# A Massive Cluster at z = 0.288 Caught in the Process of Formation: The Case of Abell 959

## TL;DR

This study reports the detection of a radio halo and relic in the galaxy cluster Abell 959 at z=0.288, providing insights into cluster merger stages and the relationship between thermal and non-thermal emissions.

## Contribution

First detection of both a radio halo and relic in Abell 959 using LOFAR data, and analysis of their relation to cluster merger stages and scaling relations.

## Key findings

- Radio halo in A959 fits known scaling relations.
- Steep-spectrum haloes are linked to early merger stages.
- Relics are associated with later merger stages.

## Abstract

The largest galaxy clusters are observed still to be forming through major cluster-cluster mergers, often showing observational signatures such as radio relics and giant radio haloes. Using LOFAR Two-meter Sky Survey data, we present new detections of both a radio halo (with a spectral index of $\alpha_{143}^{1400}=1.48^{+0.06}_{-0.23}$) and a likely radio relic in Abell 959, a massive cluster at a redshift of z=0.288. Using a sample of clusters with giant radio haloes from the literature (80 in total), we show that the radio halo in A959 lies reasonably well on the scaling relations between the thermal and non-thermal power of the system. Additionally, we find evidence that steep-spectrum haloes tend to reside in clusters with high X-ray luminosities relative to those expected from cluster LM scaling relations, indicating that such systems may preferentially lie at an earlier stage of the merger, consistent with the theory that some steep-spectrum haloes result from low-turbulence mergers. Lastly, we find that halo systems containing radio relics tend to lie at lower X-ray luminosities, relative to those expected from cluster LM scaling relations, for a given halo radio power than those without relics, suggesting that the presence of relics indicates a later stage of the merger, in line with simulations.