An elusive radio halo in the merging cluster Abell 781?

TL;DR

This study uses deep radio observations to investigate the presence of diffuse radio emission in galaxy cluster Abell 781, challenging previous claims of a radio halo and suggesting it may be a very steep spectrum source.

Contribution

The paper provides new low-frequency radio observations that contest prior detection of a radio halo in Abell 781, proposing it might be a faint, steep spectrum source instead.

Findings

Diffuse emission at cluster center is likely not a radio halo.

A prominent radio relic is detected at the outskirts.

Previous claims of a radio halo are not supported by new spectral analysis.

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 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 S(325 MHz)~15-20 mJy. Our analysis disagrees with Govoni et al. (2011), and on the basis of simple spectral considerations we do not support their claim of a radio halo with 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.