Development of the Model of Galactic Interstellar Emission for Standard Point-Source Analysis of Fermi Large Area Telescope Data

TL;DR

This paper presents the development of a comprehensive Galactic Interstellar Emission Model (GIEM) for Fermi LAT data, improving the accuracy of gamma-ray source analysis by accounting for diffuse interstellar emission components.

Contribution

The paper introduces a new, publicly available GIEM based on gas maps and inverse Compton emission, enhancing the modeling of Galactic diffuse gamma-ray emission.

Findings

Cosmic-ray proton density decreases with Galactocentric distance.

Proton spectrum softens with increasing distance from Galactic Center.

Fermi bubbles have boundaries similar to a catenary shape at low latitudes.

Abstract

Most of the celestial gamma rays detected by the Large Area Telescope (LAT) aboard the Fermi Gamma-ray Space Telescope originate from the interstellar medium when energetic cosmic rays interact with interstellar nucleons and photons. Conventional point and extended source studies rely on the modeling of this diffuse emission for accurate characterization. We describe here the development of the Galactic Interstellar Emission Model (GIEM) that is the standard adopted by the LAT Collaboration and is publicly available. The model is based on a linear combination of maps for interstellar gas column density in Galactocentric annuli and for the inverse Compton emission produced in the Galaxy. We also include in the GIEM large-scale structures like Loop I and the Fermi bubbles. The measured gas emissivity spectra confirm that the cosmic-ray proton density decreases with Galactocentric distance beyond 5 kpc from the Galactic Center. The measurements also suggest a softening of the proton spectrum with Galactocentric distance. We observe that the Fermi bubbles have boundaries with a shape similar to a catenary at latitudes below 20 degrees and we observe an enhanced emission toward their base extending in the North and South Galactic direction and located within 4 degrees of the Galactic Center.