Spectrum of the isotropic diffuse gamma-ray emission derived from first-year Fermi Large Area Telescope data

Area of Science:

• * Astrophysics
• * High-energy astrophysics
• * Gamma-ray astronomy

Background:

• * The extragalactic gamma-ray background (EGB) is a diffuse emission originating from outside the Milky Way.
• * Its precise nature and contributions from various sources are still debated.
• * Previous measurements, notably from EGRET, suggested a harder spectrum than observed here.

Purpose of the Study:

• * To present the first measurements of the EGB using data from the Fermi Large Area Telescope (LAT).
• * To characterize the spectral properties and intensity of the EGB.
• * To compare these findings with previous results and theoretical models.

Main Methods:

• * Detailed modeling of foreground Galactic diffuse gamma-ray emission.
• * Inclusion of detected LAT sources and solar gamma-ray emission in the analysis.
• * Derivation of the EGB by subtracting foreground components from the total observed gamma-ray flux.

Main Results:

• * The EGB spectrum is consistent with a power law with a differential spectral index of gamma = 2.41 ± 0.05.
• * The intensity of the EGB above 100 MeV is (1.03 ± 0.17) x 10^-5 cm^-2 s^-1 sr^-1.
• * The derived EGB spectrum is featureless, less intense, and softer than that reported by EGRET.

Conclusions:

• * The Fermi-LAT measurements provide a more precise characterization of the EGB.
• * The softer and less intense spectrum suggests a different contribution from astrophysical sources compared to earlier estimates.
• * These results offer crucial data for understanding the origin of high-energy cosmic rays and their interactions in the universe.