Abell 1033: Radio halo and gently reenergized tail at 54 MHz

TL;DR

This study investigates ultra-steep spectrum radio emissions in galaxy cluster Abell 1033 using low-frequency radio observations, revealing a spectral cutoff and providing insights into cosmic-ray acceleration mechanisms.

Contribution

First detailed low-frequency spectral analysis of Abell 1033's diffuse radio sources, highlighting a spectral cutoff and characterizing the ultra-steep spectrum radio halo.

Findings

Detected an ultra-steep spectrum radio halo with spectral index -1.65

Observed a spectral flattening in the GReET tail from -4 to -2

Identified a spectral break consistent with re-acceleration models

Abstract

Abell 1033 is a merging galaxy cluster of moderate mass ($M_{500}=3.2\times10^{14} \mathrm{M}_\odot$). It hosts a broad variety of diffuse radio sources linked to different astrophysical phenomena. The most peculiar one is an elongated feature with an ultra-steep spectrum that is the prototype of the category of gently reenergized tails (GReET). Furthermore, the cluster hosts sources that were previously classified as a radio phoenix and a radio halo. We aim to improve the understanding of the cosmic-ray acceleration mechanisms in galaxy clusters in a frequency and mass range that has been poorly explored so far. To investigate the ultra-steep synchrotron emission in the cluster, we performed a full calibration of a LOFAR observation centered at 54 MHz. We analyzed this observation together with re-calibrated data of LoTSS at 144 MHz and an archival GMRT observation at 323 MHz. We performed a spectral study of the radio galaxy tail connected to the GReET to test if the current interpretation of the source is in agreement with observational evidence below 100 MHz. Additionally, we studied the radio halo at different frequencies. We report an extreme spectral curvature for the GReET, the spectral index flattens from $\alpha_{144}^{323} = -4$ to $\alpha_{54}^{144} = -2$. This indicates the presence of a cut-off in the electron energy spectrum. At the cluster center, we detect the radio halo at 54, 144 and at lower significance at 323 MHz. We categorize it as an ultra-steep spectrum radio halo with a spectral index $\alpha = -1.65 \pm 0.17$. Additionally, it is found to be significantly above the radio power-to-cluster mass correlations reported in the literature. Furthermore, the synchrotron spectrum of the halo is found to further steepen between 144 and 323 MHz, in agreement with the presence of a break in the electron spectrum, which is a prediction of homogeneous re-acceleration models.