First evidence of diffuse ultra-steep-spectrum radio emission surrounding the cool core of a cluster

TL;DR

This paper reports the first observation of a cool-core galaxy cluster with both a mini halo and an ultra-steep-spectrum radio emission extending beyond the core, indicating complex particle acceleration processes.

Contribution

It presents the first evidence of a cluster hosting both a mini halo and an ultra-steep-spectrum halo, suggesting co-existence of different radio emission types in a single cluster.

Findings

Discovery of ultra-steep-spectrum emission beyond the core

Cluster shows signs of slight disturbance and minor merger activity

Both mini and giant halos can coexist in a single cluster.

Abstract

Diffuse synchrotron radio emission from cosmic-ray electrons is observed at the center of a number of galaxy clusters. These sources can be classified either as giant radio halos, which occur in merging clusters, or as mini halos, which are found only in cool-core clusters. In this paper, we present the first discovery of a cool-core cluster with an associated mini halo that also shows ultra-steep-spectrum emission extending well beyond the core that resembles radio halo emission. The large-scale component is discovered thanks to LOFAR observations at 144 MHz. We also analyse GMRT observations at 610 MHz to characterise the spectrum of the radio emission. An X-ray analysis reveals that the cluster is slightly disturbed, and we suggest that the steep-spectrum radio emission outside the core could be produced by a minor merger that powers electron re-acceleration without disrupting the cool core. This discovery suggests that, under particular circumstances, both a mini and giant halo could co-exist in a single cluster, opening new perspectives for particle acceleration mechanisms in galaxy clusters.