Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Significant core shift variability in parsec-scale jets of active
  galactic nuclei   The apparent position of jet base (core) in radio-loud active galactic nuclei
changes with frequency because of synchrotron self-absorption. Studying this
`core shift` effect enables us to reconstruct properties of the jet regions
close to the central engine. We report here results from core shift
measurements in AGNs observed with global VLBI at 2 and 8 GHz at epochs from
1994 to 2016. Our sample contains 40 objects observed at least 10 times during
that period. The core shift is determined using a new automatic procedure
introduced to minimize possible biases. The resulting multiple epoch
measurements of the core position are employed for examining temporal
variability of the core shift. We argue that the core shift variability is a
common phenomenon, as established for 33 of 40 AGNs we study. Our analysis
shows that the typical offsets between the core positions at 2 and 8 GHz are
about 0.5 mas and they vary in time. Typical variability of the individual core
positions is about 0.3 mas. The measurements show a strong dependence between
the core position and its flux density, suggesting that changes in both are
likely related to the nuclear flares injecting denser plasma into the flow. We
determine that density of emitting relativistic particles significantly
increases during these flares, while relative magnetic field changes less and
in the opposite direction.

12
The categories of this paper are: nucl-th
31
The categories of this paper are: ao-sci
0
The categories of this paper are: hep-ph
36
The categories of this paper are: cmp-lg
34
The categories of this paper are: chem-ph
17
The categories of this paper are: q-fin
3
The categories of this paper are: physics
20
The categories of this paper are: acc-phys
19
The categories of this paper are: eess
11
The categories of this paper are: hep-lat
25
The categories of this paper are: patt-sol
23
The categories of this paper are: solv-int
37
The categories of this paper are: supr-con
15
The categories of this paper are: nucl-ex
2
The categories of this paper are: math
33
The categories of this paper are: plasm-ph