Measurement of Very-high-energy Diffuse Gamma-ray Emissions from the Galactic Plane with LHAASO-WCDA

Abstract

The diffuse Galactic gamma-ray emission is a very important tool used to study the propagation and interaction of cosmic rays in the Milky Way. In this work, we report the measurements of the diffuse emission from the Galactic plane, covering Galactic longitudes from $15^{\circ}$ to $235^{\circ}$ and latitudes from $-5^{\circ}$ to $+5^{\circ}$, in an energy range of 1 TeV to 25 TeV, with the Water Cherenkov Detector Array (WCDA) of the Large High Altitude Air Shower Observatory (LHAASO). After masking the sky regions of known sources, the diffuse emission is detected with $24.6σ$ and $9.1σ$ significance in the inner Galactic plane and outer Galactic plane, respectively. The WCDA spectra in both regions can be well described by a power-law function, with spectral indices of $-2.67\pm0.05_{\rm stat}$ in the inner region and $-2.83\pm0.19_{\rm stat}$ in the outer region, respectively. Combined with the Square Kilometer Array (KM2A) measurements at higher energies, a clear softening of the spectrum is found in the inner region, with change of spectral indices by $\sim0.5$ at a break energy around $30$ TeV. The fluxes of the diffuse emission are higher by a factor of $1.5-2.7$ than the model prediction assuming local CR spectra and the gas column density, which are consistent with those measured by the KM2A. Along Galactic longitude, the spatial distribution of the diffuse emission shows deviation from that of the gas column density. The spectral shape of the diffuse emission are possibly variation in different longitude region. The WCDA measurements bridge the gap between the low-energy measurements by space detectors and the ultra-high-energy observations by LHAASO-KM2A and other experiments. These results suggest that improved modeling of the wide-band diffuse emission is required.

Figures (5)

• Figure 1: The significance maps in Galactic coordinate of the inner Galaxy region (top panel) and outer Galaxy region (bottom panel)) for $N_{\rm hit}\geq200$ (corresponding to energies bigger than several TeV) after masking the resolved LHAASO and TeVCat sources.
• Figure 2: SEDs of the diffuse $\gamma$-ray emission in the inner (left) and outer (right) regions. Error bars are the statistical errors (short red) and the quadratic sum of the statistical and systematic errors (long black). Dashed lines show the best-fit SBPL spectra. The grey shaded bands show the predicted diffuse fluxes from local CR intensities and the line-of-sight integral gas content, and the cyan bands are upscaled by constant factors as labelled in the plots.
• Figure 3: Fluxes of diffuse $\gamma$-ray emission in the three sub-regions of the inner Galactic plane measured by the WCDA. Only the statistical errors are shown.
• Figure 4: Galactic latitude and longitude profiles of the diffuse fluxes for $N_{\rm hit}\geq 200$ (corresponding to an energy range of about $3-25$ TeV), compared with the gas distributions as shown by solid lines.
• Figure 5: Wide-band fluxes of diffuse $\gamma$-ray emission in the inner and outer Galaxy regions. To make the comparison more directly, the Fermi-LAT results with the same mask used in this work. The ARGO-YBJ and AS$\gamma$ data in the left panel are for a slightly different region of $25^\circ<l<100^\circ$ and $|b|<5^\circ$, with different source masks 2015ApJ...806...20B2021PhRvL.126n1101A. Red dotted lines show the prediction from a conventional CR propagation and interaction model 2023ApJ...957...43Z, green dashed lines show the contribution from an assumed extra component, and black solid lines are the sum of these two components.