V. Gustafsson, M. Brüggen, C. Tasse, S. P. O'Sullivan
Magnetic fields in cluster outskirts and the intercluster medium remain poorly constrained because diffuse synchrotron emission is difficult to detect owing to its low surface brightness. Faraday rotation measures (RMs) of polarized background sources can be used to probe large-scale foreground structures. The nearby interacting Abell 3391-3395 system hosts a well-established X-ray bridge, making it a prime target to investigate magnetization in the intercluster environment. We aim to characterize the magnetized environment of Abell 3391/95 and its surroundings by constructing a dense RM grid from wide-field polarimetry. We observed Abell 3391/95 with MeerKAT in full polarization using a three-pointing mosaic. The data were calibrated with direction-independent and direction-dependent techniques and imaged using visibility plane mosaicing to obtain sensitive coverage across the field. Using Faraday synthesis, we formed Faraday cubes and measured RMs for polarized background sources. We defined on- and off-target regions using contours from a wavelet-filtered eROSITA image. We identified a total of 434 polarized sources, with a polarized source density ranging from about 30 sources per square degree in the outer regions to about 110 sources per square degree in the central part of the field, and a field-averaged density of 73 sources per square degree. The clusters show a statistically significant enhancement of RM scatter relative to the off-target region. In contrast, the bridge between the clusters shows a comparatively low RM scatter, while an RM structure-function analysis on matched angular scales yields a tentative indication of larger RM differences in the bridge than off-target. Combined with the low per-source depolarization, this points to a magnetic field in the bridge that is relatively ordered on scales on the order of 10 kpc, but less ordered on larger scales. The non-detection of diffuse synchrotron emission in the bridge yields much improved upper limits on the emissivity.