Population statistics of beamed sources. II: Evaluation of Doppler factor estimates

TL;DR

This study evaluates various methods of estimating Doppler factors in blazars by comparing model predictions with observational data, finding variability-based estimates assuming equipartition are most consistent with the statistical model.

Contribution

It assesses the accuracy of different Doppler factor estimation methods for blazars using a statistical population model and observational data comparisons.

Findings

Variability Doppler factors assuming equipartition best match the model.

Inverse Compton Doppler factors are only suitable for FSRQs.

Systematic errors dominate uncertainties in variability-based estimates.

Abstract

In a companion paper we presented a statistical model for the blazar population, consisting of distributions for the unbeamed radio luminosity function and the Lorentz factor distribution of each of the BL Lac and Flat Spectrum Radio Quasar (FSRQ) classes. Our model has been optimized so that it reproduces the MOJAVE distributions of apparent speeds and redshifts when the appropriate flux limit is applied and a uniform distribution of jet viewing angles is assumed for the population. Here we use this model to predict the Doppler factor distribution for various flux-limited samples for which Doppler factors have been estimated in a variety of ways (equipartition, variability + equipartition, inverse Compton dominance) on a blazar-by-blazar basis. By comparing the simulated and data-estimated Doppler factor distributions in each case, we evaluate the different methods of estimating blazar Doppler factors. We find that the variability Doppler factors assuming equipartition are the ones in the best agreement with our statistical model, whereas the inverse Compton Doppler factor method is only suitable for FSRQs. In the case of variability Doppler factors, we find that while random errors are relatively low ($\sim 30\%$), uncertainties are dominated by systematic effects. In the case of inverse Compton Doppler factors, random errors appear to dominate, but are significantly larger ($\sim 60\%$).