Two-component diffuse Galactic gamma-ray emission revealed with Fermi-LAT

Abstract

The enigma of cosmic ray origin and propagation stands as a key question in particle astrophysics. The precise spatial and spectral measurements of diffuse Galactic gamma-ray emission provide new avenues for unraveling this mystery. Based on 16 years of Fermi-LAT observations, we find that the diffuse gamma-ray spectral shapes are nearly identical for low energies (below a few GeV) but show significant dispersion at high energies (above a few GeV) across the Galactic disk. We further show that the diffuse emission can be decomposed into two components, a universal spectral component dominating at low energies which is consistent with the expectation from interactions of background cosmic rays and the interstellar matter, and a spatially variant component dominating at high energies which is likely due to local accelerators. These findings suggest that there is dual-origin of the Galactic diffuse emission, including the ``cosmic ray sea'' from efficient propagation of particles and the ``cosmic ray islands'' from inefficient propagation of particles, and thus shed new light on the understanding of the propagation models of Galactic cosmic rays.

Figures (6)

• Figure 1: SEDs of diffuse Galactic $\gamma$-ray emission from 18 segments of the Galactic plane. All the fluxes are normalized at 0.6 GeV. Different colors represent different Galactic longitude bins.
• Figure 2: The fluxes (a) and photon indices (b) in different sky regions. Blue color is for $0.1-1$ GeV band, and red is for $10-1000$ GeV band. In panel b, dashed lines correspond to fitting results of the spectral indices with a constant.
• Figure 3: Tow component flux which is integrate of spectrum. Component A is derived from the locally observed CR spectrum after correction of the solar modulation, and component B is the additional cutoff power-law spectrum. The red line represents the gas distributions. The blue lines represent the source distributions for supernova remnants Case:1998qg, pulsars Lorimer:2006qsYusifov:2004fr, and OB stars Bronfman:2000tw. Component A is well tracked by the gas distribution, and component B is consistent with the source distributions.
• Figure A1: The proton spectrum (a) and corresponding $\gamma$-ray spectrum (b) for component A. In panel (a), the measurements by Voyager Cummings:2016pdr, AMS-02 AMS:2021nhj, and DAMPE DAMPE:2019gys are shown. To get the proton fluxes in the local interstellar medium, the AMS-02 data are de-modulated using the force-field model with modulation potential of 0.43 GV. The blue line is the interpolation of the Voyager and de-modulated AMS-02 data below 200 GeV. Panel (b) shows the corresponding $\gamma$-ray spectrum calculated using the interpolated proton spectrum of panel (a), compared with the normalized measurements derived in this work.
• Figure A2: The two-component fitting results of the SEDs in the 18 ROIs. The black dots represent the data we analysed. The red, blue, and black lines represent the $\gamma$-ray spectra for component A, B, and their sum, respectively. In the Galactic center region, the magenta dotted line represents the spectrum of Fermi bubbles, and the brown dashed line represents the spectrum of the GCE. The open dots are the data including Fermi bubbles and the GCE, while the solid ones are fluxes excluding them.