Deep low-frequency radio observations of Abell 2256 I: The filamentary radio relic

Abstract

We present deep and high fidelity images of the merging galaxy cluster Abell 2256 at low frequencies, using the upgraded Giant Metrewave Radio Telescope (GMRT) and LOw-Frequency ARray (LOFAR). This cluster hosts one of the most prominent known relics, with a remarkably spectacular network of filamentary substructures. The new uGMRT (300-850 MHz) and LOFAR (120-169 MHz) observations combined with the archival VLA (1-4 GHz) data, allowed us to carry out the first spatially resolved spectral analysis of the exceptional relic emission down to 6 resolution over a broad range of frequencies. Our new sensitive radio images confirm the presence of complex filaments of magnetized relativistic plasma also at low frequencies. We find that the integrated spectrum of the relic is consistent with a single power law, without any sign of spectral steepening at least below 3 GHz. Unlike previous claims, the relic shows an integrated spectral index of −1.07 ± 0.02 between 144 MHz and 3 GHz, which is consistent with the (quasi)stationary shock approximation. The spatially resolved spectral analysis suggests that the relic surface very likely traces the complex shock front, with a broad distribution of Mach numbers, propagating through a turbulent and dynamically active intracluster medium. Our results show that the northern part of the relic is seen edge-on and the southern part close to face-on. We suggest that the complex filaments are regions where higher Mach numbers dominate the (re-)acceleration of electrons that are responsible for the observed radio emission.

The Coma cluster at LOFAR frequencies II: the halo, relic, and a new accretion relic

“…Recently, Rajpurohit et al (2021c) have shown that the hint for a spectral break claimed in the radio relic of Abell 2256 below 1.4 GHz (Trasatti et al 2015) is not confirmed once the analysis is performed with matching uv -coverage and unrelated sources are subtracted properly. In the case of the Coma halo, we note that the high-frequency measurements are almost a factor 10 below the value extrapolated from a power-law at low frequencies.…”