Radio view of the Peanut cluster: merger-induced stripping and a radio halo shortly after pericentric passage

Abstract

The Peanut galaxy cluster (SRGe CL0238.3+2005) offers a rare opportunity to investigate thermal and nonthermal processes only $\sim0.1$,Gyr after pericentric passage. We present a multi-frequency radio study of this system at $z=0.42$ using new MeerKAT and uGMRT observations, complemented by Chandra and MUSTANG-2 data. We identify two morphologically and spectrally distinct radio components: a double-lobed source in the northern subcluster and extended diffuse emission across the cluster center. The double-lobed source has a strongly curved, ultra-steep spectrum and is offset by $\sim22$,kpc from the nearest brightest cluster galaxy. Its spectral curvature and positional offset are consistent with remnant AGN plasma that may have been displaced during the merger or reshaped by a shock. The diffuse emission extends over $\sim1$~Mpc and is classified as an ultra-steep spectrum ($α=-1.77\pm0.11$ between 400,MHz and 1.28,GHz) radio halo. Its volume-averaged radio emissivity, $(1.1\pm0.1)\times10^{-41}$ erg s$^{-1}$ cm$^{-3}$ Hz$^{-1}$ at 1.4,GHz, is an order of magnitude higher than the characteristic value for other halos. The radial profile of the halo is well described by a single exponential fit and its radio emission correlates tightly with the X-ray, with a best-fit slope of $1.11\pm0.05$. Chandra and MUSTANG-2 observations reveal candidate surface brightness discontinuities, with the most prominent inner feature coinciding with a region of flatter spectral index, potentially tracing localized particle (re)-acceleration. The coexistence of an extended ultra-steep spectrum radio halo, high radio emissivity, remnant AGN plasma, and candidate X-ray/SZ discontinuities makes the Peanut cluster an important test case for understanding the evolution of diffuse radio emission and its connection to the dynamical state of the intracluster medium in early-stage mergers.