The Coma Cluster at LOw Frequency ARray Frequencies. I. Insights into Particle Acceleration Mechanisms in the Radio Bridge

Abstract

Radio synchrotron emission from the bridges of low-density gas connecting galaxy clusters and groups is a challenge for particle acceleration processes. In this work, we analyze the Coma radio bridge using new LOw Frequency ARray (LOFAR) observations at 144 MHz. LOFAR detects the bridge and its substructures with unprecedented sensitivity and resolution. We found that the radio emission peaks on the NGC 4839 group. Toward the halo, in front of the NGC 4839 group, the radio brightness decreases and streams of radio emission connect the NGC 4839 group to the radio relic. Using X-ray observations, we found that thermal and non-thermal plasma are moderately correlated with a sublinear scaling. We use archival radio data at 326 MHz to constrain the spectral index in the bridge, and quantify the distribution of particles and magnetic field at different frequencies. We found that the spectrum is steeper than −1.4 ± 0.2, and that the emission is clumpier at 326 MHz than at 144 MHz. Using cosmological simulations and a simplified approach to compute particle acceleration, we derive under which conditions turbulent acceleration of mildly relativistic electrons generate the radio emission in the bridge. Assuming that the initial energy ratio of the seed electrons is 3 · 10−4 with respect to the thermal gas, we are able to reproduce the observed luminosity. Our results suggest that the seed electrons released by radio galaxies in the bridge and the turbulence generated by the motion of gas and galaxies are essential to producing the radio emission.

Discovery of magnetic fields along stacked cosmic filaments as revealed by radio and X-ray emission

“…These two, as the only detected thus far with current generation radio telescopes, likely represent the peak of the distribution of filaments in terms of brightness caught in a 'short' (≤1 Gyr) phase of high dynamical activity (leading to in situ particle acceleration) before a major merger event (Brunetti & Vazza 2020). Both of these detections were shorter filaments, or intercluster birdges (1-3 Mpc), with Bonafede et al (2021) recently finding a 2 Mpc long intracluster bridge within the Coma cluster. However, fainter filaments or those between widely separated clusters with no overlap from cluster emission (which would be the majority of filaments rather than inter or intracluster bridges), in the presence of instrumental noise and source confusion from galaxies, remain below the detectable threshold.…”

Tempestuous life beyond R500: X-ray view on the Coma cluster with SRG/eROSITA

“…Massive galaxy clusters are dynamic systems that continuously accrete smaller structures. In the Coma cluster, the signatures of mergers have long been identified/suggested in the optical, radio or X-ray bands (see, e.g., Brown & Rudnick 2011;Bonafede et al 2021;Lyskova et al 2019;Malavasi et al 2020, and references therein). Apart from the NGC 4839 group, these include the structures (shocks and contact discontinuities) discussed above.…”

“…Although the multiple shocks operate in different parts of the cluster core, they are, however, synchronized by the initial merger shock. To some extent, these "coordinated" shocks could provide a similar effect as the reacceleration by the turbulence in the core, which was invoked as an alternative mechanism to keep the radio halo alive (see, e.g., Brunetti et al 2001;Bonafede et al 2021). Given the long-living perturbed state of the core predicted by simulations, a combination of both scenarios could be at work in the Coma core.…”

Pilot Study and Early Results of the Cosmic Filaments and Magnetism Survey with Nenufar: The Coma Cluster Field

“…We see in Figure 4 that the source at the centre of our NenuFAR image does indeed correspond to the known structure of the Coma cluster as seen with LOFAR. The source to the West of the cluster is 3C284, a bright radio galaxy which was subtracted from the data imaged by Bonafede et al [5]. We measure an integrated flux density of 66.0 ± 4.1 Jy for this source.…”

“…The Coma galaxy cluster is not only one of the first historically observed galaxy clusters [1], but also the first galaxy cluster with the detection of either a radio halo [2] or radio relic [3,4] in the literature. It is very well-studied at a range of frequencies [5]. As an object known to host several physical components of considerable scientific interest, it was chosen as the first galaxy cluster to be observed as part of the NenuFAR Cosmic Filaments & Magnetism Pilot Survey project.…”