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

Savini, F.; Bonafede, A; Brüggen, M; van Weeren, R; Brunetti, G; Intema, H; Botteon, A; Shimwell, T; Wilber, A; Rafferty, D; Giacintucci, S; Cassano, R and Cuciti, V (2018). First evidence of diffuse ultra-steep-spectrum radio emission surrounding the cool core of a cluster. Monthly Notices of the Royal Astronomical Society, 478(2) pp. 2234–2242.

DOI: https://doi.org/10.1093/mnras/sty1125

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.

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