[Paper Review] A GeV-TeV particle component and the barrier of cosmic-ray sea in the Central Molecular Zone

Xiaoyuan Huang, Qiang Yuan+1 more|arXiv (Cornell University)|Dec 10, 2020

Astrophysics and Cosmic Phenomena67 references15 citations

TL;DR

This study identifies a GeV-TeV cosmic ray component in the Galactic Center's Central Molecular Zone (CMZ) using Fermi-LAT γ-ray data, revealing a harder spectrum and lower flux than expected from the diffuse cosmic-ray sea. The discrepancy implies a high-energy accelerator at the Galactic center and a suppressive barrier—likely magnetic—limiting cosmic-ray sea penetration into the CMZ.

ABSTRACT

Cosmic rays are important probe of a number of fundamental physical problems such as the acceleration of high and very high energy particles in extreme astrophysical environments. The Galactic center is widely anticipated to be an important cosmic-ray source and the observations of some Imaging Atmospheric Cherenkov Telescopes did successfully reveal a component of TeV-PeV cosmic rays in the vicinity of the Galactic center. Here we report the identification of GeV-TeV cosmic rays in the central molecular zone with the $\gamma$-ray observations of the Fermi Large Area Telescope, whose spectrum and spatial gradient are consistent with that measured by the Imaging Atmospheric Cherenkov Telescopes but the corresponding cosmic-ray energy density is substantially lower than the so-called cosmic-ray sea component, suggesting the presence of a high energy particle accelerator at the Galactic center and the existence of a barrier that can effectively suppress the penetration of the particles from the cosmic-ray sea to the central molecular zone.

Motivation & Objective

• To identify and characterize a low-energy cosmic ray component in the Central Molecular Zone (CMZ) using Fermi-LAT γ-ray data.
• To determine whether the observed γ-ray emission in the CMZ arises from interactions between the cosmic-ray sea and molecular gas or from a distinct, localized source.
• To investigate the spatial and spectral properties of cosmic rays in the CMZ relative to the broader Galactic cosmic-ray distribution.
• To assess the role of the Galactic Center GeV Excess (GCE) in shaping the observed γ-ray emission and to disentangle its contribution.
• To infer the existence of a barrier suppressing cosmic-ray sea penetration into the CMZ based on flux and spectral discrepancies.

Proposed method

• Performed a morphological and spectral analysis of Fermi-LAT data in the CMZ region, using GALPROP and Planck dust opacity templates to model γ-ray emission from neutral pion decay.
• Fitted the γ-ray flux and spectral index in the CMZ and off-CMZ regions, comparing observed spectra to predictions from the cosmic-ray sea model.
• Incorporated the Galactic Center GeV Excess (GCE) component into the diffuse emission model to avoid bias in the analysis.
• Used the GALPROP code to simulate cosmic ray propagation and γ-ray production from interactions with molecular gas.
• Applied confidence intervals (1σ, 2σ) to assess uncertainties in spectral and flux measurements.
• Compared results using different gas templates (GALPROP vs. Planck) to reduce uncertainties from the XCO factor.

Experimental results

Research questions

• RQ1 Is there a distinct GeV-TeV cosmic ray component in the CMZ that differs from the diffuse cosmic-ray sea?
• RQ2 Does the spectral hardness and flux level of γ-ray emission in the CMZ match expectations from cosmic-ray sea interactions with molecular gas?
• RQ3 What is the role of the Galactic Center GeV Excess (GCE) in the observed γ-ray emission, and how should it be accounted for in the analysis?
• RQ4 Could a physical barrier—such as a strong magnetic field—suppress the penetration of cosmic rays from the sea into the CMZ?
• RQ5 What does the discrepancy between observed and predicted fluxes imply about the cosmic ray environment in the Galactic center?

Key findings

• A GeV-TeV cosmic ray component was identified in the CMZ with a spectral index of 2.49 ± 0.07, harder than the predicted 2.68 from the GALPROP Galactic Diffuse Emission model.
• The integrated γ-ray flux from 8 GeV to 500 GeV in the CMZ is (7.05 ± 0.44) × 10⁻⁵ MeV cm⁻² s⁻¹, significantly lower than the flux expected from the cosmic-ray sea interaction.
• The observed spectrum is consistent with VHE observations from H.E.S.S. and other IACTs, supporting the presence of a high-energy particle accelerator at the Galactic center.
• The inferred cosmic ray energy density in the CMZ is substantially lower than that of the extrapolated cosmic-ray sea, indicating suppression of external particle influx.
• The discrepancy is best explained by a barrier—likely magnetic—surrounding the CMZ that limits penetration of cosmic rays from the Galactic disk.
• Results remain robust when using the Planck dust opacity template instead of GALPROP, reducing uncertainty from the XCO factor in gas density conversion.