Illuminating the Diffuse Radio Emission in Low-mass Cluster A13

Abstract Recent advances in high-sensitivity radio observations have uncovered a population of faint, ultrasteep-spectrum sources in galaxy clusters, commonly known as radio phoenixes. However, their observational classification remains poorly constrained due to the limited number of confirmed detections. This study presents a detailed multifrequency, high-sensitivity, and high-resolution analysis of diffuse radio emission in the merging galaxy cluster Abell 13 (A13). Using Giant Metrewave Radio Telescope (GMRT; 147.5 MHz), upgraded GMRT (400 MHz), ASKAP-low (887.5 MHz), and MGCLS (1284 MHz) images, we detect complex, filamentary diffuse emission with a largest linear extent of 521 kpc. This emission originates from the cluster center and extends westward, confined within the X-ray-emitting intracluster medium. Chandra X-ray data confirm that A13 is undergoing a merger, and the radio morphology reflects signatures of this ongoing dynamical activity. We observed filamentary structures extending toward east–northeast and southwest directions. The spectral index across the emission appears irregular and lacks a coherent spatial gradient. The integrated spectrum reveals a steep spectral index of −1.85 ± 0.05 and a spectral curvature of −0.93 ± 0.21. These spectral properties, along with the observed morphology and brightness distribution, are consistent with a reenergization of a fossil radio plasma driven by adiabatic compression, supporting the classification of the emission as a radio phoenix.