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

Abstract

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.

accepted for publication in ApJ

References in corpus (11)

• HI4PI: A full-sky HI survey based on EBHIS and GASS
• The Parkes multibeam pulsar survey: VI. Discovery and timing of 142 pulsars and a Galactic population analysis
• The evolution of amorphous hydrocarbons in the ISM: dust modelling from a new vantage point
• Inverse Compton Origin of the Hard X-Ray and Soft Gamma-Ray Emission from the Galactic Ridge
• CO-dark gas and molecular filaments in Milky Way type galaxies
• Fermi-LAT Observations of Two Gamma-Ray Emission Components from the Quiescent Sun
• Dust variations in the diffuse interstellar medium: constraints on Milky Way dust from Planck-HFI observations
• Optically thick HI dominant in the local interstellar medium; an alternative interpretation to "dark gas"
• Gamma-ray observations of the Orion Molecular Clouds with the Fermi Large Area Telescope
• Quantifying the Interstellar Medium and Cosmic Rays in the MBM 53, 54, and 55 Molecular Clouds and the Pegasus Loop using Fermi-LAT Gamma-ray Observations
• Probing local cosmic rays using Fermi-LAT observation of a mid-latitude region in the third Galactic quadrant

Cited by in corpus (2)

• The interstellar medium in young supernova remnants: key to the production of cosmic X-rays and $\gamma$-rays
• 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