Cosmic-Ray Models of the Ridge-Like Excess of Gamma Rays in the Galactic Center

Abstract

The High-Energy Stereoscopic System (HESS) has detected diffuse TeV emission correlated with the distribution of molecular gas along the Ridge at the Galactic Center. Diffuse, non-thermal emission is also seen by the Fermi large area telescope (Fermi-LAT) in the GeV range and by radio telescopes in the GHz range. Additionally, there is a distinct, spherically symmetric excess of gamma rays seen by Fermi-LAT in the GeV range. A cosmic ray flare, occurring in the Galactic Center, $10^4$ years ago has been proposed to explain the TeV Ridge. An alternative, steady-state model explaining all three data sets (TeV, GeV, and radio) invokes purely leptonic processes. We show that the flare model from the Galactic Center also provides an acceptable fit to the GeV and radio data, provided the diffusion coefficient is energy independent. However, if Kolmogorov-type turbulence is assumed for the diffusion coefficient, we find that two flares are needed, one for the TeV data (occurring approximately $10^4$ years ago) and an older one for the GeV data (approximately $10^5$ years old). We find that the flare models we investigate do not fit the spherically symmetric GeV excess as well as the usual generalized Navarro-Frenk-White spatial profile, but are better suited to explaining the Ridge. We also show that a range of single-zone, steady-state models are able to explain all three spectral data sets. Large gas densities equal to the volumetric average in the region can be accommodated by an energy independent diffusion or streaming based steady-state model. Additionally, we investigate how the flare and steady-state models may be distinguished with future gamma-ray data looking for a spatial dependence of the gamma-ray spectral index.

V3: Minor changes made. Matches version published by MNRAS

References in corpus (12)

• Parameterization of Gamma, e^+/- and Neutrino Spectra Produced by p-p Interaction in Astronomical Environment
• Magnetic Fields in Starburst Galaxies and The Origin of the FIR-Radio Correlation
• Spatial distribution of interstellar gas in the innermost 3 kpc of our Galaxy
• Galactic electrons and positrons at the Earth:new estimate of the primary and secondary fluxes
• Cosmic Ray Protons in the Inner Galaxy and the Galactic Center Gamma-Ray Excess
• The Starburst Contribution to the Extra-Galactic Gamma-Ray Background
• Galactic Center gamma-ray "excess" from an active past of the Galactic Centre?
• Wild at Heart:-The Particle Astrophysics of the Galactic Centre
• Updated cosmic-ray and radio constraints on light dark matter: Implications for the GeV gamma-ray excess at the Galactic center
• GBT Multiwavelength Survey of the Galactic Center Region
• The Cosmic Ray Population of the Galactic Central Molecular Zone
• Diffuse TeV Emission at the Galactic Centre

Cited by in corpus (6)

• Cosmological constraints on dark matter annihilation and decay: Cross-correlation analysis of the extragalactic $\gamma$-ray background and cosmic shear
• Molecular Clouds as the Origin of the Fermi Gamma-Ray GeV-Excess
• Inverse Compton emission from millisecond pulsars in the Galactic bulge
• Does the SN rate explain the very high energy cosmic rays in the central 200 pc of our Galaxy?
• Origin of X-ray and gamma-ray emission from the Galactic central region
• High Energy Emission from the Galactic Center