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 $${X}_{\mathrm{CO}}=(0.902\pm 0.007)\times {10}^{20}$$ cm–2 (K km s–1)–1, and the dust-to-gas ratio $${X}_{\mathrm{DUST}}=(41.4\pm 0.3)\times {10}^{20}$$ 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 anticorrelationmore »
- Authors:
-
- CEA-IRFU/CNRS/Univ. Paris Diderot, Gif-sur-Yvette (France)
- Publication Date:
- Research Org.:
- SLAC National Accelerator Laboratory (SLAC), Menlo Park, CA (United States)
- Sponsoring Org.:
- USDOE
- OSTI Identifier:
- 1355151
- Grant/Contract Number:
- AC02-76SF00515
- Resource Type:
- Journal Article: Accepted Manuscript
- Journal Name:
- The Astrophysical Journal (Online)
- Additional Journal Information:
- Journal Volume: 806; Journal Issue: 2; Journal ID: ISSN 1538-4357
- Publisher:
- Institute of Physics (IOP)
- Country of Publication:
- United States
- Language:
- English
- Subject:
- 79 ASTRONOMY AND ASTROPHYSICS; cosmic rays; gamma rays: diffuse background; gamma rays: ISM
Citation Formats
Casandjian, Jean -Marc. Local H i emissivity measured with FERMI-LAT and implications for Cosmic-ray spectra. United States: N. p., 2015.
Web. doi:10.1088/0004-637X/806/2/240.
Casandjian, Jean -Marc. Local H i emissivity measured with FERMI-LAT and implications for Cosmic-ray spectra. United States. https://doi.org/10.1088/0004-637X/806/2/240
Casandjian, Jean -Marc. 2015.
"Local H i emissivity measured with FERMI-LAT and implications for Cosmic-ray spectra". United States. https://doi.org/10.1088/0004-637X/806/2/240. https://www.osti.gov/servlets/purl/1355151.
@article{osti_1355151,
title = {Local H i emissivity measured with FERMI-LAT and implications for Cosmic-ray spectra},
author = {Casandjian, Jean -Marc},
abstractNote = {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 ${X}_{\mathrm{CO}}=(0.902\pm 0.007)\times {10}^{20}$ cm–2 (K km s–1)–1, and the dust-to-gas ratio ${X}_{\mathrm{DUST}}=(41.4\pm 0.3)\times {10}^{20}$ 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. Lastly, 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.},
doi = {10.1088/0004-637X/806/2/240},
url = {https://www.osti.gov/biblio/1355151},
journal = {The Astrophysical Journal (Online)},
issn = {1538-4357},
number = 2,
volume = 806,
place = {United States},
year = {Sat Jun 20 00:00:00 EDT 2015},
month = {Sat Jun 20 00:00:00 EDT 2015}
}
Web of Science
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