Radially polarized synchrotron from galaxy-cluster virial shocks

TL;DR

This study detects radially polarized synchrotron emission near galaxy cluster virial shocks, supporting theories of shock-induced electron acceleration and magnetic field alignment at specific cluster radii.

Contribution

It provides observational evidence of polarized synchrotron signals at virial shock radii using stacking of radio data, highlighting magnetic field structures and shock properties.

Findings

Detection of excess polarized emission at 2.4R_{500}

Indication of tangential magnetic fields from shock compression

Evidence of electron acceleration consistent with strong virial shocks

Abstract

Radio-to-$\gamma$-ray signals, recently found narrowly confined near the characteristic $2.4R_{500}$ scaled radii of galaxy clusters and groups, have been associated with their virial (structure-formation accretion) shocks based on spectro-spatial characteristics. By stacking high-latitude GMIMS radio data around MCXC galaxy clusters, we identify ($3\sigma$--$4\sigma$) excess radially polarized emission at the exact same scaled radius, providing directional support, and indicating tangential magnetic fields induced by the shocked inflow. The results suggest a strong mass dependence, a flat energy spectrum, and a high polarization fraction, consistent with synchrotron emission from electrons accelerated by strong virial shocks. The narrow radial range of such stacked virial-shock signals suggests that although the shocks are theorized to have diverse, irregular morphologies, they share similar $\sim2.4R_{500}$ minimal radii.