Local H i Emissivity Measured with Fermi-LAT and Implications for Cosmic-Ray Spectra

Abstract

Cosmic-ray (CR) electrons and nuclei interact with the Galactic interstellar gas and produce high-energy γ-rays. The γ-ray emission rate per hydrogen atom, called emissivity, provides a unique indirect probe of the CR flux. We present the measurement and the interpretation of the emissivity in the solar neighborhood for γ-ray energy from 50 MeV to 50 GeV. We analyzed a subset of 4 yr of observations from the Large Area Telescope (LAT) aboard the Fermi Gamma-ray Space Telescope (Fermi) restricted to absolute latitudes 10^\circ \lt | b| \lt 70^\circ . From a fit to the LAT data including atomic, molecular, and ionized hydrogen column density templates, as well as a dust optical depth map, we derived the emissivities, the molecular hydrogen-to-CO conversion factor {X}_{CO}=(0.902+/- 0.007)× {10}²⁰ cm^-2 (K km s^-1)^-1, and the dust-to-gas ratio {X}_{DUST}=(41.4+/- 0.3)× {10}²⁰ cm^-2 mag^-1. Moreover, we detected for the first time γ-ray emission from ionized hydrogen. We compared the extracted emissivities to those calculated from γ-ray production cross sections and to CR spectra measured in the heliosphere. We observed that the experimental emissivities are reproduced only if the solar modulation is accounted for. This provides a direct detection of solar modulation observed previously through the anticorrelation between CR fluxes and solar activity. Finally, we fitted a parameterized spectral form to the heliospheric CR observations and to the Fermi-LAT emissivity and obtained compatible local interstellar spectra for proton and helium kinetic energy per nucleon between between 1 and 100 GeV and for electron-positrons between 0.1 and 100 GeV.