Speaker
Description
We present an analysis of the spectropolarimetric properties of a sample of AGN and Sgr A*, observed with the Atacama Large Millimeter/ submillimeter Array (ALMA) over five VLBI campaigns conducted between 2017 and 2023. We perform a comprehensive characterization of the compact cores in total and polarized intensity, investigating trends in the linear polarization fraction (LP), the electric vector position angle (EVPA), and the rotation measure (RM). The insights obtained from this investigation have strong implications for the nature of the magnetized plasma in the vicinity of the central object.
The data presented were acquired with ALMA during the VLBI campaigns of the Global Millimeter VLBI Array (GMVA) and the Event Horizon Telescope (EHT) in 2017, 2018, 2021, 2022, and 2023. These consist of observations at wavelengths of 3 mm (93 GHz), 1.3 mm (221 GHz), and 0.87 mm (343 GHz), targeting a variety of objects, including quasars such as 3C273 and NRAO530, BL Lacs such as OJ287 and 4C01.28, radio galaxies such as M87 and Centaurus A, and the radio source at the Galactic Center, Sgr A*.
Our investigation includes the study of the variability of the sources’ polarimetric properties, as well as their behavior across mm/sub-mm wavelengths. We report in detail the spectropolarimetric behavior of several individual sources over these five years, and we also analyze differences among the different object classes across the entire sample. Many targets exhibit depolarization along with high RM values, and we discuss the possibility that this arises from the presence of turbulent magnetic fields in the Faraday screen. We also investigate what the frequency dependence of RM can reveal about the geometry of the Faraday screen.
All sources were found to be more variable in polarization than in total intensity, sometimes showing sudden EVPA changes and RM flips from one day to the next. BL Lacs are more polarized than other quasars, and Sgr A is more polarized than radio galaxies. We discuss the RM behavior of the sources in the context of jet–sheath or accretion-flow models for the RM origin. Sgr A is the only object whose RM is consistently negative, pointing to a stable Faraday screen potentially associated with the large-scale accretion flow.