The variability angular diameter distance and the intrinsic brightness temperature of active galactic nuclei

Context. It has recently been suggested that angular diameter distances derived from comparing the variability timescales of blazars to angular size measurements with very long baseline interferometry (VLBI) may provide an alternative method to study the cosmological evolution of the Universe. Once the intrinsic brightness temperature (Tint) is known, the angular diameter distance may be found without knowledge of the relativistic Doppler factor, opening up the possibility of a single rung distance measurement method from low (zcos ≪ 1) to high (zcos > 4) redshifts. Previous studies have found Tint ≈ 1010–1011 K, with a potential frequency dependence. Aims. We aim to verify whether the variability-based estimates of the intrinsic brightness temperature of multiple active galactic nuclei (AGNs) converges to a common value. We also investigate whether the intrinsic brightness temperature changes as a function of frequency. Methods. We estimated the Tint of AGNs based on the flux variability of the radio cores of their jets. We utilized radio core light curves and size measurements of 75 sources at 15 GHz and of 37 sources at 43 GHz. We also derived Tint from a population study of the brightness temperatures of VLBI cores using VLBI survey data of more than 100 sources at 24, 43, and 86 GHz. Results. Radio core variability-based estimates of Tint constrain upper limits of log10Tint [K] < 11.56 at 15 GHz and log10Tint [K] < 11.65 at 43 GHz under a certain set of geometric assumptions. The population analysis suggests lower limits of log10Tint [K] > 9.7, 9.1, and 9.3 respectively at 24, 43, and 86 GHz. Even with monthly observations, variability-based estimates of Tint appear to be cadence-limited. Conclusions. Methods used to constrain Tint are more uncertain than previously thought. However, with improved datasets, the estimates should converge.