Tracing Magnetic Fields By the Synergies of Synchrotron Emission Gradients

Abstract

This paper studies how to employ synchrotron emission gradient techniques to reveal the properties of the magnetic field within the interstellar media. Based on data cubes of three-dimensional numerical simulations of magnetohydrodynamic turbulence, we explore spatial gradients of synchrotron emission diagnostics to trace the direction of the magnetic field. According to our simulations, multifarious diagnostics for synchrotron emission can effectively determine the potential direction of projected magnetic fields. Applying the synergies of synchrotron diagnostic gradients to the archive data from the Canadian Galactic Plane Survey, we find that multifarious diagnostic techniques make consistent predictions for the Galactic magnetic field directions. With the high-resolution data presently available from Low Frequency Array for radio astronomy and those in the future from the Square Kilometer Array, the synergies of synchrotron emission gradients are supposed to perform better in tracing the actual direction of interstellar magnetic fields, especially in the low-frequency Faraday rotation regime where traditional synchrotron polarization measure fails.

Studying the Anisotropy of Compressible Magnetohydrodynamic Turbulence by Synchrotron Polarization Intensity

“…As is shown in Figures 5, 6, 11 and 12, the anisotropic level of polarization intensity is relatively low at the frequency of 0.01 GHz, which makes the low-frequency polarized intensity more challenging to reveal the properties of MHD turbulence in the effect of the relevant noise and strong Faraday rotation. If the effect of small-scale noise-like is removed, we expect an increase in the degree of anisotropy, which should be similar to the scenario for the application of synchrotron polarization gradient techniques in the low frequency range (Zhang et al 2019a;2019b). The quadrupole ratio modulus and anisotropic coefficient used in this paper are complementary techniques to the gradient measurement.…”

Gradient measurement of synchrotron polarization diagnostic: Application to spatially separated emission and Faraday rotation regions

“…In addition, numerical resolution of data could influence the alignment measurements. We find that the projected magnetic field traced by gradient techniques is not subject to Faraday rotation, which is consistent with the situation for spatially coincident synchrotron polarization radiation and Faraday rotation (Zhang et al 2019a;Zhang, Liu & Lazarian 2019b). A comparison between the directions of projected magnetic field in the emitting-source region and the directions predicted by the gradient measurement of various diagnostics for Case A (see Figure 1).…”

“…It is shown that the gradients of diagnostic techniques (P rad , P tang ) have a better alignment measurement compared with other diagnostic techniques. In view of our findings in Zhang, Liu & Lazarian (2019b) that the gradient of P rad has a robust ability for tracing magnetic field, P rad will be chosen to trace projected magnetic field in the following studies. Throughout this paper, we use a Gaussian kernel of less than 2𝜎 to smooth small scale noise-like structures.…”

“…Therefore, the purpose of technique development of magnetic field measurement is to extract turbulence information from the integrated observation information. In view of this, we will explore in this paper the new way of studying magnetic field using synchrotron gradients technique (SGT) first proposed in Lazarian et al (2017) and Lazarian & Yuen (2018a), further explored and elaborated in Zhang et al (2019aZhang et al ( , 2019b.…”

“…However, they missed the point of the ability of these measures to trace magnetic field directions. Zhang, Liu & Lazarian (2019b) explored the flexibility of these polarization measures to robustly predict the direction of Galactic projected magnetic fields, and identified the spatial gradient from the maximum of the radial component of the polarization directional derivative which can trace the direction of magnetic field with the highest accuracy. Another new technique, synchrotron polarization derivative with respect to the squared wavelength 𝑑𝑃/𝑑𝜆 2 , has been proposed to measure the local magnetic field (Lazarian & Yuen 2018a;, the recovering of which is critical to measure the 3D magnetic field of Milky Way.…”