A Multi-Frequency View of the Radio Phoenix in the Abell 85 Cluster

Abstract

Radio phoenices are complex and filamentary diffuse radio sources found in both merging and relaxed clusters. The formation of these sources was proposed to be due to adiabatic compression of old Active Galactic Nucleus (AGN) plasma in shock waves. Most of the previous spectral studies of these sources were limited to integrated spectral indices, which were found to be very steep as well as showing a curved spectrum. Here, we have performed a multi-frequency investigation of the radio phoenix in the Abell 85 cluster. Owing to the sensitive high-resolution observations, we found some of the finer filamentary structures that were previously undetected. We produced resolved spectral index maps of the radio phoenix between 323, 700, and 1280 MHz. The orientation of the filaments, as well as the gradient across the spectral index maps, suggest the possible direction of the shock motion from northeast to southwest. The integrated spectrum of the radio phoenix was found to be very steep and curved toward high-frequencies. Furthermore, the spectral index of the filaments was found to be less steep compared to the non-filamentary regions, implying greater energy injection in the filaments. The observed features in the radio phoenix in the Abell 85 cluster seem to be in support of the adiabatic shock compression mechanism.