Deep Very Large Array observations of the merging cluster CIZA J2242.8+5301: continuum and spectral imaging

TL;DR

This study uses deep VLA observations to analyze the complex morphology and spectral properties of the merging cluster CIZA J2242.8+5301, revealing new diffuse emissions and insights into particle acceleration mechanisms.

Contribution

First detailed VLA imaging of the cluster's diffuse radio sources, uncovering filamentary structures and new faint emissions, and providing refined estimates of shock Mach numbers and particle acceleration processes.

Findings

Discovery of complex diffuse radio morphology and filamentary structures.

Detection of new faint diffuse emission extending north of the northern relic.

Evidence supporting re-acceleration of fossil electrons by merger shocks.

Abstract

Despite progress in understanding radio relics, there are still open questions regarding the underlying particle acceleration mechanisms. In this paper we present deep 1--4 GHz VLA observations of CIZA\,J2242.8+5301 ($z=0.1921$), a double radio relic cluster characterized by small projection on the plane of the sky. Our VLA observations reveal, for the first time, the complex morphology of the diffuse sources and the filamentary structure of the northern relic. We discover new faint diffuse radio emission extending north of the main northern relic. Our Mach number estimates for the northern and southern relics, based on the radio spectral index map obtained using the VLA observations and existing LOFAR and GMRT data, are consistent with previous radio and X-ray studies ($\mathcal{M}_{\rm RN}=2.58\pm0.17$ and $\mathcal{M}_{\rm RS}=2.10\pm0.08$). However, color-color diagrams and modelings suggest a flatter injection spectral index than the one obtained from the spectral index map, indicating that projection effects might be not entirely negligible. The southern relic consists of five "arms". Embedded in it, we find a tailed radio galaxy which seems to be connected to the relic. A spectral index flattening, where the radio tail connects to the relic, is also measured. We propose that the southern relic may trace AGN fossil electrons that are re-accelerated at a shock, with an estimated strength of $\mathcal{M}=2.4$. High-resolution mapping of other tailed radio galaxies also supports a scenario where AGN fossil electrons are revived by the merger event and could be related to the formation of some diffuse cluster radio emission.