A giant radio halo in the cool core cluster CL1821+643

TL;DR

This paper reports the discovery of a giant radio halo in the cool core cluster CL1821+643, challenging the typical association of such halos with merging clusters and suggesting alternative scenarios for their origin.

Contribution

It presents the first detection of a giant radio halo in a cool core cluster, indicating that such halos can exist without recent major mergers.

Findings

Giant radio halo detected in CL1821+643, a cool core cluster

Radio and X-ray properties analyzed in context of halo formation models

Possible off-axis or minor merger scenario suggested for halo origin

Abstract

Giant radio halos are Mpc-size sources found in some merging galaxy clusters. The synchrotron emitting electrons are thought to be (re)accelerated by plasma turbulence induced by the merging of two massive clusters. Cool core galaxy clusters have a low temperature core, likely an indication that a major merger has not recently occurred. CL1821+643 is one of the strongest cool core clusters known so far. Surprisingly, we detect a giant radio halo with a largest linear size of $\sim$ 1.1 Mpc. We discuss the radio and X-ray properties of the cluster in the framework of the proposed models for giant radio halos. If a merger is causing the radio emission, despite the presence of a cool-core, we suggest that it should be off-axis, or in an early phase, or a minor one.