A STUDY OF BROADBAND FARADAY ROTATION AND POLARIZATION BEHAVIOR OVER 1.3–10 GHz IN 36 DISCRETE RADIO SOURCES

Abstract

We present a broadband polarization analysis of 36 discrete polarized radio sources over a very broad, densely sampled frequency band. Our sample was selected on the basis of polarization behavior apparent in narrowband archival data at 1.4 GHz: half the sample shows complicated frequency-dependent polarization behavior (i.e., Faraday complexity) at these frequencies, while half shows comparatively simple behavior (i.e., they appear Faraday simple). We re-observed the sample using the Australia Telescope Compact Array in full polarization, with 6 GHz of densely sampled frequency coverage spanning 1.3-10 GHz. We have devised a general polarization modeling technique that allows us to identify multiple polarized emission components in a source, and to characterize their properties. We detect Faraday complex behavior in almost every source in our sample. Several sources exhibit particularly remarkable polarization behavior. By comparing our new and archival data, we have identified temporal variability in the broadband integrated polarization spectra of some sources. In a number of cases, the characteristics of the polarized emission components, including the range of Faraday depths over which they emit, their temporal variability, spectral index, and the linear extent of the source, allow us to argue that the spectropolarimetric data encode information about the magneto-ionic environment of active galactic nuclei themselves. Furthermore, the data place direct constraints on the geometry and magneto-ionic structure of this material. We discuss the consequences of restricted frequency bands on the detection and interpretation of polarization structures, and the implications for upcoming spectropolarimetric surveys.

A broad-band spectro-polarimetric view of the NVSS rotation measure catalogue – I. Breaking the nπ-ambiguity

“…We find it necessary to formally define Faraday complex sources before proceeding further. The main reason is to facilitate comparisons with the literature, as a growing number of broadband spectro-polarimetric studies of EGSs choose to extract the flux densities of their samples by integrating within a source region (e.g., Anderson et al 2016;O'Sullivan et al 2017). While this would be similar to the strategies we adopted for our point sources and extended sources, it is in contrast to our spatial doubles, for which we fitted two Gaussian functions to each image (per frequency channel and per Stokes parameter; Section 2.2) and analysed the two spatial components independently.…”

“…RM-Synthesis and QU-fitting are widely used in broadband radio polarisation studies, with a growing success in revealing the Faraday complexities of a significant number of the observed EGSs (e.g. Law et al 2011;Anderson et al 2015Anderson et al , 2016Kim et al 2016;O'Sullivan et al 2017;Kaczmarek et al 2018;Pasetto et al 2018;Schnitzeler et al 2019). The largest RM catalogue of polarised radio sources to date is the Taylor et al (2009, hereafter TSS09) catalogue, which contains RM values of 37,543 radio sources north of δ = −40 • at a source density of higher than one per square degree.…”

Radio Polarisation Study of High Rotation Measure AGNs

“…This means we took the FD of the Faraday component with the highest polarisation fraction as the source's FD. Nonetheless, we argue that the FD values obtained this way could still be reasonably compared with the NVSS RM values, and we will demonstrate the ability to apply QU-fitting (e.g., [11]) to our data to unveil the Faraday complexities (e.g., [11,12]) of our sources in Section 3.3.…”

“…We find that 12 sources had FD/RM values that agreed with each other in the two measurements within about ±100 rad m −2 , while eight had differences in the FD/RM values by about ±652.9 rad m −2 . Because the frequency coverage of the two observations were vastly different, Faraday complexities could cause deviations of the PA from the simple linear relationship with λ 2 , and for such cases, the FD/RM values determined could vary depending on the frequency coverages (e.g., [12]). More care is needed to facilitate a direct comparison between our broadband data and the narrowband NVSS data.…”

An In-Depth Investigation of Faraday Depth Spectrum Using Synthetic Observations of Turbulent MHD Simulations

“…In order to physically interpret such data, the technique of Faraday rotation measure (RM) synthesis [10,11] and direct fitting of the Stokes Q and U spectra of a polarized source with models of the magneto-ionic media, known as Stokes Q, U fitting [6,12,13] have been developed. It is often not straightforward to interpret the results from these techniques and connect them to the physical properties of the magnetized plasmas being investigated [14].Currently, several large-scale spectro-polarimetric campaigns are underway, mostly in the 1 to 5 GHz frequency range. On one hand, dedicated surveys with interferometers are being conducted to study the broad-band polarization properties of millions of extragalactic sources.…”