Constraining the Cosmic-ray Energy Based on Observations of Nearby Galaxy Clusters by LHAASO

Abstract

Galaxy clusters act as reservoirs of high-energy cosmic rays (CRs). As CRs propagate through the intracluster medium, they generate diffuse ${\hat{I}}^3$-rays detectable by arrays such as LHAASO. These ${\hat{I}}^3$-rays result from proton-proton ($pp$) collisions of very high-energy cosmic rays (VHECRs) or inverse Compton (IC) scattering of positron-electron pairs created by $p{\hat{I}}^3$ interactions of ultra-high-energy cosmic rays (UHECRs). We analyzed diffuse ${\hat{I}}^3$-ray emission from the Coma, Perseus, and Virgo clusters using LHAASO data. Diffuse emission was modeled as a disk of radius $R_{500}$ for each cluster while accounting for point sources. No significant diffuse emission was detected, yielding 95\% confidence level (C.L.) upper limits on the ${\hat{I}}^3$-ray flux: for WCDA (1-25~TeV) and KM2A ($>25$~TeV), less than $(49.4,13.7,54.0)$ and $(1.34,1.14,0.40)\times 10^{-14}$~ph~cm${}^{-2}$~s${}^{-1}$ for Coma, Perseus, and Virgo, respectively. The ${\hat{I}}^3$-ray upper limits can be used to derive model-independent constraints on the integral energy of CRp above 10~TeV (corresponding to the LHAASO observational range $>1$~TeV under the $pp$ scenario) to be less than $(1.96,0.59,0.08)\times 10^{61}$~erg. The absence of detectable annuli/ring-like structures, indicative of cluster accretion or merging shocks, imposes further constraints on models in which the UHECRs are accelerated in the merging shocks of galaxy clusters.