Synchrotron emission from virial shocks around stacked OVRO-LWA galaxy clusters

Galaxy clusters accrete mass through large-scale, strong, structure-formation shocks. Such a virial shock is thought to deposit fractions ζe and ζB 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 m ξe ξB≃ (1-4)× 10-4, where m ≡ 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 m ξe∼ 0.6 per cent inferred from the Fermi signal yields ξ B ≃ (2-9) per cent, corresponding to B (0.1-0.3) μG magnetic fields downstream of typical virial shocks. Preliminary evidence suggests non-spherical shocks, with factor 2-3 elongations.