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The ultra-steep diffuse radio emission observed in the cool-core cluster RX J1720.1+2638 with LOFAR at 54 MHz

Biava, N; de Gasperin, F; Bonafede, A; Edler, HW; Giacintucci, S; Mazzotta, P; Brunetti, G; Botteon, A; Brüggen, M; Cassano, R; Drabent, A; Edge, AC; Enßlin, T; Gastaldello, F; Riseley, CJ; Rossetti, M; Rottgering, HJA; Shimwell, TW; Tasse, C; van Weeren, RJ

The ultra-steep diffuse radio emission observed in the cool-core cluster RX J1720.1+2638 with LOFAR at 54 MHz Thumbnail


N Biava

F de Gasperin

A Bonafede

HW Edler

S Giacintucci

P Mazzotta

G Brunetti

A Botteon

M Brüggen

R Cassano

A Drabent

T Enßlin

F Gastaldello

CJ Riseley

M Rossetti

HJA Rottgering

TW Shimwell

C Tasse

RJ van Weeren


Diffuse radio emission at the centre of galaxy clusters has been observed both in merging clusters on scales of Mpc, called giant radio haloes, and in relaxed systems with a cool-core on smaller scales, named mini haloes. Giant radio haloes and mini haloes are thought to be distinct classes of sources. However, recent observations have revealed the presence of diffuse radio emission on Mpc scales in clusters that do not show strong dynamical activity. RX J1720.1+2638 is a cool-core cluster, presenting both a bright central mini halo and a fainter diffuse, steep-spectrum emission extending beyond the cluster core that resembles giant radio halo emission. In this paper, we present new observations performed with the LOw Frequency ARray Low Band Antennas (LBA) at 54 MHz. These observations, combined with data at higher frequencies, allow us to constrain the spectral properties of the radio emission. The large-scale emission presents an ultrasteep spectrum with α14454∼3.2⁠. The radio emission inside and outside the cluster core have strictly different properties, as there is a net change in spectral index and they follow different radio-X-ray surface brightness correlations. We argue that the large-scale diffuse emission is generated by particles re-acceleration after a minor merger. While for the central mini halo, we suggest that it could be generated by secondary electrons and positrons from hadronic interactions of relativistic nuclei with the dense cool-core gas, as an alternative to re-acceleration models.


Biava, N., de Gasperin, F., Bonafede, A., Edler, H., Giacintucci, S., Mazzotta, P., …van Weeren, R. (2021). The ultra-steep diffuse radio emission observed in the cool-core cluster RX J1720.1+2638 with LOFAR at 54 MHz. Monthly Notices of the Royal Astronomical Society, 508(3), 3995-4007.

Journal Article Type Article
Acceptance Date Sep 28, 2021
Online Publication Date Oct 1, 2021
Publication Date 2021-12
Deposit Date Oct 27, 2021
Publicly Available Date Jan 28, 2022
Journal Monthly Notices of the Royal Astronomical Society
Print ISSN 0035-8711
Electronic ISSN 1365-2966
Publisher Royal Astronomical Society
Peer Reviewed Peer Reviewed
Volume 508
Issue 3
Pages 3995-4007
Related Public URLs


Published Journal Article (6.7 Mb)

Copyright Statement
This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society. © 2021 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.

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