A view of the CIZA J2242.8+5301 galaxy cluster at very low radio frequencies

¹ Hamburger Sternwarte, University of Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
² INAF – Istituto di Radioastronomia di Bologna, Via Gobetti 101, 40129 Bologna, Italy
³ Thüringer Landessternwarte, Sternwarte 5, 07778 Tautenburg, Germany
⁴ Leiden Observatory, Leiden University, PO Box 9513 2300 RA Leiden, The Netherlands

^⋆ Corresponding author: giulia.lusetti@hs.uni-hamburg.de

Received: 13 March 2025
Accepted: 18 April 2025

Abstract

Context. The galaxy cluster CIZA J2242.8+5301 is a well-studied merging galaxy cluster that hosts prominent double radio relics including the famous sausage relic, as well as other diffuse radio sources. Observations at frequencies below 100 MHz are essential for investigating the physics of radio relics as they provide unique access to the low-energy population of cosmic-ray electrons.

Aims. We aim to study the morphology, spectral characteristics, and physical processes that produce relics.

Methods. We present the first observations of the Sausage cluster at 45 MHz, the lowest radio frequency at which this cluster has been studied to date, using the Low Band Antenna (LBA) of the LOFAR radio interferometer. We made use of ten hours of LOFAR LBA observations, from which we achieved a thermal-noise limited radio image with a noise level of 1.5 mJy/beam at a resolution of 15″. These data were combined with existing multi-frequency measurements at higher frequencies: LOFAR High Band Antenna (HBA: 145 MHz); Giant Metrewave Radio Telescope (GMRT: 325, 610 MHz); Westerbork Synthesis Radio Telescope (WSRT: 1.2, 1.4 GHz); and Karl G. Jansky Very Large Array (VLA: 1.5, 3 GHz). This broad frequency coverage allowed us to derive integrated spectral indices, spectral index and curvature maps, and Mach number distributions across the relics.

Results. We derived Mach numbers from the local injection index measure using low-frequency data with ℳ_N = 2.9 ± 0.5 for the northern relic and ℳ_S = 2.9 ± 0.8 for the southern relic. LOFAR LBA observations reveal a remarkably symmetric surface brightness profile across the eastern part of the northern relic, with wings extending on either side of the peak. This discovery is contrary to the expectation of particle acceleration at a single, sharp shock and the subsequent downstream advection of accelerated electrons. We modelled the surface brightness profile, including the effects of projection, magnetic field variation, and shock deformation.