41 research outputs found
Opacity, variability and kinematics of AGN jets
Synchrotron self-absorption in active galactic nuclei (AGN) jets manifests
itself as a time delay between flares observed at high and low radio
frequencies. It is also responsible for the observing frequency dependent
change in size and position of the apparent base of the jet, aka the core shift
effect, detected with very long baseline interferometry (VLBI). We measure the
time delays and the core shifts in 11 radio-loud AGN to estimate the speed of
their jets without relying on multi-epoch VLBI kinematics analysis. The 158
GHz total flux density time lags are obtained using Gaussian process
regression, the core shift values are measured using VLBI observations and
adopted from the literature. A strong correlation is found between the apparent
core shift and the observed time delay. Our estimate of the jet speed is higher
than the apparent speed of the fastest VLBI components by the median
coefficient of 1.4. The coefficient ranges for individual sources from 0.5 to
20. We derive Doppler factors, Lorentz factors and viewing angles of the jets,
as well as the corresponding de-projected distance from the jet base to the
core. The results support evidence for acceleration of the jets with bulk
motion Lorentz factor on de-projected scales
of 0.5500 parsecs.Comment: Accepted by MNRAS; 11 pages, 11 figures, 3 table
Effelsberg Monitoring of a Sample of RadioAstron Blazars: Analysis of Intra-Day Variability
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. -ray loud sources were found to vary by
up to a factor 4 more than -ray quiet ones, with 4
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 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.Comment: 23 pages, 9 figures, published online by MDPI Galaxie