- The paper identifies two symmetric radio relics, each about 1.3 Mpc long and separated by 2.6 Mpc, challenging standard acceleration models.
- The paper employs uGMRT, VLA, and LOFAR data to reveal high polarization (up to 42%) and varying spectral indices that imply dynamic shock interactions.
- The paper demonstrates that merger dynamics and non-thermal processes in low-mass clusters complicate relic formation, urging further multi-wavelength research.
Radio Analysis of PSZ2\,G181.06+48.47: Unveiling the Physics of a Low-Mass Galaxy Cluster with Distant Radio Relics
This paper presents an in-depth paper of the low-mass galaxy cluster PSZ2\,G181.06+48.47 through broadband radio observations spanning from 0.3 to 2 GHz, utilizing data from the upgraded Giant Metrewave Radio Telescope (uGMRT), the Karl G. Jansky Very Large Array (VLA), and LOFAR. The focal point of this research is the discovery and analysis of two enigmatic radio relics situated at an unprecedented separation of 2.6 Mpc, each approximately 1.3 Mpc in length, providing a unique laboratory for studying particle acceleration in low-density environments typical of galaxy cluster outskirts.
Key observations feature the detection of two giant radio relics located symmetrically about the cluster center. The northeastern (NE) relic exhibits high polarization (up to 42%) and a flat integrated spectral index of -0.92, which challenges the standard diffusive shock acceleration (DSA) model from the thermal pool. The southwestern (SW) relic shows a steepening spectral index from -0.6 at the shock front to -1.6 downstream, suggesting spectral aging and an inference of a high Mach number shock (∼4.8) based on radio analysis.
The implications of these observations are manifold:
- Polarization and Magnetic Fields: The aligned magnetic field vectors and high polarization fraction in the NE relic support a scenario where magnetic fields are compressed and ordered by the passage of a shock front. The degree of polarization also provides clues about the Mach number and shock orientation relative to the line of sight.
- Particle Acceleration Mechanisms: The discrepancies between observed spectral indices and those predicted by simple DSA models suggest that pre-existing relativistic electrons, possibly from active galactic nuclei (AGN) or past merger shocks, are being re-accelerated. This aligns with scenarios involving re-acceleration of seed electrons or fossil radio plasma.
- Merger Dynamics: The morphology and orientation of the relics suggest a complex merger dynamics with a merger axis inclined to the line of sight, leading to a larger three-dimensional separation. This is consistent with a merging scenario that is not aligned with the plane of the sky, challenging previous assumptions of relic formation in post-core passage mergers.
The presence of a possible radio halo with steep spectral characteristics further complicates the picture, suggesting the cluster hosts multiple non-thermal phenomena usually associated with dynamically disturbed systems.
Overall, this research highlights the importance of low-frequency radio observations in unveiling the complex interplay between turbulence, magnetic fields, and particle acceleration in galaxy clusters. The findings from PSZ2\,G181.06+48.47 invite further multi-wavelength analysis to enhance our understanding of the physics governing radio relics and halos, particularly in low-mass clusters where observational constraints have previously limited such studies. Future high-sensitivity and high-resolution observations, potentially complemented by simulations, will be crucial in advancing the current paradigm of cosmic ray acceleration and magnetic field evolution in cluster environments.