First uGMRT Constraints on Rotation Measure in the Radio-Quiet Seyfert Galaxy NGC 4235

The gist: This study presents the first-ever analysis of polarization across two frequency bands of the uGMRT and provides constraints on rotation measure (RM) at lower frequencies for a weak radio source in NGC 4235, demonstrating the potential of uGMRT broadband polarimetry for probing magnetized environments in and around such AGN.

Observations & Data Reduction

The researchers observed the source NGC 4235 in full polar mode at 1.3 GHz and 663 MHz using the uGMRT, employing different feed configurations for each frequency The uGMRT at ∼660 MHz (Band-4) has native linear feeds along with a ‘quadrature hybrid,’ aka polarizer, converting linear to circularly polarized signals At ∼1.3 GHz (Band-5), due to the absence of such a polarizer, linearly polarized signals are received The data reduction involved using their own (publicly available) uGMRT-based polarization data reduction pipelines using Common Astronomical Software Application (CASA), for reducing uGMRT Band-4 (circular basis) and Band-5 (linear basis) data In a circular basis, the polarization calibration includes cross-hand delay calibration, leakage (or Dterm) calibration, and cross-hand phase calibration solved using CASA tasks gaincal and polcal They used the polarized calibrator 3C 286 for polarization angle, delay and leakage calibration The final outputs from the data reduction procedure include Stokes I, Q, U, and V images The linear polarization intensity is computed as Ip = pQ squared + U squared

RM Estimation Methodology

The rotation measure (RM) is a powerful diagnostic of both the intrinsic magneto-ionic environment near the AGN and the intervening media The observed polarization angle (φobs) changes from the intrinsic angle of polarization (φint) by rotation measure (RM) times wavelength squared (λ 2) as: φobs = φint + RMλ squared RM depends on the electron density (ne) and the magnetic field (Bk) integrated along the line of sight as: RM = e(3/2πm 2e/c4)Z L 0 ne(l) Bk(l) dl Assuming a uniform electron density and magnetic field along a path length L, Equation 3 becomes: RM (rad m−2) = 812 ne(cm−3) Bk (µG)L (kpc) Measurements of RM therefore offer a powerful diagnostic of both the intrinsic magneto-ionic environment near the AGN and the intervening media They aimed to constrain the RM to the lower uGMRT frequencies at the ‘core’3, which is well-fitted with a single Gaussian component4 having a size of ∼ 9'' or ∼ 2 kpc in the 10 GHz VLA image and ∼ 6'' or ∼ 1 kpc in the 660 MHz GMRT image The task rmfit uses at least three well-separated frequency bands to resolve the potential nπ ambiguities in the polarization angle by χ squared minimisation, and to obtain a unique solution of RM for each pixel

Core Analysis and Interpretation

The study found that NGC 4235 exhibits extended radio emission with an overall S-shaped/bubble morphology (as often observed in Seyfert galaxies; Kharb et al., 2006, 2010) The radio ‘core’ at 10 GHz shows an α of +0.11 ± 0.02 (Ghosh et al., 25) This spectral index suggests that the nuclear emission is optically thick due to synchrotron self-absorption In such optically thick regions, the projected magnetic field is expected to be oriented parallel to the observed EVPAs The EVPAs at 10 GHz indicate that the B-field in the inner optically thick core is predominantly perpendicular to the outflow axis, consistent with a toroidal or helical B-field configuration This geometry is expected in wide-angle disk winds or magnetically driven outflows launched from the accretion disk, which can collimate a weak jet from parsec to kpc scales The global minimum of χ squared was obtained for branch offsets (relative to the ∼ 8 − 12 GHz data) of n= 0, 0, 0, +3, +13 at 8.68, 9.87, and 11.2 GHz for the EVPAs The corresponding best-fit parameters are RM = 198.4 ± 0.7 rad m−2, and intrinsic polarization angle φint = −54.5◦ ± 2.9◦ The results highlight the reliability of uGMRT polarimetric measurements, which provide more constraints on the polarimetric modelling and better information about AGN jets and their environments

Physical Implications

The modest RM value for NGC 4235 suggests that there is an absence of a strong depolarization in the ‘core’, indicating that the Faraday-rotating medium is thin and uniform The consistency between the VLA-scale ordered magnetic fields and the uGMRT-scale RM values indicates that the kpc-scale radio emission in NGC 4235 is Faraday-rotated by the magnetized plasma in the host galaxy’s interstellar medium The Galactic RM along this line of sight is found to be 5.5 ± 1.5 rad m−2 Subtracting this from the observed RM at the ‘core’, the resultant ‘core’ RM becomes 192.9 ± 1.7 rad m−2 This suggests a low electron density (≈ 3 × 10−2cm−3) and small perturbation length scale (∼ 0.3 parsec) within an assumed magnetized region of 1.1 − 1.5 kpc, for the Faraday-rotating medium These properties are consistent with a diffuse magnetized sheath or AGN-driven wind surrounding the radio jet-like outflow The relatively small RM further indicates that significant internal Faraday depolarization within the core is unlikely, suggesting that the observed Faraday rotation predominantly originates in an external, low-density magneto-ionized medium

Magnetic Field Estimation

For an estimate of the magnetic field strength, the researchers adopted the standard equipartition assumption commonly used in radio-emitting AGN They estimate the B-field to be ∼ 5 − 8 µG in the core using both the VLA 10 GHz and uGMRT 660 MHz data Assuming that the characteristic line-of-sight field strength is the same as the equipartition B-field, they infer an average electron density of ne = (2.4 − 2.

1. This system can be improved to perform low-frequency radio astronomy polarization analysis by implementing a robust method for resolving nπ ambiguity in Rotation Measure (RM) fitting, as detailed in Table 3 of the paper. The improved AI will use a global χ2 minimisation to obtain the RM value rather than relying on a simple linear fit, ensuring a unique solution for RM from multi-frequency EVPA data.

2. The system can be upgraded to accurately model the physical environment surrounding AGN outflows by integrating the relationship between observed RM and physical parameters, specifically RM (rad m−2) = 812 ne(cm−3) Bk (µG)L (kpc) as presented in Equation 3. This allows the AI to estimate an average electron density of the Faraday-rotating medium of ∼ 3 × 10−2 cm−3 for a characteristic perturbation scale of ∼ 0.3 parsec within an assumed magnetized region of size ∼ 1.3 kpc.

3. The system can be enhanced to differentiate between external and internal Faraday rotation mechanisms by analyzing the linearity of the polarization angle versus wavelength squared plot, which shows no significant curvature for the 'core'. This capability allows the AI to infer that the observed Faraday rotation predominantly originates in an external, low-density magneto-ionized medium.

We present a multi-frequency polarimetric study of the radio-quiet Seyfert galaxy NGC 4235 using the upgraded Giant Metrewave Radio Telescope (uGMRT), aimed at constraining the Faraday rotation properties of its kpc-scale radio emission. This marks the first-ever analysis of polarization across two frequency bands of the uGMRT and the first constraints on rotation measure (RM) at lower frequencies for a weak radio source (17.1 plus or minus 1.7 mJy at 1.3 GHz, 27.7 plus or minus 2.8 mJy at 663 MHz). We detect linearly polarized radio emission associated with the `core' and the extended lobes of NGC 4235. Broadband polarimetric analysis yields an RM value of about200 rad m-2 in the `core', indicating Faraday rotation by magnetized plasma external to the synchrotron-emitting regions. Using these RM constraints, we estimate an average electron density of the Faraday-rotating medium of about 3 times10-2 cm-3 for a characteristic perturbation scale of about 0.3 parsec within an assumed magnetized region of size about 1.3 kpc, consistent with a diffuse magnetized sheath or AGN-driven wind. These results extend classical Faraday rotation diagnostics to the low-power, radio-quiet AGN regime and demonstrate the potential of uGMRT broadband polarimetry for probing magnetized environments in and around such AGN.