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

¹ Dipartimento di Fisica e Astronomia, Universitát di Bologna, Via P. Gobetti 93/2, 40129 Bologna, Italy
e-mail: kamlesh.rajpurohit@unibo.it
² INAF-Istituto di Radio Astronomia, Via Gobetti 101, 40129 Bologna, Italy
³ Thüringer Landessternwarte, Sternwarte 5, 07778 Tautenburg, Germany
⁴ Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
⁵ Leiden Observatory, Leiden University, PO Box 9513 2300 RA Leiden, The Netherlands
⁶ CSIRO Astronomy and Space Science, PO Box 1130 Bentley, WA 6102, Australia
⁷ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
⁸ Astronomy & Astrophysics Division, Physical Research Laboratory, Ahmedabad 380009, India
⁹ Department of Physics, School of Natural Sciences UNIST, Ulsan 44919, Korea

Received: 12 April 2021
Accepted: 23 August 2021

Abstract

We present the first deep low frequency radio observations of the massive and highly disturbed galaxy cluster Abell 2744 using the upgraded Giant Metrewave Radio Telescope (uGMRT). The cluster is experiencing a very complex multiple merger and hosts a giant halo and four radio relics. The uGMRT observations, together with existing VLA (1−4 GHz) and Chandra observations, allow us to study the complexity of the physical mechanisms active in this system. Our new images reveal that the central halo emission is more extended toward low frequencies. We find that 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. The halo also shows local regions in which the spectral index is significantly different from the average value. 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 comparison of the radio surface brightness and spectral index with the X-ray brightness and temperature reveals for the first time different trends, indicating that the halo consists of two main components with distinct evolutionary signatures. All four relics in this system follow a power-law radio spectrum, compatible with shocks with Mach numbers in the range 3.0 − 4.5. All relics are also highly polarized from 1−4 GHz and show low Faraday dispersion measures, suggesting that they are located in the outermost regions of the cluster. The complexity in the distribution and properties of nonthermal components in Abell 2744 supports a multiple merger scenario, as also highlighted by previous X-ray and lensing studies. Our unique results demonstrate the importance of sensitive and high-resolution, multifrequency radio observations for understanding the interplay between the thermal and nonthermal components of the ICM.