2025MNRAS.538.3326W

Ultra-steep spectrum (USS) radio sources with complex filamentary morphologies are a poorly understood subclass of diffuse radio sources found in galaxy clusters. They are characterized by power-law spectra with spectral indices less than -1.5, and are typically located in merging clusters. We present XMM-Newton observations of the galaxy cluster A272, containing a USS diffuse radio source. The system is an ongoing major cluster merger with an extended region of bright X-ray emission south of the core. Surface brightness analysis yields a $3\sigma$ detection of a merger shock front in this region. We obtain shock Mach numbers $M_\rho = 1.20 \pm 0.09$ and $M_T = 1.7 \pm 0.3$ from the density and temperature jumps, respectively. Optical data reveal that the system is a merger between a northern cool core cluster and a southern non-cool core cluster. We find that the USS source, with spectral index $\alpha ^{\text{74 MHz}}_{\text{1.4 GHz}} = -1.9 \pm 0.1$, is located in the X-ray bright southern region. Radio observations show that the source has a double-lobed structure with complex filaments, and is centred on the brightest cluster galaxy of the southern subcluster. We provide two suggestions for the origin of this source; the first posits the source as an active galactic nucleus (AGN) relic that has been re-energized by the passing of a merger shock front, while the second interprets the complex structure as the result of two overlapping AGN radio outbursts. We also present constraints on the inverse Compton emission at the location of the source.