Monitoring of 3C 286 with ALMA, IRAM, and SMA from 2006 to 2025: Stability, Synchrotron Ages, and Frequency-Dependent Polarization Attributed to Core-Shift

TL;DR

The paper presents two decades of multi-instrument monitoring of 3C 286 across 5–343.4 GHz with ALMA, IRAM, and SMA to assess flux stability, polarization, and EVPA. It finds that the intrinsic total flux remains stable up to 229 GHz, while ALMA flux trends may be influenced by the time-varying brightness of Uranus, the primary calibrator; polarization properties are stable and EVPA shows a frequency-dependent rotation well described by core-shift in a conical jet, with RM increasing as RM$_{ m core}\\propto\nu^{2.05}$. The observed spectral break around $ u_{ m br}\approx67$ GHz and a derived equipartition magnetic field of $B_{ m eq}\approx4.4$ mG yield a synchrotron age of about $\tau_{\rm syn}\approx450$ years, though a multi-zone jet interpretation may be required. Overall, the work provides a precise mm/submm calibration framework, constrains jet physics, and supports using model EVPA curves for absolute calibration across GHz to submillimeter frequencies.

Abstract

We present the results of multi-frequency monitoring of the radio quasar 3C 286, conducted using three instruments: ALMA at 91.5, 103.5, 233.0, and 343.4 GHz, the IRAM 30-m Telescope at 86 and 229 GHz, and SMA 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 dataset. ALMA data from 2018 to 2024 exhibit a decrease in flux, 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 increases as a function of frequency up to (3.2+/-1.5)x10^4 rad m^-2, following RM proportional to nu^alpha with alpha = 2.05+/-0.06. This trend is consistent with the core-shift effect expected in a conical jet.

Figures (10)

• Figure 1: Total flux (top), fractional polarization (middle), and EVPA (bottom) of 3C 286 obtained with ALMA at 91.5, 103.5, 233.0, and 343.4 GHz. The horizontal dotted lines represent the average values at each frequency.
• Figure 2: The total flux of 3C 286 measured with ALMA in Band 3 (top) and Bands 6 and 7 (bottom). Filled circles without error bars represent the full dataset. Larger highlighted circles with error bars indicate the average flux and the standard deviation at each frequency (Table \ref{['tab:app_flux']}), displayed at the median date of the corresponding period: Period I (MJD 58200--59000), Period II (MJD 59200--59900), and Period III (MJD $>$ 60100). Shaded regions correspond to these three periods. Dashed horizontal lines indicate the mean flux values over the entire monitoring period.
• Figure 3: Total flux (top), fractional polarization (middle), and EVPA (bottom) of 3C 286 obtained with IRAM at 86 and 229 GHz and SMA at 225 GHz. The horizontal dotted lines represent the average values at each frequency. Note that the y-axis scales are identical to those in Figure \ref{['fig:amapola_all']}.
• Figure 4: Total flux of 3C 286 measured with IRAM at 86 GHz and ALMA in Band 3 (top), and with IRAM at 229 GHz and ALMA in Bands 6 and 7 (bottom). Dashed horizontal lines indicate the mean flux values measured with IRAM over its monitoring period for each frequency. Shaded regions correspond to these three periods shown in Figure \ref{['fig:alma_flux']}.
• Figure 5: Top: The total flux of Uranus measured with IRAM at 86 GHz. The blue dashed curve represents the ESA4 model. Bottom: The residual flux, i.e., the excess flux relative to the model normalized by the observed flux, is shown with cross marks. The light blue points represent the average residual flux in 1000-day bins. The blue dashed line represents the best-fit lines to the entire residual flux. The ESA4 model appears to agree best with the data around 2017.