Linear polarization structures in LOFAR observations of the interstellar medium in the 3C 196 field

Aims. This study aims to characterize linear polarization structures in LOFAR observations of the interstellar medium (ISM) in the 3C 196 field, one of the primary fields of the LOFAREpoch of Reionization key science project. Methods. We have used the high band antennas (HBA) of LOFAR to image this region and rotation measure (RM) synthesis to unravel the distribution of polarized structures in Faraday depth. Results. The brightness temperature of the detected Galactic emission is 5-15 K in polarized intensity and covers the range from -3 to +8 rad m^-2 in Faraday depth. The most interesting morphological feature is a strikingly straight filament at a Faraday depth of +0.5 rad m^-2 running from north to south, right through the centre of the field and parallel to the Galactic plane. There is also an interesting system of linear depolarization canals conspicuous in an image showing the peaks of Faraday spectra. We used the Westerbork Synthesis Radio Telescope (WSRT) at 350 MHz to image the same region. For the first time, we see some common morphology in the RM cubes made at 150 and 350 MHz. There is no indication of diffuse emission in total intensity in the interferometric data, in line with results at higher frequencies and previous LOFAR observations. Based on our results, we determined physical parameters of the ISM and proposed a simple model that may explain the observed distribution of the intervening magnetoionic medium. Conclusions. The mean lineofsight magnetic field component, B_||, is determined to be 0.3 ± 0.1 μG and its spatial variation across the 3C 196 field is 0.1 μG. The filamentary structure is probably an ionized filament in the ISM, located somewhere within the Local Bubble. This filamentary structure shows an excess in thermal electron density (n_eB_||> 6.2 cm^-3μG) compared to its surroundings.