PI survey, the Planck dust-emission model, and the Fermi-LAT $γ$-ray data. We found the fractions of optical depth correction to the HI column density and CO-dark H2 to be nearly equal. We fitted the CR spectra directly measured at/near the Earth and the measured $γ$-ray emissivity spectrum simultaneously. We obtained a spectral break in the interstellar proton spectrum at ${\sim}$7~GeV, and found the $γ$-ray emissivity normalization agrees with the AMS-02 proton spectrum within 10\%, relaxing the tension with the CR spectra previously claimed.