Deep low-frequency radio observations of Abell 2256

II. The ultra-steep spectrum radio halo^⋆

¹ Dipartimento di Fisica e Astronomia, Università di Bologna, via P. Gobetti 93/2, 40129 Bologna, Italy
² INAF – Istituto di Radio Astronomia, Via Gobetti 101, 40129 Bologna, Italy
e-mail: kamlesh.rajpurohit@unibo.it
³ Thüringer Landessternwarte (TLS), Sternwarte 5, 07778 Tautenburg, Germany
⁴ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
⁵ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, via Gobetti 93/3, 40129 Bologna, Italy
⁶ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
⁷ CSIRO Space & Astronomy, PO Box 1130 Bentley, WA 6102, Australia
⁸ Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
⁹ Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA
¹⁰ INAF – IASF Milano, via A. Corti 12, 20133 Milano, Italy
¹¹ Astronomy & Astrophysics Division, Physical Research Laboratory, Ahmedabad 380009, India
¹² Lehrstuhl für Astronomie, Universität Würzburg, Campus Hubland Nord, Emil-Fischer-Strasse 31, 97074 Würzburg, Germany

Received: 8 September 2022
Accepted: 19 October 2022

Abstract

We present the first detailed analysis of the radio halo in the merging galaxy cluster Abell 2256 using the LOw Frequency ARray, the upgraded Giant Metrewave Radio Telescope, and the Karl G. Jansky Very Large Array. Radio observations (120 MHz–2 GHz) combined with archival Chandra and XMM-Newton X-ray data allowed us to study the central radio halo emission with unprecedented detail. The integrated radio emission from the entire halo is characterized by an ultra-steep spectrum, which can be described by a power law with α_144 MHz^1.5 GHz = −1.63 ± 0.03 and radial steepening in the outer regions. The halo is significantly underluminous according to the current scaling relations between radio power and mass at 1.4 GHz, not at 150 MHz; ultra-steep spectrum halos are predicted to be statistically underluminous. Despite the complex structure of this system, the halo morphology is remarkably similar to that of the X-ray emission. The radio surface brightness distribution across the halo is strongly correlated with the X-ray brightness of the intracluster medium. The derived correlations show sublinear slopes and distinct structures: the core is I_R ∝ I_X^1.51, the outermost region I_R ∝ I_X^0.41, and we find radio morphological connections with X-ray discontinuities. We also find a strong anticorrelation between the radio spectral index and the X-ray surface brightness, implying radial steepening. We suggest that the halo core is either related to old plasma from previous active galactic nuclei activity, being advected, compressed, and reaccelerated by mechanisms activated by the cold front or less turbulent with strong magnetic field in the core. The change in the radio versus X-ray correlation slopes in the outer regions of the halo could be due to a radial decline of the magnetic field, the increase in the number density of seed particles, or increasing turbulence. Our findings suggest that the emitting volume is not homogenous according to turbulent reacceleration models.