A helical magnetic field in quasar NRAO 150 revealed by Faraday rotation

Context. Active galactic nuclei (AGN) are some of the most luminous and extreme environments in the Universe. The central engines of AGN are believed to be super-massive black-holes (SMBHs) are fed by accretion discs threaded by magnetic fields within a dense magneto-ionic medium. Aims. We report our findings from polarimetric very-long-baseline interferometry (VLBI) observations of quasar NRAO 150 taken in October 2022 using a combined network of the Very Long Baseline Array (VLBA) and Effelsberg 100-m Radio Telescope. These observations comprise the first co-temporal multi-frequency polarimetric VLBI observations of NRAO 150 at frequencies above 15 GHz. Methods. We used the new VLBI polarization calibration procedure, <ns0:monospace>GPCAL</ns0:monospace>, with polarization observations of frequencies of 12 GHz, 15 GHz, 24 GHz, and 43 GHz of NRAO 150. From these observations, we were able to measure the Faraday rotation and use it to derive the intrinsic electric vector position angle (EVPA0) for the source. As a complementary measurement, we determined the behavior of polarization as a function of observed frequency. Results. The polarization from NRAO 150 only comes from the core region, with a peak polarization intensity occurring at 24 GHz. Across the core region of NRAO 150, we see clear gradients in Faraday rotation and EVPA0 values that are aligned with the direction of the jet curving around the core region. We find that for the majority of the polarized region the polarization fraction is greater at higher frequencies, with intrinsic polarization fractions in the core ≈3%. Conclusions. The Faraday rotation gradients and circular patterns in EVPA0 offer strong evidence to support the presence of a helical+toroidal magnetic field. Furthermore, the presence of low intrinsic polarization fractions indicate that the polarized emission and, hence, the helical+toroidal magnetic field, is present within the innermost jet.