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

TL;DR

This study detects a significant synchrotron emission signal around galaxy clusters, supporting the presence of virial shocks depositing energy into cosmic rays and magnetic fields, consistent with theoretical models.

Contribution

First stacking analysis of low-frequency radio data around galaxy clusters to detect virial shock synchrotron emission, confirming theoretical predictions.

Findings

Significant excess emission detected at ~2.5 R500 around clusters.

Inferred magnetic fields of 0.1-0.3 microGauss downstream of shocks.

CRE spectral index consistent with acceleration in strong shocks.

Abstract

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.