  Galaxy clusters accrete mass through large scale, strong, structure-formation
shocks. Such a virial shock is thought to deposit fractions $\xi_e$ and $\xi_B$
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, $R_{500}$ radii. Both high (73 MHz) and
co-added low ($36\text{--}68\text{ MHz}$) frequency channels separately
indicate a significant ($4\text{--}5\sigma$) excess peaked at $(2.4 \text{--}
2.6) R_{500}$, coincident with a previously stacked Fermi $\gamma$-ray signal
interpreted as inverse-Compton emission from virial-shock CREs. The stacked
radio signal is well fit (TS-test: $4$--$6\sigma$ at high frequency,
$4$--$8\sigma$ at low frequencies, and $8$--$10\sigma$ joint) by virial-shock
synchrotron emission from the more massive clusters, with
$\dot{m}\xi_e\xi_B\simeq (1\text{--}4)\times 10^{-4}$, 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, $p \simeq 2.0
\pm 0.2$, consistent with acceleration in a strong shock. Assuming
equipartition or using $\dot{m}\xi_e\sim0.6\%$ inferred from the Fermi signal
yields $\xi_B\simeq (2\text{--}9)\%$, corresponding to $B \simeq
(0.1\text{--}0.3)~\mu\text{G}$ magnetic fields downstream of typical virial
shocks. Preliminary evidence suggests non-spherical shocks, with factor
$2$--$3$ elongations.
