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

Abstract

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 LOFAR-Epoch 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 magneto-ionic medium. Conclusions. The mean line-of-sight 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 e B > 6.2 cm −3 µG) compared to its surroundings.

Anisotropies in the HI gas distribution toward 3C 196

“…This field is one of the primary fields of the LOFAR-Epoch of Reionization key science project. Linear polarization structures have been observed with LOFAR by Jelić et al (2015). Strikingly straight filaments were found to be aligned with the magnetic field (Zaroubi et al 2015) and Hi shows a similar filamentary structure (Fig.…”

“…Moreover, there are also linear depolarization canals conspicuous in images at the peaks of the Faraday spectra. Jelić et al (2015) conclude that the filamentary structure is probably caused by an ionized filament in the ISM, located somewhere in the foreground.…”

“…Jelić et al (2015) derived from the RM to dispersion measure ratio a mean line-of-sight magnetic field component of B = 0.3 ± 0.1µG and its variations across the 3C 196 field to 0.1µG. The 3C 196 field is well studied and we hope that follow-up investigations can lead to more precise results.…”

“…Linear polarization structures in LOFAR observations of the interstellar medium in the 3C 196 field have been observed by Jelić et al (2015). This is one of the three primary fields of the LOFAR-Epoch of Reionization key science project.…”

A Space-based Observational Strategy for Characterizing the First Stars and Galaxies Using the Redshifted 21 cm Global Spectrum

“…Finally, we note that the low-foreground areas of the sky are polarized to a few percent (5%) (Jelić et al 2014(Jelić et al , 2015Lenc et al 2016). Our dual polarization instrument directly measures this intrinsic sky polarization.…”