arXiv subjects

I. Liodakis

Publications and source records attributed to I. Liodakis.

94 records · Page 6Linked to original sources

arXiv2016

RoboPol: optical polarization-plane rotations and flaring activity in blazars

We present measurements of rotations of the optical polarization of blazars during the second year of operation of RoboPol, a monitoring programme of an unbiased sample of gamma-ray bright blazars specially designed for effective detection of such events, and we analyse the large set of rotation events discovered in two years of observation. We investigate patterns of variability in the polarization parameters and total flux density during the rotation events and compare them to the behaviour in a non-rotating state. We have searched for possible correlations between average parameters of the polarization-plane rotations and average parameters of polarization, with the following results: (1) there is no statistical association of the rotations with contemporaneous optical flares; (2) the average fractional polarization during the rotations tends to be lower than that in a non-rotating state; (3) the average fractional polarization during rotations is correlated with the rotation rate of the polarization plane in the jet rest frame; (4) it is likely that distributions of amplitudes and durations of the rotations have physical upper bounds, so arbitrarily long rotations are not realised in nature.

astro-ph.HE↗

arXiv2015

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

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 ($\sim 60\%$).

astro-ph.HE↗

arXiv2015

Population statistics of beamed sources. I: A new model for blazars

Observations of blazar jets are shrouded in relativistic effects, thus hindering our understanding of their intrinsic properties and dominant physical processes responsible for their generation, evolution,radiation and particle emission. In this work we focus on extracting information about timescales in the jet rest frame using a population-modeling approach. We employ Monte Carlo simulations to derive a simple population model for the intrinsic unbeamed luminosities and the Lorentz $Γ$ of blazar jets that adequately describe the observed redshift and apparent superluminal speed distributions for flux-limited blazar samples. We derive separate models for BL Lacs and Flat Spectrum Radio Quasars. We then use these models to compute the predicted distribution of Doppler factors in each blazar class, and address the following questions: (a) What is the relativistically induced spread in observed timescales (e.g., event duration, time lags) in a flux-limited sample of relativistic jets? (b) Could differences between BL Lacs and FSRQs observed in the time domain be attributed to differences in beaming between the two populations? (c) Is there a statistically preferred amount of beaming in a flux-limited sample? How large are statistical deviations from that preferred value? We use our findings to propose promising approaches in phenomenological studies of timescales in blazar jets.

astro-ph.HE↗

arXiv2014

Jet bending and velocity profiles

In this study, we consider the problem of geometry of curved jets. We work with a simplified two-dimensional kinematical model assuming that the jet velocity is varying. We prescribe the velocity variation and study two cases concerning parsec and kilo-parsec scale jets by applying some simplified numerical simulations. For parsec scale jets, we use some general expressions and derive the shape of jets for certain numerical cases. For kilo-parsec scale jets, we study the cases of 4C53.37 and 1159+583 of Abell 2220 and Abell 1446 galaxy clusters, respectively, in an effort to test our model and compare our numerical results with observational data.

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