Study of the Cosmic Rays and Interstellar Medium in Local HI Clouds using Fermi-LAT Gamma-Ray Observations

An accurate estimate of the interstellar gas density distribution is crucial to understanding the interstellar medium (ISM) and Galactic cosmic rays (CRs). To comprehend the ISM and CRs in a local environment, a study of the diffuse $\gamma$-ray emission in a mid-latitude region of the third quadrant was performed. The $\gamma$-ray data in the 0.1--25.6~GeV energy range of the Fermi Large Area Telescope (LAT) and other interstellar gas tracers such as the HI4PI survey data and the Planck dust thermal emission model were used, and the northern and southern regions were analyzed separately. The variation of the dust emission Dem with the total neutral gas column density NH was studied in high dust-temperature areas, and the NH/Dem ratio was calibrated using $\gamma$-ray data under the assumption of a uniform CR intensity in the studied regions. The measured integrated $\gamma$-ray emissivities above 100~MeV are $(1.58\pm0.04)\times10^{-26}~\mathrm{photons~s^{-1}~sr^{-1}~H\mbox{-}atom^{-1}}$ and $(1.59\pm0.02)\times10^{-26}~\mathrm{photons~s^{-1}~sr^{-1}~H\mbox{-}atom^{-1}}$ in the northern and southern regions, respectively, supporting the existence of a uniform CR intensity in the vicinity of the solar system. While most of the gas can be interpreted to be HI with a spin temperature of $T_\mathrm{S} = 125~\mathrm{K}$ or higher, an area dominated by optically thick HI with $T_\mathrm{S} \sim 40~\mathrm{K}$ was identified.