Title: Measurement of Very-high-energy Diffuse Gamma-ray Emissions from the Galactic Plane with LHAASO-WCDA
Authors: The LHAASO Collaboration
First Author’s Institution: Key Laboratory of Particle Astrophysics & Experimental Physics Division & Computing Center, Institute of High Energy Physics, Chinese Academy of Sciences, 100049 Beijing, China
Status: Published in PRL [open access]
Cosmic Rays (CRs) are high-energy protons and electrons racing through space at nearly the speed of light. Their origins and the mechanisms governing their journey across the universe remain some of the biggest unsolved mysteries in astrophysics. To unravel these mysteries, scientists turn to gamma-ray emissions—produced when CRs collide with the interstellar medium or interact with radiation fields—offering crucial clues about how these energetic particles propagate through galaxies.
These gamma-ray photons have energies that are comparable to or exceed the maximum proton energy (7 TeV) achievable by the Large Hadron Collider (LHC), and detecting them is not an easy task: space-based telescopes have a limited field of view, and ground-based observatories struggle with sensitivity constraints. Among the most well-known gamma-ray instruments are the Fermi Space Telescope, which detects photons below the TeV range, and the Square Kilometer Array (KM2A) of the Large High Altitude Air Shower Observatory (LHAASO), which captures gamma rays between 10 and 1000 TeV. A crucial energy gap remains in the several-TeV range. Filling this gap is essential for constructing a complete picture of diffuse gamma-ray emissions in the Milky Way and refining cosmic ray transport models (see e.g., this review paper). Now, with the Water Cherenkov Detector Array (WCDA) of the LHAASO experiment, this study presents the first detection of gamma rays in the 1–25 TeV range from the Galactic plane, shedding new light on high-energy processes in our galaxy.
The authors present the spectrum of diffuse gamma-ray emission from both the inner and outer regions of the Galactic plane, using data collected over two years. To isolate diffuse emission from CRs, they carefully mask out all known gamma-ray sources along the disk, ensuring that only the background gamma-ray glow remains. In Figure 1, the gray band represents the gamma-ray flux predicted by theoretical models based on local CR intensities and gas column density. However, the actual measurements by LHAASO—marked by red squares—reveal a striking discrepancy: the observed flux is 2.7 times higher in the inner disk and 1.5 times higher in the outer disk than what the model predicts.
Figure 1. Spectral energy distribution of diffuse gamma-ray emission in the inner (left panel) and outer (right panel) Galactic disk. The gray bands represent the predicted flux based on local cosmic ray intensities and gas column densities. These predictions have been scaled up by the factors indicated in the figure (shown in cyan) for comparison. The observed data points exceed the model predictions, highlighting an underestimation in the expected flux. (Fig. 2 from the original paper)
To explore this underestimation, the authors incorporated lower-energy data from the Fermi Telescope in Figure 2 and found that introducing an additional energy component brings the model into excellent agreement with observations across a wide energy range. This discovery hints at the presence of hidden gamma-ray sources that have yet to be identified. What could be responsible for this extra energy? Possible candidates include unresolved, low-surface-brightness sources such as pulsar wind nebulae, young massive star clusters, or even interactions between CRs and the medium where they get accelerated. These findings suggest that the high-energy landscape of our galaxy is more complex than previously thought, with potential new sources shaping the diffuse gamma-ray emission.
Figure 2. Combined gamma-ray spectra of measurements from Fermi and LHAASO for the inner (left panel) and outer (right panel) Galactic disk. Comparison with the model shows that introducing an additional energy component leads to a better agreement with the observed data. (Fig. 4 from the original paper)
To investigate possible spectral breaks, the authors also fit the spectrum in Fig. 1 using a smoothly broken power-law (SBPL) function, which transits gradually between two different slopes rather than shifting abruptly at a single energy. Their analysis reveals a significant break around 30 TeV that steepens the slope of the spectrum by 0.5 in the inner region, while the outer region exhibits a more gradual decline with no significant break.
In order to explore this further, they divide the inner disk into three subregions and re-fit the spectra separately. Interestingly, the third subregion behaves differently from the others, hinting at localized variations in the diffuse gamma-ray emission. This finding suggests that the cosmic ray spectrum may not be uniform across the Galactic plane, potentially influenced by regional differences in CR sources, propagation effects, or interactions with the interstellar medium. These spectral variations provide new insight into the complex structure of high-energy processes in our galaxy.
Figure 3. Energy spectra for different regions of the inner Galactic disk, separated by Galactic longitude. The third region (blue line) exhibits a noticeably different spectral index compared to the others, suggesting potential spectral variations across the Galactic plane. (Fig. 3 from the original paper)
This study provides a crucial step toward understanding the diffuse gamma-ray emission in our Milky Way, revealing the excesses in cosmic-ray-induced gamma rays and showing spectral variations across the Galactic plane. The results suggest the presence of hidden gamma-ray sources and highlight the complexity of high-energy interactions in our galaxy. Future observations with upcoming telescopes, combined with improved theoretical modeling, will be essential to identifying the nature of these hidden contributors and refining our understanding of cosmic ray propagation. With each new discovery, we move closer to unraveling the mysteries of the high-energy universe.
Astrobite edited by Anavi Uppal.
Author
I’m a PhD candidate at the University of Michigan, Ann Arbor. I’m interested in numerical simulations of cosmic rays feedback in galaxies and their comparison with observation.
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