2023MNRAS.521.5786H

Galaxy clusters accrete mass through large-scale, strong, structure-formation shocks. Such a virial shock is thought to deposit fractions ξ_e and ξ_B of the thermal energy in cosmic-ray electrons (CREs) and magnetic fields, respectively, thus generating a leptonic virial ring. However, the expected synchrotron signal was not convincingly established until now. We stack low-frequency radio data from the OVRO-LWA around the 44 most massive, high latitude, extended MCXC clusters, enhancing the ring sensitivity by rescaling clusters to their characteristic, R₅₀₀ radii. Both high (73 MHz) and co-added low (36-68 MHz) frequency channels separately indicate a significant (4-5σ) excess peaked at (2.4-2.6)R₅₀₀, coincident with a previously stacked Fermi γ-ray signal interpreted as inverse-Compton emission from virial-shock CREs. The stacked radio signal is well fit (TS-test: 4-6σ at high frequency, 4-8σ at low frequencies, and 8-10σ joint) by virial-shock synchrotron emission from the more massive clusters, with $\dot{m}\xi _e\xi _B\simeq (1\!-\!4)\times 10^{-4}$, where $\dot{m}\equiv \dot{M}/(MH)$ is the dimensionless accretion rate for a cluster of mass M and a Hubble constant H. The inferred CRE spectral index is flat, p ≃ 2.0 ± 0.2, consistent with acceleration in a strong shock. Assuming equipartition or using $\dot{m}\xi _e\sim 0.6~{{\ \rm per\ cent}}$ inferred from the Fermi signal yields $\xi _B\simeq (2\!-\!9)~{{\ \rm per\ cent}}$, corresponding to B ≃ (0.1-0.3) $\mu$G magnetic fields downstream of typical virial shocks. Preliminary evidence suggests non-spherical shocks, with factor 2-3 elongations.