Diffuse Cosmic Rays Shining in the Galactic Center: A Novel Interpretation of H.E.S.S. and Fermi-LAT $\gamma$-Ray Data

We present a novel interpretation of the $\gamma$-ray diffuse emission measured by Fermi-LAT and H.E.S.S. in the Galactic center (GC) region and the Galactic ridge (GR). In the first part we perform a data-driven analysis based on PASS8 Fermi-LAT data: We extend down to a few GeV the spectra measured by H.E.S.S. and infer the primary cosmic-ray (CR) radial distribution between 0.1 and 3 TeV. In the second part we adopt a CR transport model based on a position-dependent diffusion coefficient. Such behavior reproduces the radial dependence of the CR spectral index recently inferred from the Fermi-LAT observations. We find that the bulk of the GR emission can be naturally explained by the interaction of the diffuse steady-state Galactic CR sea with the gas present in the central molecular zone. Although we confirm the presence of a residual radial-dependent emission associated with a central source, the relevance of the large-scale diffuse component prevents to claim a solid evidence of GC pevatrons.

Figure 1

The $\gamma$-ray spectrum in the GR region ($|l|<0.8°$, $|b|<0.3°$). Fermi-LAT data, shown here for the first time, and H.E.S.S. data from Ref. [4] are compared with the contribution of the Galactic CR sea as computed with the gamma and base models discussed in the text. The single power-law best fit of the combined data is also reported. We have subtracted the contribution of point sources from Fermi-LAT data.

Figure 2

Same as for Fig. 1 but for the pacman region defined in the text.

Figure 3

The CR energy density radial profiles for $E_{\mathrm{CR}}>10\mathrm{TeV}$, as determined by H.E.S.S. [1], and for $0.1\le E_{\mathrm{CR}}\le 3\mathrm{TeV}$, as determined here from Fermi-LAT data, are reported. Those data are compared with the gamma model predictions (solid lines). The regions of the sky used for deriving the data are represented in the inset. The model energy density profiles on Galactic scales are reported in the Supplemental Material [20].