Effelsberg Monitoring of a Sample of RadioAstron Blazars: Analysis of Intra-Day Variability

Jun Liu,Emmanouil Angelakis,J Zensus,Xiang Liu,Hayley Bignall,Thomas Krichbaum,Kirill Sokolovsky,Yuri Kovalev,Alex Kraus

doi:10.3390/galaxies6020049

Abstract

We present the first results of an ongoing intra-day variability (IDV) flux density monitoring program of 107 blazars, which were selected from a sample of RadioAstron space very long baseline interferometry (VLBI) targets. The~IDV observations were performed with the Effelsberg 100-m radio telescope at 4.8\,GHz, focusing on the statistical properties of IDV in a relatively large sample of compact active galactic nuclei (AGN). We investigated the dependence of rapid ($<$3 day) variability on various source properties through a likelihood approach. We found that the IDV amplitude depends on flux density and that fainter sources vary by about a factor of 3 more than their brighter counterparts. We also found a significant difference in the variability amplitude between inverted- and flat-spectrum radio sources, with the former exhibiting stronger variations. $\gamma$-ray loud sources were found to vary by up to a factor 4 more than $\gamma$-ray quiet ones, with 4$\sigma$ significance. However a galactic latitude dependence was barely observed, which suggests that it is predominantly the intrinsic properties (e.g., angular size, core-dominance) of the blazars that determine how they scintillate, rather than the directional dependence in the interstellar medium (ISM). We showed that the uncertainty in the VLBI brightness temperatures obtained from the space VLBI data of the RadioAstron satellite can be as high as $\sim$70\% due to the presence of the rapid flux density variations. Our statistical results support the view that IDV at centimeter wavelengths is predominantly caused by interstellar scintillation (ISS) of the emission from the most compact, core-dominant region in an AGN.

Highlights

RadioAstron is an international collaborative mission with a 10-m radio telescope onboard the SPEKTR-R spacecraft launched in July 2011 [1,2,3]
We presented statistical results based on five observing sessions of an intra-day variability (IDV) monitoring program with the Effelsberg 100-m radio telescope at 4.8 GHz
We found significant differences in the strength of IDV, dependent on the source flux density, spectral index and γ-ray loudness

Summary

Introduction

RadioAstron is an international collaborative mission with a 10-m radio telescope onboard the SPEKTR-R spacecraft launched in July 2011 [1,2,3]. The space telescope observes at wavelengths of 92 cm (324 MHz, P-band), 18 cm (1.7 GHz, L-band), 6 cm (4.8 GHz, C-band), and 1.3 cm (22.2 GHz, K-band), forming space very long baseline interferometry (space VLBI, or SVLBI) together with ground-based radio telescopes, with the highest angular resolution achievable (e.g., ~7 μ as at the K-band) so far. Coordinated ground-based flux density monitoring of the RadioAstron targets at centimeter wavelengths is essential to estimate the effect of interstellar scintillation (ISS) on the SVLBI visibilities. In order to measure the magnitudes and timescales of ISS of blazars observed by RadioAstron, in 2014 we started a RadioAstron target-triggered flux monitoring program with the Effelsberg 100-m radio telescope at 4.8 GHz. The single dish ISS monitoring and space VLBI with RadioAstron offer independent probes of the structure of blazar ‘cores’ at microarcsecond angular scales

Results

Discussion

Conclusion