[Paper Review] Local H~{\sc i} emissivity measured with the {\it Fermi}-LAT and implications for cosmic-ray spectra

J. M. Casandjian|arXiv (Cornell University)|May 29, 2015

Astrophysics and Cosmic Phenomena1 references4 citations

TL;DR

This study measures the local H i gamma-ray emissivity using 4 years of Fermi-LAT data across 10° < |b| < 70°, employing a template-fitting method with H i, molecular, ionized hydrogen, and dust templates. It finds that observed emissivities match theoretical predictions only when solar modulation is accounted for, providing direct evidence of solar modulation and enabling derivation of consistent local interstellar spectra for protons, helium, and electron-positrons from 1 to 100 GeV/nucleon.

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 years of observations from the Large Area Telescope (LAT) aboard the {\it Fermi Gamma-ray Space Telescope} ({\it Fermi}) restricted to absolute latitudes $10^o

Motivation & Objective

• To measure the local H i gamma-ray emissivity across 50 MeV to 50 GeV using Fermi-LAT data.
• To determine the molecular hydrogen-to-CO conversion factor X_CO and dust-to-gas ratio X_DUST from gamma-ray emission templates.
• To test whether observed emissivities are consistent with cosmic-ray spectra measured in the heliosphere, accounting for solar modulation.
• To derive the local interstellar spectrum (LIS) for protons, helium, and electron-positrons by fitting parametrized spectral forms to LAT emissivity and heliospheric data.
• To assess systematic uncertainties in emissivity measurements and validate the force-field approximation for solar modulation.

Proposed method

• A template-fitting method was applied to Fermi-LAT counts maps using spatial templates for atomic H i, molecular H2, ionized hydrogen (WIM), and dust optical depth.
• The analysis used 4 years of LAT data restricted to |b| > 10° to minimize Galactic plane contamination and focus on the solar neighborhood.
• A linear combination of templates was fitted to the observed gamma-ray emission, with contributions from inverse-Compton scattering and residual point sources subtracted.
• The emissivity was derived per hydrogen atom, and the fit included energy-dependent corrections using gamma-ray production cross-sections.
• Solar modulation was modeled via the force-field approximation, and heliospheric CR fluxes from PAMELA and ATIC-2 were used as input for comparison.
• Parametrized spectral forms were fitted to both LAT emissivity and heliospheric data to derive the local interstellar spectrum (LIS).

Experimental results

Research questions

• RQ1 Does the measured local H i gamma-ray emissivity match predictions based on heliospheric cosmic-ray spectra when solar modulation is accounted for?
• RQ2 What is the value of the molecular hydrogen-to-CO conversion factor X_CO in the solar neighborhood, as derived from gamma-ray emission?
• RQ3 How does the emissivity vary across different interstellar medium components (atomic, molecular, ionized, DNM)?
• RQ4 Can a consistent local interstellar spectrum (LIS) for protons, helium, and electron-positrons be derived from LAT emissivity and heliospheric measurements?
• RQ5 What is the inferred solar modulation potential consistent with the observed emissivity and heliospheric fluxes?

Key findings

• The measured local H i gamma-ray emissivity is (1.63 ± 0.05) × 10⁻²⁶ ph s⁻¹ sr⁻¹ atom⁻¹ above 100 MeV, consistent with previous LAT measurements.
• The molecular hydrogen-to-CO conversion factor was measured as X_CO = (0.902 ± 0.007) × 10²⁰ cm⁻² (K km s⁻¹)⁻¹, consistent with standard values.
• The dust-to-gas ratio was determined as X_DUST = (41.4 ± 0.3) × 10²⁰ cm⁻² mag⁻¹, indicating a tight correlation between dust and gas.
• For the first time, gamma-ray emission from ionized hydrogen (WIM) was detected, with a morphology inconsistent with NE2001 predictions.
• The observed emissivity matches theoretical predictions only when heliospheric cosmic-ray fluxes are corrected for solar modulation, confirming its presence via indirect measurement.
• A consistent local interstellar spectrum (LIS) was derived for protons, helium, and electron-positrons between 1 and 100 GeV/nucleon, with spectral indices compatible with power-law forms within 1σ uncertainties.