  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 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 show 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 148, 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 spectral index of the radio phoenix was found to be very steep
with a break at around 700 MHz, indicating the re-energization of fossil
electrons being recent. 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.
