A Linearly Polarized Merger Shock Down to 550 MHz: A uGMRT Study of the Merging Cluster Abell 746

TL;DR

This study reports the first detection of polarized radio relic emission below 1 GHz in the galaxy cluster Abell 746, providing new insights into magnetic field structures in merging clusters through low-frequency polarization measurements.

Contribution

It presents the first polarization detection of a radio relic at 650 MHz, demonstrating the potential of low-frequency observations to probe magnetic field complexities in galaxy clusters.

Findings

Polarization detected at 650 MHz with ~18% fractional polarization.

Magnetic fields align with shock emission, consistent with relic characteristics.

Depolarization spectra suggest non-uniform magnetic fields or complex polarized regions.

Abstract

Radio relics, arc-like polarized sources with highly aligned magnetic fields, are typically found on the outskirts of merging galaxy clusters. The magneto-ionic media responsible for the significant coherence observed in radio relics remain poorly understood. Low-frequency measurements of radio relics are essential for constraining depolarization models, which provide crucial insights into the magnetic field distribution. However, these measurements are challenging due to the emission properties and interferometer systematics. We have detected polarization signals from the northwest radio relic in Abell 746 at 650 MHz with the upgraded Giant Meterwave Radio Telescope, marking the first-ever detection of polarisation from radio relics below 1 GHz. At this frequency, the average Rotation Measure (RM) corrected magnetic fields align well with shock radio emission, typical of radio relics. The fractional polarization at 650 MHz is $\sim 18\pm4 \%$. Our results indicate that a single internal depolarization model cannot explain the observed depolarization spectra, suggesting a non-uniform magnetic field distribution or complex contribution of different polarized regions in the radio relic. Our detection of polarization signals at 650 MHz reveals critical insights into radio relic magnetic field structures, offering a low-frequency approach to understanding ICM magnetic fields in merging galaxy clusters.