How unusual is the cool-core radio halo cluster CL1821+643?

Massive galaxy clusters with cool cores typically host diffuse radio sources called mini-haloes, whereas, those with non-cool cores host radio haloes. We attempt to understand the unusual nature of the cool-core galaxy cluster CL1821+643, which hosts a megaparsec-scale radio halo, using new radio observations and morphological analysis of its intra-cluster medium. We present the Giant Metrewave Radio Telescope (GMRT) 610-MHz image of the radio halo. The spectral index α, defined as S ∝ ν−α, of the radio halo is 1.0 ± 0.1 over the frequency range of 323–610–1665 MHz. Archival Chandra X-ray data were used to make surface brightness and temperature maps. The morphological parameters Gini, M 20 and concentration (C) were calculated on X-ray surface brightness maps by including and excluding the central quasar (H1821+643) in the cluster. We find that the cluster CL1821+643, excluding the quasar, is a non-relaxed cluster as seen in the morphological parameter planes. It occupies the same region as other merging radio halo clusters in the temperature versus morphology parameter plane. We conclude that this cluster has experienced a non-core-disruptive merger.