2026ApJ...996...24A

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.