Polarization and Faraday rotation measure imaging at metre wavelengths with sub-arcsecond resolution: a foundational calibration strategy

Abstract

ABSTRACT Low-frequency radio polarimetric observations provide a powerful probe of magnetic fields in astrophysical sources and the intervening medium, as well as magnetospheric emission from compact objects such as pulsars, magnetically active stars, brown dwarfs, and planetary aurorae. With baselines of up to 2000 km, LOFAR offers a unique opportunity to study the low-frequency polarized Universe at sub-arcsecond resolution. However, polarimetric studies with LOFAR have so far been limited to angular resolutions of ${\sim }6,\mathrm{ arcsec}$, resulting in stronger beam depolarization. Here, we present a calibration strategy that enables full-resolution polarimetric imaging with the LOFAR pan-European array. Our method applies full-Jones corrections to the international stations using an in-field unpolarized calibrator. In addition, when a sufficiently bright polarized source is present in the field, multi-epoch observations can be aligned in Faraday depth using a visibility-based correction that accounts for polarization angle and rotation measure offsets. This approach enables deeper combined imaging and deconvolution. We apply this strategy to the LOFAR ELAIS-N1 field, combining four 8-h observations for a total integration time of 32 h. At $0.3,\mathrm{ arcsec}$ resolution, we detect two previously known polarized sources identified in lower-resolution studies, resolve additional polarized components, and localize emission regions with sub-arcsecond precision. We also identify a new polarized source and detect circularly polarized emission from the binary M-dwarf system CR Draconis, measuring its proper motion across epochs. These results demonstrate that sub-arcsecond polarimetry at metre wavelengths is now feasible with LOFAR, opening new science opportunities in the LOFAR2.0 era.