Linear polarization structures in LOFAR observations of the interstellar
  medium in the 3C196 field
  This study aims to characterize linear polarization structures in LOFAR
observations of the interstellar medium (ISM) in the 3C196 field, one of the
primary fields of the LOFAR-Epoch of Reionization key science project. 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. The brightness temperature of the detected Galactic emission is
$5-15~{\rm K}$ in polarized intensity and covers the range from -3 to +8 ${\rm
rad~m^{-2}}$ in Faraday depth. The most interesting morphological feature is a
strikingly straight filament at a Faraday depth of $+0.5~{\rm rad~m^{-2}}$
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~{\rm 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 magneto-ionic medium. The
mean line-of-sight magnetic field component, $B_\parallel$, is determined to be
$0.3\pm0.1~{\rm \mu G}$ and its spatial variation across the 3C196 field is
$0.1~{\rm \mu G}$. 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 ($n_e B_\parallel>6.2~{\rm
cm^{-3}\mu G}$) compared to its surroundings.
