# Particle re-acceleration and diffuse radio sources in the galaxy cluster Abell 1550

**Authors:** T. Pasini, H. W. Edler, M. Brüggen, F. de Gasperin, A. Botteon, K. Rajpurohit, R. J. van Weeren, F. Gastaldello, M. Gaspari, G. Brunetti, V. Cuciti, C. Nanci, G. di Gennaro, M. Rossetti, D. Dallacasa. D. N. Hoang, C. J. Riseley  
**arXiv:** [2205.12281](https://arxiv.org/abs/2205.12281)  
**Date:** 2022-05-24  
**Summary page:** https://www.summarizepaper.com/en/paper/2205.12281v1/

## Key points

- Study focused on diffuse radio emission in galaxy cluster A1550 to constrain particle re-acceleration in intra-cluster medium
- Detected ultra-steep spectrum radio halo with spectral index $S_\nu \propto \nu^{-1.6}$ extending over 1.2 Mpc at 54 MHz, morphology closely follows thermal intra-cluster medium distribution
- Identified radio relic adjacent to centrally located head-tail radio galaxy, connected to radio halo by a bridge spanning 600 kpc
- Observed potential radio phoenix between relic and radio galaxy, linked through two arms with nearly constant spectral index for 300 kpc
- Analysis suggests turbulence induced by major merger contributed to formation of radio halo within A1550, with axis aligned NE-SW; relic position aligns with this scenario, potentially originating from shock along merger axis

## Summary

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Our study of diffuse radio emission in the galaxy cluster A1550 aimed to constrain particle re-acceleration in the intra-cluster medium. Utilizing observations at four different frequencies (54, 144, 400, and 1400 MHz) along with archival Chandra X-ray data, we detected an ultra-steep spectrum radio halo with a spectral index of $S_\nu \propto \nu^{-1.6}$ extending over 1.2 Mpc at 54 MHz. The morphology of this halo closely follows the distribution of the thermal intra-cluster medium as inferred from Chandra observations. Adjacent to the centrally located head-tail radio galaxy within A1550, we identified a radio relic with a projected extent of 500 kpc. From this relic, a bridge spanning 600 kpc connects it to the radio halo. Additionally, between the relic and the radio galaxy, we observed what appears to be a radio phoenix based on its curved spectrum. This phoenix is linked to the tail of the radio galaxy through two arms exhibiting a nearly constant spectral index for 300 kpc. Our analysis suggests that turbulence induced by a major merger may have contributed to the formation of the radio halo within A1550, with its axis aligned in the NE-SW direction. The position of the relic aligns with this scenario, potentially originating from a shock propagating along the merger axis. It is plausible that this same shock also produced the radio phoenix through adiabatic compression while generating electrons pre-accelerated by turbulence that were subsequently re-accelerated to form the bridge connecting various components. Furthermore, hints of gentle re-energization were detected in two arms extending from the tail of the radio galaxy. These findings provide valuable insights into particle acceleration processes and merger dynamics within galaxy clusters like A1550.