Advanced Diagnostics for the Study of Linearly Polarized Emission. II. Application to Diffuse Interstellar Radio Synchrotron Emission

Abstract

Diagnostics of polarized emission provide us with valuable information on the Galactic magnetic field and the state of turbulence in the interstellar medium, which cannot be obtained from synchrotron intensity alone. In Paper I (Herron et al. 2017b), we derived polarization diagnostics that are rotationally and translationally invariant in the Q-U plane, similar to the polarization gradient. In this paper, we apply these diagnostics to simulations of ideal magnetohydrodynamic turbulence that have a range of sonic and Alfvénic Mach numbers. We generate synthetic images of Stokes Q and U for these simulations, for the cases where the turbulence is illuminated from behind by uniform polarized emission, and where the polarized emission originates from within the turbulent volume. From these simulated images we calculate the polarization diagnostics derived in Paper I, for different lines of sight relative to the mean magnetic field, and for a range of frequencies. For all of our simulations, we find that the polarization gradient is very similar to the generalized polarization gradient, and that both trace spatial variations in the magnetoionic medium for the case where emission originates within the turbulent volume, provided that the medium is not supersonic. We propose a method for distinguishing the cases of emission coming from behind or within a turbulent, Faraday rotating medium, and a method to partly map the rotation measure of the observed region. We also speculate on statistics of these diagnostics that may allow us to constrain the physical properties of an observed turbulent region.

Tracing Magnetic Fields By the Synergies of Synchrotron Emission Gradients

“…It is the coordinate system selection that causes the reduced AM values for U and QU , which will be improved in the case of strong Faraday rotation (see Section 3.3.2) or of rotated mean magnetic field orientations (Section 3.4). The need for rotationally-invariant quantities should be emphasized as an important part of this work and of Gaensler et al (2011) and Herron et al (2018a). With the increase of M A , all the effective AMs experience slight reduction, which is in agreement with simulations presented by Zhang et al (2019) in the case of low resolution numerical simulations.…”

Practical Modeling of Large-Scale Galactic Magnetic Fields: Status and Prospects

“…The next step will be to include more information than simply the two-point statistics of the magnetic field. Studies of the ISM turbulence have begun to characterize a variety of its statistical properties based on the diffuse synchrotron emission in total [77] or polarized intensity [78][79][80]. Likewise, for dust, the Planck collaboration has opened a new window into the turbulence in the colder phase of the ISM with high-resolution maps of the polarized dust emission ( [81] and references therein).…”

Synchrotron Polarization Radiative Transfer: Relativistic Thermal Electron Contribution

“…The circular polarization of pulsar wind nebula (PWN) can also be further checked if we consider the thermal electron contribution (Bucciantini & Olmi 2018). We may even consider performing our model for the diffuse synchrotron emission if the electrons in the interstellar medium also have a thermal component (Herron et al 2018).…”