TY - JOUR
T1 - Catching the radio flare in CTA 102
T2 - III. Core-shift and spectral analysis
AU - Fromm, C. M.
AU - Ros, E.
AU - Perucho, M.
AU - Savolainen, T.
AU - Mimica, P.
AU - Kadler, M.
AU - Lobanov, A. P.
AU - Zensus, J. A.
PY - 2013
Y1 - 2013
N2 - Context. The temporal and spatial spectral evolution of the jets of
active galactic nuclei (AGN) can be studied with multi-frequency,
multi-epoch very-long-baseline-interferometry (VLBI) observations. The
combination of both morphological (kinematical) and spectral parameters
can be used to derive source-intrinsic physical properties, such as the
magnetic field and the nonthermal particle density. Such a study is of
special interest during the high states of activity in AGNs, since VLBI
observations can provide estimates of the location of the flaring site.
Furthermore, we can trace the temporal variations in the
source-intrinsic parameters during the flare, which may reflect the
interaction between the underlying plasma and a traveling shock wave.
The source CTA 102 exhibited such a radio flare around 2006.
Aims: In the first two papers of this series (Papers I and II), we
analyzed the single-dish light curves and the VLBI kinematics of the
blazar CTA 102 and suggested a shock-shock interaction between a
traveling and a standing shock wave as a possible scenario to explain
the observed evolution of the component associated to the 2006 flare. In
this paper we investigate the core shift and spectral evolution to test
our hypothesis of a shock-shock interaction. Methods: We used
eight multi-frequency Very Long Baseline Array (VLBA) observations to
analyze the temporal and spatial evolution of the spectral parameters
during the flare. We observed CTA 102 between May 2005 and April 2007
using the VLBA at six different frequencies spanning from 2 GHz up to 86
GHz. After the calibrated VLBA images were corrected for opacity, we
performed a detailed spectral analysis. We developed methods for
aligning the images and extracting the uncertainties in the spectral
parameters. From the derived values we estimated the magnetic field and
the density of the relativistic particles and combined those values with
the kinematical changes provided from the long-term VLBA monitoring
(Paper II) and single-dish measurements (Paper I). Results: The
detailed analysis of the opacity shift reveals that the position of the
jet core is proportional to ν-1 with some temporal
variations. The value suggests possible equipartition between magnetic
field energy and particle kinetic energy densities at the most compact
regions. From the variation in the physical parameters we deduced that
the 2006 flare in CTA 102 is connected to the ejection of a new
traveling feature (tej = 2005.9) and to the interaction
between this shock wave and a stationary structure (interpreted as a
recollimation shock) around 0.1 mas from the core (de-projected 18 pc at
a viewing angle of ϑ = 2.6°). The source kinematics, together
with the spectral and structural variations, can be described by helical
motions in an overpressured jet.
Appendices are available in electronic form at http://www.aanda.org
AB - Context. The temporal and spatial spectral evolution of the jets of
active galactic nuclei (AGN) can be studied with multi-frequency,
multi-epoch very-long-baseline-interferometry (VLBI) observations. The
combination of both morphological (kinematical) and spectral parameters
can be used to derive source-intrinsic physical properties, such as the
magnetic field and the nonthermal particle density. Such a study is of
special interest during the high states of activity in AGNs, since VLBI
observations can provide estimates of the location of the flaring site.
Furthermore, we can trace the temporal variations in the
source-intrinsic parameters during the flare, which may reflect the
interaction between the underlying plasma and a traveling shock wave.
The source CTA 102 exhibited such a radio flare around 2006.
Aims: In the first two papers of this series (Papers I and II), we
analyzed the single-dish light curves and the VLBI kinematics of the
blazar CTA 102 and suggested a shock-shock interaction between a
traveling and a standing shock wave as a possible scenario to explain
the observed evolution of the component associated to the 2006 flare. In
this paper we investigate the core shift and spectral evolution to test
our hypothesis of a shock-shock interaction. Methods: We used
eight multi-frequency Very Long Baseline Array (VLBA) observations to
analyze the temporal and spatial evolution of the spectral parameters
during the flare. We observed CTA 102 between May 2005 and April 2007
using the VLBA at six different frequencies spanning from 2 GHz up to 86
GHz. After the calibrated VLBA images were corrected for opacity, we
performed a detailed spectral analysis. We developed methods for
aligning the images and extracting the uncertainties in the spectral
parameters. From the derived values we estimated the magnetic field and
the density of the relativistic particles and combined those values with
the kinematical changes provided from the long-term VLBA monitoring
(Paper II) and single-dish measurements (Paper I). Results: The
detailed analysis of the opacity shift reveals that the position of the
jet core is proportional to ν-1 with some temporal
variations. The value suggests possible equipartition between magnetic
field energy and particle kinetic energy densities at the most compact
regions. From the variation in the physical parameters we deduced that
the 2006 flare in CTA 102 is connected to the ejection of a new
traveling feature (tej = 2005.9) and to the interaction
between this shock wave and a stationary structure (interpreted as a
recollimation shock) around 0.1 mas from the core (de-projected 18 pc at
a viewing angle of ϑ = 2.6°). The source kinematics, together
with the spectral and structural variations, can be described by helical
motions in an overpressured jet.
Appendices are available in electronic form at http://www.aanda.org
KW - galaxies: active
KW - radiation mechanisms: non-thermal
KW - galaxies: clusters: individual: CTA 102
KW - galaxies: jets
KW - radio continuum: galaxies
U2 - 10.1051/0004-6361/201321784
DO - 10.1051/0004-6361/201321784
M3 - Article
SN - 0004-6361
VL - 557
JO - Astronomy & Astrophysics
JF - Astronomy & Astrophysics
M1 - A105
ER -