Ultra-Steep Spectrum Radio `Jellyfish' Uncovered in Abell 2877
Torrance Hodgson, Iacopo Bartalucci, Melanie Johnston-Hollitt,, Benjamin McKinley, Franco Vazza, Denis Wittor

TL;DR
This paper reports the discovery of an ultra-steep spectrum radio source in galaxy cluster Abell 2877, resembling a jellyfish, likely caused by reacceleration of aged electron populations through a weak cluster-scale mechanism.
Contribution
It presents the first example of a polyphoenix, suggesting a new large-scale reacceleration process in galaxy clusters, supported by observational data and simulations.
Findings
The source has the steepest known spectral index for a cluster synchrotron source.
The source's morphology resembles a jellyfish with tentacles and peaks.
Simulations show such emission can be a transient phase from interacting AGN remnants.
Abstract
We report on the discovery of a mysterious ultra-steep spectrum (USS) synchrotron source in the galaxy cluster Abell 2877. We have observed the source with the Murchison Widefield Array at five frequencies across 72-231 MHz and have found the source to exhibit strong spectral curvature over this range as well the steepest known spectra of a synchrotron cluster source, with a spectral index across the central three frequency bands of . Higher frequency radio observations, including a deep observation with the Australia Telescope Compact Array, fail to detect any of the extended diffuse emission. The source is approximately 370 kpc wide and bears an uncanny resemblance to a jellyfish with two peaks of emission and long tentacles descending south towards the cluster centre. Whilst the `USS Jellyfish' defies easy classification, we here propose that the…
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