Fermi Large Area Telescope Measurements of the Diffuse Gamma-Ray Emission at Intermediate Galactic Latitudes

The diffuse galactic $\gamma$-ray emission is produced by cosmic rays (CRs) interacting with the interstellar gas and radiation field. Measurements by the Energetic Gamma-Ray Experiment Telescope (EGRET) instrument on the Compton Gamma-Ray Observatory indicated excess $\gamma$-ray emission $\gtrsim 1\mathrm{GeV}$ relative to diffuse galactic $\gamma$-ray emission models consistent with directly measured CR spectra (the so-called “EGRET GeV excess”). The Large Area Telescope (LAT) instrument on the Fermi Gamma-Ray Space Telescope has measured the diffuse $\gamma$-ray emission with improved sensitivity and resolution compared to EGRET. We report on LAT measurements for energies 100 MeV to 10 GeV and galactic latitudes $10°\le |b|\le 20°$. The LAT spectrum for this region of the sky is well reproduced by a diffuse galactic $\gamma$-ray emission model that is consistent with local CR spectra and inconsistent with the EGRET GeV excess.

Figure 1

Diffuse emission intensity averaged over all galactic longitudes for latitude range $10°\le |b|\le 20°$. Data points: LAT, red dots; EGRET, blue crosses. Systematic uncertainties: LAT, red; EGRET, blue.

Figure 2

LAT data with model, source, and UIB components for sky region in Fig. 1. Model (lines): ${\pi }^0$-decay, red; bremsstrahlung, magenta; IC, green. Shaded/hatched regions: UIB, grey/solid; source, blue/hatched; total ($\mathrm{\text{model}}+\mathrm{UIB}+\mathrm{\text{source}}$), black/hatched.