Properties of intrinsic polarization angle ambiguities in Faraday tomography

Abstract

Faraday tomography is a powerful method to diagnose polarizations and Faraday rotations along the line of sight. The quality of Faraday tomography is, however, limited by several conditions. Recently, it is reported that Faraday tomography indicates false signals in some specific situations. In this paper, we systematically investigate the condition of the appearance of false signals in Faraday tomography. We study this by pseudo-observing two sources within a beam, and change in the intrinsic polarization angles, rotation measures, intensities, and frequency coverage. We find that false signals arise when rotation measure between the sources is less than 1.5 times the full width at half maximum of the rotation measure spread function. False signals also depend on the intensity ratio between the sources and are reduced for large ratio. On the other hand, the appearance of false signals does not depend on frequency coverage, meaning that the uncertainty should be correctly understood and taken into consideration even with future wide-band observations such as Square Kilometer Array (SKA).

Study of the Vertical Magnetic Field in Face-on Galaxies Using Faraday Tomography

“…In addition, when F(f) is constructed from an observed polarization spectrum, effects such as a false signal in RM CLEAN (Farnsworth et al 2011;Kumazaki et al 2014;Miyashita et al 2016) as well as the limited frequency coverage and noises in observation need to be considered. For instance, the shape of F(f) could depend on the wavelength because of an imperfect Fourier transform due to the limited sampling of the squared wavelength.…”

Strategy to Explore Magnetized Cosmic Web with Forthcoming Large Surveys of Rotation Measure

“…The following works do not address the technical problems of Faraday tomography. Improvement of Faraday tomography such as RM CLEAN and QU-fitting [60][61][62][63][64] is another important work; see also some articles in this volume.…”

The Power of Low Frequencies: Faraday Tomography in the Sub-GHz Regime

“…An observation from 1-2 GHz, possible with the VLA L-band receivers has a Faraday depth resolution of 50 rad m −2 , while a typical observation in the LOFAR high-band from 110-170 MHz has a resolution of 1 rad m −2 , under the assumption no bandwidth is lost to radio frequency interference. While it is possible to identify Faraday depth structure on scales smaller than the resolution [26], it is difficult to measure this structure unambiguously [27] and it requires high signal-to-noise. Low frequency observations are thus very desirable for studies of objects with small Faraday depth variations, such as giant radio galaxies or local interstellar medium (ISM) structure in the Milky Way.…”