VLBI core fluxes and brightness temperatures

We modeled the source structure of seven AGN in the EHT 2017 data set using linearly polarized circular Gaussian components (1749+096, 1055+018, BL Lac, J0132-1654, J0006-0623, CTA 102, and 3C 454.3) and collected results for the other nine AGN from dedicated EHT publications, complemented by lower frequency data in the 2-86GHz range. Combining these data into a multifrequency EHT+ data set, we studied the dependences of the VLBI core component flux density, size, and brightness temperature on the frequency measured in the AGN host frame (and hence on the distance from the central black hole), characterizing them with power law fits.

We compared the observations with the BK jet model and estimated the magnetic field strength dependence on the distance from the central black hole. Our observations spanning event horizon to parsec scales indicate a deviation from the standard BK model, particularly in the decrease of the brightness temperature with the observing frequency. Only some of the discrepancies may be alleviated by tweaking the model parameters or the jet collimation profile.

Either bulk acceleration of the jet material, energy transfer from the magnetic field to the particles, or both are required to explain the observations. For our sample, we estimate a general radial dependence of the Doppler factor {delta}{prop.to}r^<=0.5^. This interpretation is consistent with a magnetically accelerated sub-parsec jet.

We also estimate a steep decrease of the magnetic field strength with radius B{prop.to}r^-3^, hinting at jet acceleration or efficient magnetic energy dissipation.