# The structure of the giant radio fossil in the Ophiuchus galaxy cluster

## TL;DR

This study uses upgraded GMRT observations to reveal detailed structures in the giant fossil radio lobe of the Ophiuchus galaxy cluster, including steep-spectrum filaments and a spectral break indicating the lobe's age.

## Contribution

It provides high-resolution, low-frequency imaging of the fossil radio lobe, uncovering complex filamentary structures and spectral features not previously observed.

## Key findings

- Detection of narrow, long radio filaments with steep spectra within the lobe
- Identification of a spectral break indicating a radiative cooling age of approximately 174 Myr
- Extended emission traced up to 820 kpc from the cluster center

## Abstract

We present high-sensitivity follow-up observations of the giant fossil radio lobe in the Ophiuchus galaxy cluster with the upgraded Giant Metrewave Radio Telescope (uGMRT) in the 125-250 MHz and 300-500 MHz frequency bands. The new data have sufficient angular resolution to exclude compact sources and enable us to trace the faint extended emission from the relic lobe to a remarkable distance of 820 kpc from the cluster center. The new images reveal intricate spatial structure within the fossil lobe, including narrow (5-10 kpc), long (70-100 kpc) radio filaments embedded within the diffuse emission at the bottom of the lobe. The filaments exhibit a very steep spectrum ($S_\nu\propto \nu^{-\alpha}$ with $\alpha \sim 3$), significantly steeper than the ambient synchrotron emission from the lobe ($\alpha \sim 1.5-2$); they mostly disappear in recently-published MeerKAT images at 1.28 GHz. Their origin is unclear; similar features observed in some other radio lobes typically have a spectrum flatter than that of their ambient medium. These radio filaments may trace regions where the magnetic field has been stretched and amplified by gas circulation within the rising bubble. The spectrum of the brightest region of the radio lobe exhibits a spectral break, which corresponds to a radiative cooling age of the fossil lobe of approximately 174 Myr, giving a date for this most powerful AGN explosion.