Direction-Dependent Faraday Synthesis

Abstract

Modern radio interferometers enable high-resolution polarization imaging, offering insights into cosmic magnetism through Rotation Measure (RM) synthesis. Traditional 2+1D RM synthesis treats the 2D spatial and 1D spectral transforms separately. A fully 3D approach transforms data directly from visibility-frequency space to sky-Faraday depth space using a 3D Fourier transform. Faraday synthesis uses the full dataset for improved reconstruction but requires a 3D deconvolution algorithm to subtract artifacts from the residual image. Applying this method to modern interferometers also requires corrections for direction-dependent effects (DDEs). We extend Faraday synthesis by incorporating DDE corrections, enabling accurate polarized imaging in the presence of instrumental and ionospheric effects. We implement this method within DDFACET, introducing a direction-dependent deconvolution algorithm (DDFSCLEAN) that applies DDE corrections in a faceted framework. Additionally, we parameterize the CLEAN components and evaluate the model over a larger set of frequency channels, naturally correcting for bandwidth depolarization. The method is tested on both synthetic and real data. Our results show that Faraday synthesis enables deeper deconvolution, reduces artifacts, and increases dynamic range. The depolarization correction improves recovery of polarized flux, allowing coarser frequency resolution without loss of sensitivity at high Faraday depths. From the 3D reconstruction, we identify a polarized source in a LOFAR Surveys pointing not detected by earlier RM surveys. Faraday synthesis is memory-intensive due to the large transforms between the visibility domain and the Faraday cube, and is only now becoming practical. Nevertheless, our implementation achieves comparable or faster runtimes than the 2+1D approach, making it a competitive alternative for polarization imaging.