Faraday tomography of the local interstellar medium with LOFAR: Galactic foregrounds towards IC342

Abstract

The new generation of low-frequency radio telescopes, such as the Low Frequency Array (LOFAR: a Square Kilometre Array-low pathfinder), provides advancements in our capability of probing Galactic magnetism through low-frequency polarimetry. Maps of diffuse polarized radio emission and Faraday rotation can be used to infer properties of, and trace structure in, the magnetic fields in the ISM. However, to date very little of the sky has been probed at high angular and Faraday depth resolution. We observed a 5x5 degree region centred on the nearby galaxy IC342 using LOFAR in the frequency range 115-178 MHz at 4 arcmin resolution and performed Faraday tomography to detect foreground Galactic polarized synchrotron emission separated by Faraday depth (different amounts of Faraday rotation). Our Faraday depth cube shows rich polarized structure, with up to 30 K of polarized emission at 150 MHz. We detect two overlapping diffuse polarized features that are clearly separated in Faraday depth. Faraday-thick structures at such low frequencies would be too strongly depolarized to explain the observations and are therefore rejected. Only Faraday thin structures will not be strongly depolarized; producing such structures requires localized variations in the ratio of synchrotron emissivity to Faraday depth per unit distance, which can arise from several physical phenomena, such as a transition between regions of ionized and neutral gas. We conclude that the observed polarized emission is Faraday thin, and propose that the emission originates from two neutral clouds in the local ISM. We have modeled the Faraday rotation for this line of sight and estimated that the line of sight component of magnetic field of the local ISM for this direction varies between -0.86 and +0.12 uG. We propose that this may be a useful method for mapping magnetic fields within the local ISM.

17 pages, 12 figures, accepted for publication in A&A

References in corpus (11)

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• Initial LOFAR observations of Epoch of Reionization windows: II. Diffuse polarized emission in the ELAIS-N1 field
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• The Fractional Ionization of the Warm Neutral Interstellar Medium
• 74 MHz Discrete HII Absorption Regions Towards The Inner Galaxy
• Polarization signatures of unresolved radio sources
• Plasma Turbulence in the Local Bubble
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• The challenges of low-frequency radio polarimetry: lessons from the Murchison Widefield Array
• The Galactic Magneto-Ionic Medium Survey: Moments of the Faraday Spectra
• Diffuse polarized emission in the LOFAR Two-meter Sky Survey
• Verifying reddening and extinction for Gaia DR1 TGAS main sequence stars
• S-PASS/ATCA: a window on the magnetic universe in the southern hemisphere
• Through thick or thin: Multiple components of the magneto-ionic medium towards the nearby $\mathrm{H}\mathrm{II}$ region Sharpless 2-27 revealed by Faraday tomography
• Tracing Magnetic Field with Synchrotron Polarization Gradients: Parameter Study
• Advanced Diagnostics for the Study of Linearly Polarized Emission. I: Derivation
• Source Finding in Linear Polarization for LOFAR, and SKA Predecessor Surveys, using Faraday Moments
• A dense plasma globule in the solar neighborhood