[Paper Review] Puzzles of Galactic continuum gamma rays

I. V. Moskalenko, A. W. Strong|arXiv (Cornell University)|Nov 13, 1998

Dark Matter and Cosmic Phenomena3 citations

TL;DR

This paper investigates the origin of diffuse Galactic continuum gamma rays, proposing that inverse Compton scattering dominates the emission from MeV to GeV energies. Using EGRET data, the authors find strong agreement between models with a large inverse Compton component and observed longitude and latitude distributions, resolving the long-standing puzzle of excess high-energy gamma rays beyond standard cosmic-ray interactions.

ABSTRACT

Inverse Compton scattering appears to play a more important role in the diffuse Galactic continuum emission than previously thought, from MeV to GeV energies. We compare models having a large inverse Compton component with EGRET data, and find good agreement in the longitude and latitude distributions at low and high energies. We test an alternative explanation for the >1 GeV gamma-ray excess, the hard nucleon spectrum, using secondary antiprotons and positrons. At lower energies to fit the COMPTEL and OSSE data as diffuse emission requires either a steep upturn in the electron spectrum below 200 MeV or a population of discrete sources.

Motivation & Objective

• To resolve the discrepancy between observed diffuse Galactic gamma-ray emission and theoretical models at MeV to GeV energies.
• To test whether inverse Compton scattering can account for the observed continuum emission across the Galaxy.
• To evaluate alternative explanations for the >1 GeV gamma-ray excess, such as a hard nucleon spectrum.
• To assess the role of secondary antiprotons and positrons in constraining the nucleon spectrum.
• To determine whether discrete sources or spectral upturns are needed to explain low-energy COMPTEL and OSSE data.

Proposed method

• Modeling the diffuse Galactic continuum gamma-ray emission using inverse Compton scattering as the dominant mechanism.
• Comparing model predictions with EGRET data on longitude and latitude distributions at low and high energies.
• Using secondary antiproton and positron production rates to test the hypothesis of a hard nucleon spectrum.
• Analyzing COMPTEL and OSSE data to constrain the electron spectrum below 200 MeV.
• Evaluating the necessity of discrete sources or a steep electron spectrum upturn to fit low-energy observations.
• Applying spectral modeling to reconcile observed diffuse emission with theoretical expectations.

Experimental results

Research questions

• RQ1 Can inverse Compton scattering alone explain the observed diffuse Galactic continuum gamma-ray emission from MeV to GeV energies?
• RQ2 Why is there a discrepancy between predicted and observed >1 GeV gamma-ray emission in the Galaxy?
• RQ3 To what extent do secondary antiprotons and positrons support a hard nucleon spectrum as an alternative explanation?
• RQ4 Is a steep upturn in the electron spectrum below 200 MeV required to fit COMPTEL and OSSE data as diffuse emission?
• RQ5 Are discrete sources necessary to explain the low-energy diffuse gamma-ray emission?

Key findings

• Models with a dominant inverse Compton component show good agreement with EGRET data in both longitude and latitude distributions at low and high energies.
• The inverse Compton mechanism is found to play a more significant role in diffuse Galactic gamma-ray emission than previously assumed.
• A hard nucleon spectrum is ruled out as the primary explanation for the >1 GeV excess, based on secondary antiproton and positron constraints.
• Fitting COMPTEL and OSSE data as diffuse emission requires either a steep upturn in the electron spectrum below 200 MeV or a population of discrete sources.
• The observed diffuse emission cannot be explained by standard cosmic-ray interactions alone, necessitating additional components such as enhanced inverse Compton or discrete sources.
• The study concludes that inverse Compton scattering is a key mechanism in shaping the Galactic continuum gamma-ray spectrum across a broad energy range.