Local H i emissivity measured with the Fermi-LAT and implications for cosmic-ray spectra

Original · EN

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 years of observations from the Large Area Telescope (LAT) aboard the Fermi Gamma-ray Space Telescope (Fermi) restricted to absolute latitudes 10ᵒ<|b| <70ᵒ. 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 XCO=(0.902±0.007) × 10²⁰ cm⁻² (K km s⁻¹)⁻¹ and the dust-to-gas ratio XDUST=(41.4±0.3) × 10²⁰ 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 parametrized spectral form to the heliospheric CR observations as well as 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.