Diffuse Galactic continuum gamma rays. A model compatible with EGRET data and cosmic-ray measurements

TL;DR

This study tackles the GeV excess in the diffuse Galactic gamma-ray emission by testing CR propagation models against EGRET data across the full sky up to 100 GeV. It employs the GALPROP framework to compute gamma-ray components from protons, electrons, and secondary species, constraining the proton spectrum with secondary antiprotons and the electron spectrum with the gamma-ray data. An optimized model with modest deviations from locally measured spectra provides a good fit across 30 MeV–100 GeV and across latitude and longitude profiles, highlighting the pivotal role of inverse Compton emission and secondary leptons in shaping the diffuse emission. The results improve consistency with CR measurements and have implications for the extragalactic gamma-ray background estimates, demonstrating that a physically motivated propagation model can account for the observed emission without invoking drastic spectral changes or exotic sources.

Abstract

We present a study of the compatibility of some current models of the diffuse Galactic continuum gamma rays with EGRET data. A set of regions sampling the whole sky is chosen to provide a comprehensive range of tests. The range of EGRET data used is extended to 100 GeV. The models are computed with our GALPROP cosmic-ray propagation and gamma-ray production code. We confirm that the "conventional model" based on the locally observed electron and nucleon spectra is inadequate, for all sky regions. A conventional model plus hard sources in the inner Galaxy is also inadequate, since this cannot explain the GeV excess away from the Galactic plane. Models with a hard electron injection spectrum are inconsistent with the local spectrum even considering the expected fluctuations; they are also inconsistent with the EGRET data above 10 GeV. We present a new model which fits the spectrum in all sky regions adequately. Secondary antiproton data were used to fix the Galactic average proton spectrum, while the electron spectrum is adjusted using the spectrum of diffuse emission itself. The derived electron and proton spectra are compatible with those measured locally considering fluctuations due to energy losses, propagation, or possibly details of Galactic structure. This model requires a much less dramatic variation in the electron spectrum than models with a hard electron injection spectrum, and moreover it fits the gamma-ray spectrum better and to the highest EGRET energies. It gives a good representation of the latitude distribution of the gamma-ray emission from the plane to the poles, and of the longitude distribution. We show that secondary positrons and electrons make an essential contribution to Galactic diffuse gamma-ray emission.

Figures (13)

• Figure 1: B/C ratio as calculated in reacceleration model. Lower curve -- LIS, upper -- modulated ($\Phi=450$ MV). Data below 200 MeV/nucleon: ACE davis, Ulysses *ulysses_bc, Voyager *voyager; high energy data: HEAO-3 Engelmann90, for other references see StephensStreitmatter98.
• Figure 2: Proton spectra as calculated in conventional (solid lines) and optimized (dots) models compared with the data (upper curve -- LIS, lower -- modulated to 650 MV). Thin dotted line shows the LIS spectrum best fitted to the data above 20 GeV M02. Data: AMS p_ams, BESS 98 sanuki00, CAPRICE 94 Boez99, IMAX 92 Menn00, LEAP 87 p_leap.
• Figure 3: Electron spectra for conventional (solid), hard electron (dashes), and optimized models (dots), compared with data (upper curve -- LIS, lower -- modulated to 600 MV). Data: AMS leptons_ams, CAPRICE 94 Boez00, HEAT 94-95 duvernois01 MASS 91 grimani02, Sanriku kobayashi99.
• Figure 4: $\gamma$-ray spectrum of conventional model (44-500180) for the sky regions described in Table \ref{['sky_regions']}: top row H--A--B, middle row C--D--E, bottom F. The model components are: $\pi^0$-decay (dots, red), IC (dashes, green), bremsstrahlung (dash-dot, cyan), EGRB (thin solid, black), total (thick solid, blue). EGRET data: red vertical bars. COMPTEL data: green vertical bars. NB EGRB is added to the total prediction for the EGRET energy range only.
• Figure 5: Antiproton flux as calculated in conventional and optimized models compared with the data (upper curve -- LIS, lower -- modulated to 550 MV). The lines are coded as in Fig. \ref{['fig:protons']}. Data: BESS 95-97 Orito00, BESS 98 Asaoka02, MASS 91 basini99, CAPRICE 98 boezio01.