New Insights into Cosmic Ray Dynamics in Galaxy Clusters

A recent paper titled "Low injection rate of cosmic-ray protons in the turbulent reacceleration model of radio halos in galaxy clusters" by Kosuke Nishiwaki and Katsuaki Asano explores the dynamics of cosmic rays within galaxy clusters. The study focuses on giant radio halos (RHs), which are large-scale synchrotron emissions found in the central regions of merging galaxy clusters. These emissions are thought to arise from the re-energization of older populations of cosmic ray electrons (CREs) and may also involve secondary leptons produced from hadronic collisions of cosmic ray protons (CRPs).

The authors developed a new method to analyze the reacceleration model that includes both primary and secondary CREs, assuming that primary cosmic rays originate from internal galaxies. Their findings indicate that a primary CRE model with a reacceleration duration of approximately 3 billion years can accurately reproduce the statistical properties of radio halos observed in recent LOFAR surveys, as well as the spectrum and profile of the Coma cluster.

A significant aspect of the research is the limitation placed on the cosmic-ray proton injection rate, which is found to be less than or equal to $10^{41}$ erg/s. This rate is considerably lower than what would be expected from early starburst activities or jets from active galactic nuclei. The authors suggest that this discrepancy necessitates a reevaluation of either the model for cosmic ray supply from galaxies or the turbulent reacceleration model itself.

The implications of this study are significant for understanding cosmic ray dynamics and the evolution of galaxy clusters, as well as for refining models that predict cosmic ray behavior in these massive structures. The research was submitted on August 25, 2024, and can be cited as arXiv:2408.13846.