Faraday Depolarization Study of a Radio Galaxy Using LOFAR Two-metre Sky Survey: Data Release 2

TL;DR

This study addresses how Faraday depolarization in a low-frequency radio galaxy can be disentangled from instrumental leakage to reveal the magneto-ionic environment. It applies qu-fitting to LOFAR LoTSS-DR2 polarimetric data for ILTJ012215.21+254334.8, testing a five-model suite that combines Faraday-thin, Faraday-thick, and external Faraday dispersion components, and uses MultiNest-based Bayesian evidence to select the best model. The main finding is that a three-component model (1T+2ED) best describes the data, consisting of a near-zero RM instrumental leakage and two external depolarization components around RM $\approx -47$ rad m$^{-2}$, with $\chi^2_{red}=2.12$, implying a turbulent, inhomogeneous magneto-ionic medium along the line of sight. The approach demonstrates the power of LOFAR polarization studies for probing galactic and intergalactic magnetic fields and motivates expanding the analysis to the full FR-I subset of LoTSS-DR2.

Abstract

We present a detailed depolarization analysis of a radio galaxy ILTJ012215.21+254334.8, utilizing polarimetric data from the LOFAR Two-metre Sky Survey (LoTSS) Data Release 2 (DR2) catalogue. The selected source exhibits a rotation measure (RM) of ~ - 47 rad/m^2 and a projected linear size of 335 kpc at a redshift z ~ 0.05. Depolarization model fitting was performed on LOFAR High Band Antenna data (120 - 168 MHz), with fractional polarization detected at 3.0%. Five depolarization models were tested, and Bayesian qu-fitting revealed that the three-component model (1T+2ED) best describes the data, with a reduced chi-squared value of 2.12 and a logarithmic Bayesian evidence of 1384.82. This model includes a Faraday-thin component at RM ~ - 0.3 rad/m^2 (instrumental leakage) and two external Faraday dispersion astrophysical emission at RM ~ - 47 rad/m^2. The results demonstrate that depolarization in low-frequency radio galaxies requires multi-component modelling and is driven by turbulence and inhomogeneity in the magneto-ionic medium. Our findings highlight the potential of LOFAR polarization studies for probing galactic and intergalactic magnetic fields with high precision.

Figures (7)

• Figure 1: The data points with $1\sigma$ error bars show the fractional q, u, and p plotted against $\lambda^2$ for the source. The best-fit model is shown as a solid line.
• Figure 2: The polarization angle $\psi$ is plotted against $\lambda^2$ for the source ILTJ012215.21+254334.8, with the best-fit model shown as a solid line.
• Figure 3: Polarization data, with $1 \sigma$ error bars, q as a function of u for the source, and the corresponding best-fitting model 1T+2ED. The decrease in the radius indicates depolarization. The data points are coloured by observing frequency. Higher frequency points are coloured red. Lower frequency points are coloured violet/blue. Red points close to the centre indicate relatively strong and coherent polarization at higher frequencies. Lighter points spread outward suggest depolarization and more complex polarization behaviour at lower frequencies. This effect is a signature of Faraday depolarization in polarized radio sources.
• Figure 4: The posterior distribution of the best-fit model parameters are shown using one- and two-dimensional projections in a corner plot. This visualization illustrates both the individual distributions and the correlations among the parameters in the dataset. The diagonal sections display histograms for each parameter's distribution with $1\sigma$ error bars. Off-diagonal panels display scatter plots that reveal correlations and covariances between pairs of parameters, illustrating how they jointly vary in the dataset.
• Figure 5: Same as Figure \ref{['fig:pqu']}, but for $\mathrm{1Tk+2ED}$ model.