2025ApJ...994..196K

We present the results of multifrequency monitoring of the radio quasar 3C 286, conducted using three instruments: the Atacama Large Millimeter/submillimeter Array (ALMA) at 91.5, 103.5, 233.0, and 343.4 GHz, the IRAM 30 m Telescope at 86 and 229 GHz, and the Submillimeter Array at 225 GHz. The IRAM measurements from 2006 to 2024 show that the total flux of 3C 286 is stable within measurement uncertainties, indicating long-term stability up to 229 GHz, when applying a fixed kelvin-to-jansky conversion factor throughout its data set. ALMA data from 2018 to 2024 exhibit a decrease in flux, of which up to 4% could be attributed to an apparent increase in the absolute brightness of Uranus, the primary flux calibrator for ALMA with the ESA4 model. Taken together, these results suggest that the intrinsic total flux of 3C 286 has remained stable up to 229 GHz over the monitoring period. The polarization properties of 3C 286 are stable across all observing frequencies. The electric vector position angle (EVPA) gradually rotates as a function of wavelength squared, which is well described by a single power law over the full frequency range. We therefore propose using the theoretical EVPA values from this model curve for absolute EVPA calibration between 5 and 343.4 GHz. The Faraday rotation measure (RM) increases as a function of frequency up to (3.2 ± 1.5) × 10⁴ rad m^–2, following ∣RM∣ ∝ ν^α^ with α = 2.05 ± 0.06. This trend is consistent with the core-shift effect expected in a conical jet.