Multi-Wavelength Signatures of a Giant Cometary Radio Halo in MACSJ0417-1154

Abstract

Galaxy clusters hosting diffuse non-thermal radio emission offer direct insight into plasma processes of the intracluster medium (ICM). We present the first multi-frequency study of the radio halo in MACSJ0417 (z = 0.445) using uGMRT (300-850 MHz), MeerKAT (900-1670 MHz), and archival \textit{XMM-Newton} data. The halo extends to $\sim$1.75 Mpc at 400 MHz, while two candidate relics (R1 and R2) are detected at 2.9 Mpc. The integrated spectra follow single power-laws with spectral indices $\alpha \simeq -1.3$ for the halo and $\alpha \simeq -1.6$ for the relics. Sensitive uGMRT imaging reveals a radio surface brightness edge $\sim$43$''$ SE of the cluster centre, which coincides with an X-ray discontinuity. Resolved spectral maps (400--1280 MHz) show significant fluctuations and a clear radial steepening of the spectral index. X-ray analysis reveals an elongated SE-NW morphology and high-temperature regions ($\sim$11 keV) along this axis. A strong radio and X-ray surface brightness correlation is found (correlation coefficient $\sim$ 0.85), with the correlation slope evolving from sublinear at 400 MHz to linear at 1280 MHz. These results, together with the spectral properties, support the turbulent re-acceleration model and point to inhomogeneous ICM conditions. The pure hadronic model is excluded owing to unrealistic energy requirements for cosmic-ray protons. We propose that MACSJ0417 is undergoing a minor off-axis dissociative merger (mass ratio $\sim$6:1) along the SE-NW axis, which has preserved its cool core while driving turbulence that powers the giant radio halo.