The magnetic mayhem in Abell 2199: discovery of synchrotron threads and homogeneous diffuse radio lobes

Abstract

Sensitive low-frequency radio observations have started uncovering examples of synchrotron-emitting threads, isolated from the rest of radio emission in galaxy clusters. As the bridge of radio emission previously detected between the radio lobes of 3C 338 in Abell 2199 is a candidate of such a structure, we observed this galaxy cluster using the International LOFAR Telescope. These observations revealed the presence of multiple narrow isolated synchrotron threads in 3C 338: east, west and north of the AGN and its radio lobes. Chandra X-ray observations show that these structures most likely do not reside within cavities in the intracluster medium (ICM), and are therefore considered to be distinct structures from the radio lobes. Non-detections in 1.5 GHz Very Large Array observations imply that the spectral index of these newly-discovered isolated threads is likely $\alpha _{1500}^{144} < -3.0$ or steeper. We consider these isolated synchrotron threads to most likely display examples of magnetic threads within the ICM that have captured synchrotron-emitting plasma, as has recently been proposed. Furthermore, our observations reveal the radio lobes to show an almost perfectly uniform spectral index, unlike what would be expected if substantial age differences are present in the radio lobes according to standard spectral ageing models. We find that the relativistic plasma in 3C 338 is consistent with a homogeneous cosmic ray electron population, with the spectral variations dependent on the local magnetic field strength. Finally, we explore the various models that could explain this trend in the radio lobes.