Kavli Affiliate: Li Xin Li
| First 5 Authors: Zhen Cao, F. Aharonian, Q. An, Axikegu, Y. X. Bai
| Summary:
The diffuse Galactic $gamma$-ray emission, mainly produced via interactions
between cosmic rays and the diffuse interstellar medium, is a very important
probe of the distribution, propagation, and interaction of cosmic rays in the
Milky Way. In this work we report the measurements of diffuse $gamma$-rays
from the Galactic plane between 10 TeV and 1 PeV energies, with the square
kilometer array of the Large High Altitude Air Shower Observatory (LHAASO).
Diffuse emissions from the inner ($15^{circ}<l<125^{circ}$, $|b|<5^{circ}$)
and outer ($125^{circ}<l<235^{circ}$, $|b|<5^{circ}$) Galactic plane are
detected with $29.1sigma$ and $12.7sigma$ significance, respectively. The
outer Galactic plane diffuse emission is detected for the first time in the
very- to ultra-high-energy domain ($E>10$~TeV). The energy spectrum in the
inner Galaxy regions can be described by a power-law function with an index of
$-2.99pm0.04$, which is different from the curved spectrum as expected from
hadronic interactions between locally measured cosmic rays and the
line-of-sight integrated gas content. Furthermore, the measured flux is higher
by a factor of $sim3$ than the prediction. A similar spectrum with an index of
$-2.99pm0.07$ is found in the outer Galaxy region, and the absolute flux for
$10lesssim Elesssim60$ TeV is again higher than the prediction for hadronic
cosmic ray interactions. The latitude distributions of the diffuse emission are
consistent with the gas distribution, while the longitude distributions show
slight deviation from the gas distribution. The LHAASO measurements imply that
either additional emission sources exist or cosmic ray intensities have spatial
variations.
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