Low Injection Rate of Cosmic-Ray Protons in the Turbulent Reacceleration Model of Radio Halos in Galaxy Clusters

Abstract A giant radio halo (RH) is a diffuse synchrotron emission observed on the scale of megaparsecs, typically found in the central region of merging galaxy clusters. Its large size and steep spectrum suggest that it originates from the reenergization of an aged population of cosmic-ray electrons (CREs), while the secondary leptons produced in the pp hadronic collision of cosmic-ray protons (CRPs) may contribute to the emission. In this study, we investigate the reacceleration model including both primary and secondary CREs, assuming that the primary cosmic rays (CRs) originate from internal galaxies. In our new method, we follow the cosmological evolution of each cluster and calculate the energy spectra and 1D spatial distributions of CRs. The primary CRE model with a ∼3 Gyr duration of reacceleration successfully reproduces the statistical properties of the RHs observed in the recent LOFAR survey, as well as the spectrum and profile of the Coma cluster. The gamma-ray and neutrino emissions produced by reaccelerated CRPs are consistent with the upper limits. However, if the CRP injection rate is high and the secondary CREs become significant, the model with the required ∼3 Gyr reacceleration overproduces the number of RHs. The limit on the CRP injection rate, L p ≲ 10 41 erg s −1 , is significantly lower than that expected from the early starburst activity or jets from active galactic nuclei. This discrepancy requires a revision of either the model of CR supply from galaxies or the turbulent reacceleration model.