Dissecting nonthermal emission in the complex multiple-merger galaxy cluster Abell 2744: Radio and X-ray analysis

Abstract

We report upgraded Giant Metrewave Radio Telescope observations of the massive and highly disturbed galaxy cluster Abell 2744. The cluster is experiencing a very complex multiple merger and hosts a giant halo and four radio relics. To study the spectral and polarization properties, and radio versus X-ray correlations of the diffuse sources, we complement our analysis with existing VLA (1-4 GHz) and Chandra observations. In our new images, the central halo emission is observed to be more extended than detected previously. The integrated spectrum of the halo follows a power-law between 150 MHz and 3 GHz, while its subregions show significantly different spectra also featuring high frequency spectral steepening. Our results highlight that an overall power-law spectrum, as observed in many radio halos, may also arise from the superposition of different subcomponents. The radio brightness distribution of the halo strongly correlates with the X-ray brightness of the intracluster medium (ICM) with a constant sublinear slope from 675 MHz to 3 GHz. The comparison of the spectral index with the X-ray brightness and temperature reveals different trends indicating that the halo consists of two main components with distinct evolutionary signatures, possibly related to different cluster subcomponents. All four relics in this system follow a power-law radio spectrum, compatible with shocks with Mach number in the range 3.0 − 4.5. All relics are also highly polarized between 1 − 4 GHz and show Faraday dispersion measures below 10 rad m −2 , suggesting that they are located in the low density ICM. The complexity in the distribution and properties of nonthermal components in Abell 2744 support a multiple merger scenario, as also highlighted by previous X-ray and lensing studies. Our results demonstrate the importance of sensitive and high-resolution, multi-frequency radio observations for understanding the interplay between the thermal and non-thermal components of the ICM.

A MeerKAT-meets-LOFAR Study of MS 1455.0+2232: A 590 kiloparsec 'Mini'-Halo in a Sloshing Cool-Core Cluster

“…However, we note that this behaviour is in contrast with what has typically been seen for radio haloes, which show a generally similar radial profile at both radio and X-ray wavelengths (e.g. Hoang et al 2021;Rajpurohit et al 2021a).…”

“…For radio haloes, few studies have the necessarily high-resolution multi-frequency radio data required to investigate this. However, from these few studies, the picture is also mixed: the radio haloes in Abell 520 and Abell 2744 show no change in the correlation slope with frequency (respectively Hoang et al 2019;Rajpurohit et al 2021a) whereas the radio haloes hosted by ClG 0217+70, MACS J0717.5+3745 and Abell 2256 do exhibit a change in correlation slope with frequency (respectively Hoang et al 2021;Rajpurohit et al 2021b, Rajpurohit et al submitted). For MACS J0717.5+3745 and Abell 2256, the correlation slope steepens toward higher frequencies, which implies spectral index steepening, in line with the turbulent (re-)acceleration scenario under certain physical conditions.…”

“…For radio haloes, the picture is mixed. Some haloes show positive correlation (Botteon et al 2020b), some show an anticorrelation (Rajpurohit et al 2021b) and others show both correlation and anticorrelation for different sub-regions (Rajpurohit et al 2021a). We quantified the relation between radio spectral index 𝛼 and X-ray surface brightness in by fitting a power-law in linear-log space as follows:…”

Exploring the spectral properties of radio relics I: Integrated spectral index and Mach number

“…If one of these measurements is shallow, improperly deconvolved, or performed without any matching uv-coverages, the observed spectrum might appear steeper. Recent observations by Loi et al (2020), Rajpurohit et al (2020b) and Rajpurohit et al (2021a) highlighted the importance of enough frequency coverage, when computing integrated radio spectra. In addition, proper subtraction of unrelated sources is very important which may also steepen the spectrum.…”

Modelling the Energy Spectra of Radio Relics

“…As of writing this paper, about 60 relics and candidates are known, and radio observations are detecting more relics. Furthermore, high-resolution observations show that relics are nonuniform structures and they show threads and filaments on kpc-scales [6,7]. The origin of these filaments is still unknown.…”