An elusive radio halo in the merging cluster Abell 781?

Abstract

Deep radio observations of the galaxy cluster Abell 781 have been carried out using the Giant Metrewave Radio Telescope at 325 MHz and have been compared to previous 610-MHz observations and to archival Very Large Array (VLA) 1.4-GHz data. The radio emission from the cluster is dominated by a diffuse source located at the outskirts of the X-ray emission, which we tentatively classify as a radio relic. We detected residual diffuse emission at the cluster centre at the level of S325 MHz ∼ 15–20 mJy. Our analysis disagrees with Govoni et al., and on the basis of simple spectral considerations, we do not support their claim of a radio halo with the flux density of 20–30 mJy at 1.4 GHz. Abell 781, a massive and merging cluster, is an intriguing case. Assuming that the residual emission is indicative of the presence of a radio halo barely detectable at our sensitivity level, it could be a very steep spectrum source.