Faraday Complexity and Depolarisation in a High-Rotation-Measure Radio Galaxy from the Spectra and Polarisation In Cutouts of Extragalactic Sources (SPICE-RACS) DR2

Abstract

We present a broadband spectro-polarimetric analysis of the extragalactic radio source \texttt{RACS\_0900-28\_7036} using SPICE-RACS DR2 observations with the Australian Square Kilometre Array Pathfinder (ASKAP). The source was selected for its large rotation measure (${\rm RM}=345.7\pm0.2~{\rm rad~m^{-2}}$), substantial excess relative to the local foreground ($Δ{\rm RM}\approx171~{\rm rad~m^{-2}}$), and strong evidence of Faraday complexity ($σ_{\rm add}/δσ_{\rm add}\approx8.6$). Observations span 803--1083~MHz in 36 spectral channels, enabling detailed characterization of Faraday rotation and wavelength-dependent depolarization. One-dimensional QU-fitting and Bayesian model selection identify a multi-component model comprising one Burn-slab component and two external Faraday dispersion components (1 Slab + 2 EFD) as the preferred description. The dominant astrophysical component exhibits ${\rm RM}\approx345.5~{\rm rad~m^{-2}}$ with modest Faraday dispersion ($σ_{\rm RM}\approx3~{\rm rad~m^{-2}}$), consistent with the Galactic foreground rotation measure at the source position (${\rm RM}_{\rm Gal}=331.9\pm33.1~{\rm rad~m^{-2}}$). A secondary broader component at ${\rm RM}\approx131.5~{\rm rad~m^{-2}}$ shows strong depolarization ($σ_{\rm RM}\approx19.5~{\rm rad~m^{-2}}$), indicating an additional turbulent Faraday-active medium along the line of sight. The fractional polarization spectrum and $q$--$u$ plane evolution further confirm multiple Faraday-active regions along the line of sight. These results demonstrate that ASKAP broadband spectropolarimetry can resolve complex Faraday structures and probe turbulent magnetized environments, providing a framework for systematic depolarization studies across the full SPICE-RACS catalog and enabling statistical investigations of Faraday complexity in diverse extragalactic radio sources.