Multimessenger Concordance for the Cygnus Region as the Source of the Cosmic-Ray Knee

Abstract

The origin of the cosmic-ray (CR) knee remains one of the central open questions in particle astrophysics. Recent measurements by the Large High Altitude Air Shower Observatory revealed a pronounced feature in the proton spectrum at $\sim3-4$~PeV, while observations of diffuse gamma rays above $100$~TeV do not exhibit a corresponding spectral break. This apparent discrepancy challenges the standard interpretation, in which the local CR distribution is representative of the Galactic CR sea. Here, we investigate whether the CR knee can instead originate from the Cygnus region as a nearby PeVatron. By combining CR measurements at Earth with very-high-energy gamma-ray observations from LHAASO and the Tibet-AS$γ$ experiment, we identify an additional hard gamma-ray component in the inner Galaxy consistent with a source located in the Cygnus region. We show that our results provide a concordance multimessenger picture. The required properties are compatible with the PeVatron candidate detected by LHAASO in the Cygnus bubble and with the Galactic neutrino flux observed by the IceCube Neutrino Observatory.

Figures (8)

• Figure 1: Summary plot of our results on contributions from the Cygnus region to the CR proton flux (solid green line), diffuse gamma-ray and neutrino fluxes (red and blue solid lines, respectively), as observed in different sky regions. The Galactic CR sea component is shown by the dotted green line and its diffuse gamma-ray and neutrino components are shown with red and blue dotted lines, respectively. Total gamma-ray and neutrino fluxes (corresponding to the sum of the Galactic and Cygnus components, as well as the contribution from unresolved sources) are displayed with red and blue dashed lines, respectively. For details on the computation of each flux component, see Sections \ref{['sec:Methods']} and \ref{['sec:Results']}. For gamma rays, the flux is computed for the LHAASO inner Galactic region ($|b|<5^\circ$, $15^\circ<l<125^\circ$); while for neutrinos, the flux is computed for the Galactic plane region of IceCube ($|b|<90^\circ$, $0^\circ<l<360^\circ$). Cygnus proton fluxes are compared to observations by direct missions (DAMPE An_2019, CALET Adriani_2019, CREAM Yoon_2017 and ATIC-2 Panov_2009) and ground-bases experiments (IceTop Aartsen_2019 and LHAASO Cao_2025bLHAASO2025), shown with green scatter points. Observations of diffuse gamma rays in the corresponding LHAASO sky region Cao_2025a displayed as red scatter points. Best-fit Galactic diffuse (all-flavor) neutrino flux based on IceCube observations IceCube:2023ame for three different templates ($\pi^0$ and KRA$^5$ and KRA$^{50}$) are shown as blue shaded area, spanned by the minimum and maximum best-fit fluxes.
• Figure 2: Schematic representation (not in scale) of the Cygnus region, as located in the context of the two LHAASO and Tibet-AS$\gamma$ regions and with respect to the Earth position. With a green circle we illustrate the slow diffusion zone (the Cygnus bubble) and with a dashed red lines we show the diffusion region of PeV protons.
• Figure 3: Flux of CR protons (left panel) and helium (right panel) as a function of energy per nuclei. Observational data by direct detection experiments (PAMELA Adriani_2011, AMS-02 Aguilar_2015aAguilar_2015b, DAMPE An_2019Alemanno_2021, CALET Adriani_2019Adriani_2023, CREAM Yoon_2017 and ATIC-2 Panov_2009) as well as ground-based observatories (IceTop Aartsen_2019, KASCADE-Grande Finger_2011 and LHAASO Cao_2025bLHAASO2025) are shown with colored scatter points. Predicted Cygnus contributions are represented with black dotted-line for source injection parameters, $W_p=2.8\times10^{51}$ erg, $s=2.1$ and $E^{\rm max}=Z \cdot 5$ PeV, for atomic number $Z$. Best-fit of the Galactic population shown with black dashed-line. Black solid lines correspond to sum of both (Galactic and Cygnus) components and shaded areas indicate the energy range of interest for the production of TeV-PeV gamma rays and neutrinos.
• Figure 4: Diffuse gamma-ray flux in LHAASO inner ($|b|<5^\circ$, $15^\circ<l<125^\circ$, top panel) and outer ($|b|<5^\circ$, $125^\circ<l<235^\circ$, bottom panel) Galaxy regions after LHAASO's mask is applied. The gamma-flux expectations for the Galactic component $\phi_\gamma^{\text{Gal}}(E_\gamma,\hat{n}_\gamma)$ the and Cygnus contribution $\phi_\gamma^{\text{Cyg}}(E_\gamma,\hat{n}_\gamma)$ are shown in blue bands and green lines, respectively. We also show with a yellow line, the contribution expected from unresolved sources in these regions as described in the text. The total diffuse gamma-ray flux predicted at Earth $\phi_\gamma (E_\gamma,\hat{n}_\gamma)$ is represented by red bands. Observational data by LHAASO Cao_2025a are added with black scatter points.
• Figure 5: Ratio of diffuse gamma-ray flux between LHAASO inner and outer Galaxy regions. The ratio between LHAASO observation data after the mask is applied is shown with black scatter points, while the observations before the mask are shown with blue scatter points. The ratio from our computed diffuse fluxes are shown with red solid and dashed lines, for masked and unmasked cases, respectively.