Understanding the radio relic emission in the galaxy cluster MACS J0717.5+3745: Spectral 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 Minnesota Institute for Astrophysics, University of Minnesota, 116 Church St. S.E., Minneapolis, MN 55455, USA
⁷ Department of Physics, New Mexico Tech, Socorro, NM 87801, USA
⁸ National Radio Astronomy Observatory, Socorro, NM 87801, USA
⁹ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
¹⁰ INAF-Osservatorio Astronomico di Cagliari, Via della Scienza 5, 09047 Selargius, CA, Italy
¹¹ Astronomy & Astrophysics Division, Physical Research Laboratory, Ahmedabad 380009, India
¹² U.S. Naval Research Laboratory, 4555 Overlook Avenue SW, Washington DC 20375, USA
¹³ INAF-IASF Milano, Via A. Corti 12, 20133 Milano, Italy

Received: 14 September 2020
Accepted: 2 December 2020

Abstract

Radio relics are diffuse, extended synchrotron sources that originate from shock fronts generated during cluster mergers. The massive merging galaxy cluster MACS J0717.5+3745 hosts one of the more complex relics known to date. We present upgraded Giant Metrewave Radio Telescope band 3 (300−500 MHz) and band 4 (550−850 MHz) observations. These new observations, combined with published VLA and the new LOFAR HBA data, allow us to carry out a detailed, high spatial resolution spectral analysis of the relic over a broad range of frequencies. The integrated spectrum of the relic closely follows a power law between 144 MHz and 5.5 GHz with a mean spectral slope α = −1.16 ± 0.03. Despite the complex morphology of this relic, its subregions and the other isolated filaments also follow power-law behaviors, and show similar spectral slopes. Assuming diffusive shock acceleration, we estimated a dominant Mach number of ∼3.7 for the shocks that make up the relic. A comparison with recent numerical simulations suggests that in the case of radio relics, the slopes of the integrated radio spectra are determined by the Mach number of the accelerating shock, with α nearly constant, namely between −1.13 and −1.17, for Mach numbers 3.5 − 4.0. The spectral shapes inferred from spatially resolved regions show curvature, we speculate that the relic is inclined along the line of sight. The locus of points in the simulated color-color plots changes significantly with the relic viewing angle. We conclude that projection effects and inhomogeneities in the shock Mach number dominate the observed spectral properties of the relic in this complex system. Based on the new observations we raise the possibility that the relic and a narrow-angle-tailed radio galaxy are two different structures projected along the same line of sight.