First Detection of sub-PeV Diffuse Gamma Rays from the Galactic Disk: Evidence for Ubiquitous Galactic Cosmic Rays beyond PeV Energies

We report, for the first time, the long-awaited detection of diffuse gamma rays with energies between 100 TeV and 1 PeV in the Galactic disk. Particularly, all gamma rays above 398 TeV are observed apart from known TeV gamma-ray sources and compatible with expectations from the hadronic emission scenario in which gamma rays originate from the decay of ${\pi }^0$’s produced through the interaction of protons with the interstellar medium in the Galaxy. This is strong evidence that cosmic rays are accelerated beyond PeV energies in our Galaxy and spread over the Galactic disk.

Figure 1

The arrival direction of each gamma-ray-like event observed with (a) $100<E<158\mathrm{TeV}$, (b) $158<E<398\mathrm{TeV}$, and (c) $398<E<1000\mathrm{TeV}$, respectively, in the equatorial coordinate. The blue solid circles show arrival directions of gamma-ray-like events observed by the Tibet $\mathrm{AS}+\mathrm{MD}$ array. The area of each circle is proportional to the measured energy of each event. The red plus marks show directions of the known Galactic TeV sources (including the unidentified sources) listed in the TeV gamma-ray catalog [9]. The solid curve indicates the Galactic plane, while the shaded areas indicate the sky regions outside the field of view of the Tibet $\mathrm{AS}+\mathrm{MD}$ array.

Figure 2

Gamma-ray excess counts as a function of Galactic latitude with (a) $100<E<158\mathrm{TeV}$, (b) $158<E<398\mathrm{TeV}$, and (c) $398<E<1000\mathrm{TeV}$. The excess count is calculated from the observed event number after subtracting the estimated background event number (see the text). The Galactic longitude of the arrival direction in each figure is integrated across our FOV (approximately $22°<l<225°$). The solid circles show the experimental data, while the shaded histograms display the model profile [8] rebinned in every (a),(b) 1° and (c) 5° of the Galactic latitude. The downward arrows show upper limits of excess at 68% confidence level. The number of excess in the model, which is independent of energy, is normalized to the observed number within $|b|<5°$.

Figure 3

The distribution of the angular distance between the arrival direction of each observed gamma-ray-like event with $E>398\mathrm{TeV}$ and the direction of its closest known TeV source listed in the TeV gamma-ray catalog [9]. The red solid circles show the observed data, while the dashed and solid histograms display the MC results expected from the isotropic event distribution and the diffuse gamma-ray model [8], respectively, to be observed with our geometrical exposure.

Figure 4

Differential energy spectra of the diffuse gamma rays from the Galactic plane in the regions of (a) $|b|<5°$, $25°<l<100°$ and (b) $|b|<5°$, $50°<l<200°$, respectively. The solid circles show the observed flux after excluding the contribution from the known TeV sources listed in the TeV gamma-ray catalog [9], while the solid and dashed curves display the predicted energy spectra by the space-independent and space-dependent models by Lipari and Vernetto [8], respectively (see the text). The dotted curve in (a) shows the flux predicted by a leptonic model [36] in which gamma rays are induced by relativistic electrons and positrons from pulsars. Solid squares in (a) and triangles with arrows in (b) indicate the flux measured by ARGO-YBJ [17] and the flux upper limit by the CASA-MIA experiment [18], respectively. The error bar shows $1\sigma$ statistical error.