Young-Bin Shin | Searcharxiv

arXiv2026

Cosmological Distance Measurements of High Redshift Blazars: OJ 248 (z=0.939) and 4C +38.41 (z=1.814)

In this study, we estimated the angular diameter distances to two high-redshift active galactic nuclei (AGNs), OJ 248 (z = 0.939) and 4C +38.41 (z = 1.814), using 43 GHz radio light curves. The aim of this work is to extend AGN variability-based distance measurement methods to the high-redshift regime. The distance estimates were analyzed under two assumptions for the maximum intrinsic brightness temperature TB,int: (1) the equipartition temperature, and (2) the sample-averaged TB,int. As a representative result, when adopting the equipartition temperature, the angular diameter distances to OJ 248 and 4C +38.41 are estimated to be 7592.7 +/- 396.7 Mpc and 11069.8 +/- 1216.9 Mpc, respectively. In addition, Doppler factors were calculated using the inverse-Compton method, and additional distance estimates were obtained based on these values. Our error budget analysis shows that the most significant systematic uncertainty arises from epoch selection. These results indicate that systematic effects have a substantial impact on the derived distance estimates and limit their reliability, particularly in the high-redshift regime. Reducing these uncertainties will require improved observational cadence and reduced post-fit noise. AGN variability provides an alternative approach to distance estimation. However, our results reveal significant limitations in the current methodology and indicate that further methodological and observational improvements are required before its cosmological applicability can be reliably assessed.

astro-ph.GA↗

arXiv2025

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 ($T_{\rm int}$) 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 $(z_{\rm cos}\ll1)$ to high $(z_{\rm cos}>4)$ redshifts. 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 $T_{\rm int}$ 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 $T_{\rm int}$ 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 $T_{\rm int}$ constrain upper limits of $\log_{10}T_{\rm int}$ [K]$<11.56$ at 15~GHz and $\log_{10}T_{\rm int}$ [K]$<11.65$ at 43 GHz under a certain set of geometric assumptions. The population analysis suggests lower limits of $\log_{10}T_{\rm int}$ [K]$>9.7$, $9.1$, and $9.3$ respectively at 24, 43, and 86 GHz. Even with monthly observations, variability-based estimates of $T_{\rm int}$ appear to be cadence-limited. Conclusions. Methods used to constrain $T_{\rm int}$ are more uncertain than previously thought. However, with improved datasets, the estimates should converge.

astro-ph.GA↗