Discovery of a double radio relic in ZwCl1447.2+2619: A rare testbed for shock acceleration models with a peculiar surface brightness ratio

TL;DR

This paper reports the discovery of a rare double radio relic in galaxy cluster ZwCl1447.2+2619, providing a unique testbed for shock acceleration models due to its unusual surface brightness ratio and weak shock signatures.

Contribution

It presents the first detailed analysis of an atypical double radio relic with unusual brightness and spectral properties, challenging existing models of shock acceleration in galaxy clusters.

Findings

Both relics indicate weak shocks with Mach numbers around 2-3.

The northern relic's brightness suggests possible cosmic ray re-acceleration.

The system is a post-merger cluster approximately 0.7 Gyr after collision.

Abstract

We report a discovery of a double radio relic in the cluster merger ZwCl1447.2+2619 ($z=0.376$) with uGMRT observations at $420\rm~MHz$ and $700\rm~MHz$. The linear sizes of the northern and southern relics are $\sim0.3~$Mpc and $\sim1.2~$Mpc, respectively, which is consistent with the theoretical expectation that a larger relic is produced in the less massive subcluster side. However, ZwCl1447.2+2619 is unlike other known double radio relic systems, where the larger relics are much more luminous by several factors. In this merger the higher surface brightness of the smaller northern relic makes its total radio luminosity comparable to that of the much larger southern relic. The surface brightness ratio $\sim0.1$ between the two radio relics differs significantly from the relation observed in other double radio relic systems. From our radio spectral analysis, we find that both relics signify similar weak shocks with Mach numbers of $2.9\pm0.8$ and $2.0\pm0.7$ for the northern and southern relics, respectively. Moreover, the northern relic is connected to a discrete radio source with an optical counterpart, which indicates the possible presence of cosmic ray injection and re-acceleration. Therefore, we propose that this atypical surface brightness ratio can be explained with the particle acceleration efficiency precipitously dropping in the weak shock regime and/or with re-acceleration of fossil cosmic rays. Our multi-wavelength analysis and numerical simulation suggest that ZwCl1447.2+2619 is a post-merger, which has experienced a near head-on collision $\sim0.7\rm~Gyr$ ago.