Abstract
In this paper, we present a detailed analysis of the Faraday depth (FD) spectrum and its clean components obtained through the application of the commonly used technique of Faraday rotation measure synthesis to analyze spectro-polarimetric data. To directly compare the Faraday depth spectrum with physical properties of a magneto-ionic medium, we generated synthetic broad-bandwidth spectro-polarimetric observations from magnetohydrodynamic (MHD) simulations of a transonic, isothermal, compressible turbulent medium. We find that correlated magnetic field structures give rise to a combination of spiky, localized peaks at certain FD values, and broad structures in the FD spectrum. Although most of these spiky FD structures appear narrow, giving an impression of a Faraday thin medium, we show that they arise from strong synchrotron emissivity at that FD. Strong emissivity at a FD can arise because of both strong spatially local polarized synchrotron emissivity at a FD or accumulation of weaker emissions along the distance through a medium that have Faraday depths within half the width of the rotation measure spread function. Such a complex Faraday depth spectrum is a natural consequence of MHD turbulence when the lines of sight pass through a few turbulent cells. This therefore complicates the convention of attributing narrow FD peaks to the presence of a Faraday-rotating medium along the line of sight. Our work shows that it is difficult to extract the FD along a line of sight from the Faraday depth spectrum using standard methods for a turbulent medium in which synchrotron emission and Faraday rotation occur simultaneously.
Highlights
The advent of broad-band receivers on all major radio telescopes in the last decade have opened up new avenues for investigating the properties of synchrotron-emitting relativistic plasma in astrophysical objects
Starting with magnetohydrodynamic (MHD) simulations of isothermal, transonic, compressible turbulent plasma, similar to that observed in the Galactic interstellar medium (ISM) [32,33], we use raytracing to simulate broad-band spectro-polarimetric observations
We have investigated in detail various features in Faraday depth spectra obtained from synthetic broad-band spectro-polarimetric observations in the frequency range 0.5 to 6 GHz sampled with frequency channels
Summary
The advent of broad-band receivers on all major radio telescopes in the last decade have opened up new avenues for investigating the properties of synchrotron-emitting relativistic plasma in astrophysical objects. To study the diffuse Galactic magneto-ionic medium, broad-band, large sky-area surveys below ∼15 GHz with single-dish telescopes have been undertaken, e.g., the Global Magneto-Ionic Medium Survey (GMIMS; [22]), the GALFA Continuum Transit Survey (GALFACTS; [23]), the S-band Polarization All Sky Survey (S-PASS; [24]), survey with the SKA-MPG prototype telescope [25], the C-Band All Sky Survey (C-BASS; [26]) and the Q-U-I JOint TEnerife (QUIJOTE; [27,28]) With these broad-band polarization surveys, pressing astrophysical problems, such as black hole accretion and its connection to AGN jet launching mechanism, cosmic evolution of magnetic fields in galaxies, structure and strength of magnetic fields in the interstellar, intra-cluster and intergalactic medium will be investigated (see e.g., [29]).
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