Polarized radiative transfer, rotation measure fluctuations, and large-scale magnetic fields

Abstract

Faraday rotation measure at radio wavelengths is commonly used to diagnose largescale magnetic fields. It is argued that the length-scales on which magnetic fields vary in large-scale diffuse astrophysical media can be inferred from correlations in the observed RM. RM is a variable which can be derived from the polarised radiative transfer equations in restrictive conditions. This paper assesses the usage of RMF (rotation measure fluctuation) analyses for magnetic field diagnostics in the framework of polarised radiative transfer. We use models of various magnetic field configurations and electron density distributions to show how density fluctuations could affect the correlation length of the magnetic fields inferred from the conventional RMF analyses. We caution against interpretations of RMF analyses when a characteristic density is ill defined, e.g. in cases of log-normal distributed and fractal-like density structures. As the spatial correlations are generally not the same in the line-of-sight longitudinal direction and the sky plane direction, one also needs to clarify the context of RMF when inferring from observational data. In complex situations, a covariant polarised radiative transfer calculation is essential to capture all aspects of radiative and transport processes, which would otherwise ambiguate the interpretations of magnetism in galaxy clusters and larger-scale cosmological structures.

Properties of polarized synchrotron emission from fluctuation-dynamo action -- I. Application to galaxy clusters

“…In this paper we explored in detail the properties of polarized emission, particularly the synchrotron emissivity and the polarization signals that arises from such turbulent dynamo action. Even though, a great deal of work has been done on Faraday RM both on the observational and theoretical front (Clarke et al 2001;Carilli & Taylor 2002;Vogt & Enßlin 2003;Subramanian et al 2006;Cho & Ryu 2009;Bonafede et al 2010;Bhat & Subramanian 2013;Böhringer et al 2016;Marinacci et al 2018;Sur et al 2018;Vazza et al 2018;On et al 2019), little attention has been paid to understand the emissivity and polarization signals generated by the intermittent magnetic field structures produced by fluctuation dynamos. This is primarily because detection of polarized signals from radio halos has so far proved to be an arduous task with current radio interferometers (Vacca et al 2010).…”

Observables for moving, extremely charged primordial extremely massive black holes

“…To estimate the maximum effect, we assume that all free electrons in the gas cloud are thermal electrons with negligible thermal velocity and we assume photon conservation of the radiation traversing the gas cloud and the magnetic field. Linearly polarised emission from quasars or fast radio bursts can serve to probe such Faraday rotation effects, see, for instance, [18] and references therein. If the linearly polarised radiation has a frequency much larger than the electron gyro-frequency, ω ω B = eB/m e , and the plasma frequency, ω ω p = n e e 2 /(m e 0 ), the rotation measure is given by…”