2025A&A...693A...9L

Context. The advancement of the Event Horizon Telescope has enabled the study of relativistic jets in active galactic nuclei down to sub-parsec linear scales even at high redshift. Quasi-simultaneous multifrequency observations provide insights into the physical conditions in compact regions and allow accretion theories to be tested.
Aims. Initially, we aimed to measure the magnetic field strength close to the central supermassive black hole in NRAO 530 (1730-130) by studying the frequency-dependent opacity of the jet matter, Faraday rotation, and the spectral index in the millimeter-radio bands.
Methods. NRAO 530 was observed quasi-simultaneously at 15, 22, 43, 86, and 227 GHz at four different very long baseline interferometer (VLBI) networks. By means of imaging and model-fitting, we aligned the images, taken at different frequencies. We explored opacity along the jet and the distribution of the linearly polarized emission in it.
Results. Our findings reveal that the jet of NRAO 530 at 86 and 227 GHz is transparent down to its origin, with 70 mJy emission detected at 227 GHz potentially originating from the accretion disk. The magnetic field strength near the black hole, estimated at 5r_g, is 3 × 10³ - 3 × 10⁴ G (depending on the central black hole mass). These values represent some of the highest magnetic field strengths reported for active galaxies. We also report the first ever VLBI measurement of the Faraday rotation at 43-227 GHz, which reveals rotation measure values as high as -48 000 rad/m², consistent with higher particle density and stronger magnetic fields at the jet's outset. The complex shape of the jet in NRAO 530 is in line with the expected behavior of a precessing jet, with a period estimated to be around 6 ± 4 years.