251 research outputs found
Catching the Radio Flare in CTA 102 III. Core-Shift and Spectral Analysis
The temporal and spatial spectral evolution of the jets of AGN can be studied
with multi-frequency, multi-epoch VLBI observations. The combination of both,
morphological and spectral parameters can be used to derive source intrinsic
physical properties such as the magnetic field and the non-thermal particle
density. In the first two papers of this series, 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. We used 8
multi-frequency 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
- 86 GHz. After the calibrated VLBA images were corrected for opacity, we
performed a detailed spectral analysis. From the derived values we estimated
the magnetic field and the density of the relativistic particles. The detailed
analysis of the opacity shift reveals that the position of the jet core is
proportional to nu^-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 of the
physical parameters we deduced that the 2006 flare in CTA 102 is connected to
the ejection of a new traveling feature (t=2005.9) and the interaction between
this shock wave and a stationary structure around 0.1 mas from the core. The
source kinematics together with the spectral and structural variations can be
described by helical motions in an over-pressured jet.Comment: 35 pages, 46 figure
On the phenomenological classification of continuum radio spectra variability patterns of Fermi blazars
The F-GAMMA program is a coordinated effort to investigate the physics of
Active Galactic Nuclei (AGNs) via multi-frequency monitoring of {\em Fermi}
blazars. The current study is concerned with the broad-band radio spectra
composed of measurement at ten frequencies between 2.64 and 142 GHz. It is
shown that any of the 78 sources studied can be classified in terms of their
variability characteristics in merely 5 types of variability. The first four
types are dominated by spectral evolution and can be reproduced by a simple
two-component system made of the quiescent spectrum of a large scale jet
populated with a flaring event evolving according to Marscher & Gear (1985).
The last type is characterized by an achromatic change of the broad-band
spectrum which must be attributed to a completely different mechanism. Here are
presented, the classification, the assumed physical system and the results of
simulations that have been conducted.Comment: 2011 Fermi Symposium proceedings - eConf C11050
Catching the radio flare in CTA 102 I. Light curve analysis
Context: The blazar CTA 102 (z=1.037) underwent a historical radio outburst
in April 2006. This event offered a unique chance to study the physical
properties of the jet. Aims: We used multifrequency radio and mm observations
to analyze the evolution of the spectral parameters during the flare as a test
of the shock-in-jet model under these extreme conditions. Methods: For the
analysis of the flare we took into account that the flaring spectrum is
superimposed on a quiescent spectrum. We reconstructed the latter from archival
data and fitted a synchrotron self-absorbed distribution of emission. The
uncertainties of the derived spectral parameters were calculated using Monte
Carlo simulations. The spectral evolution is modeled by the shock-in-jet model,
and the derived results are discussed in the context of a geometrical model
(varying viewing angle) and shock-shock interaction. Results: The evolution of
the flare in the turnover frequency-turnover flux density plane shows a double
peak structure. The nature of this evolution is dicussed in the frame of
shock-in-jet models. We discard the generation of the double peak structure in
the turnover frequency-turnover flux density plane purely based on geometrical
changes (variation of the Doppler factor). The detailed modeling of the
spectral evolution favors a shock-shock interaction as a possible physical
mechanism behind the deviations from the standard shock-in-jet model.Comment: 15 pages, 12 figure
Sub-milliarcsecond imaging of a bright flare and ejection event in the extragalactic jet 3C 111
Flares in radio-loud AGN are thought to be associated with the injection of
fresh plasma into the compact jet base. Such flares are usually strongest and
appear earlier at shorter radio wavelengths. Hence, VLBI at mm-wavelengths is
best suited to study the earliest structural changes of compact jets associated
with emission flares. We study the morphological changes of the parsec-scale
jet in the nearby (z=0.049) gamma-ray bright radio galaxy 3C111 following a
flare that developed into a major radio outburst in 2007. We analyse three
successive observations of 3C111 at 86 GHz with the Global mm-VLBI Array (GMVA)
between 2007 and 2008 which yield a very high angular resolution of ~45muas. In
addition, we make use of single-dish radio flux density measurements from the
F-GAMMA and POLAMI programmes, archival single-dish and VLBI data. We resolve
the flare into multiple plasma components with a distinct morphology resembling
a bend in an otherwise remarkably straight jet. The flare-associated features
move with apparent velocities of ~4.0c to ~4.5c and can be traced also at lower
frequencies in later epochs. Near the base of the jet, we find two bright
features with high brightness temperatures up to ~10^11K, which we associate
with the core and a stationary feature in the jet. The flare led to multiple
new jet components indicative of a dynamic modulation during the ejection. We
interpret the bend-like feature as a direct result of the outburst which makes
it possible to trace the transverse structure of the jet. In this scenario, the
components follow different paths in the jet stream consistent with
expectations for a spine-sheath structure, which is not seen during
intermediate levels of activity. The possibility of coordinated multiwavelength
observations during a future bright radio flare in 3C111 makes this source an
excellent target for probing the radio-gamma-ray connection.Comment: 11 pages, 7 figures, accepted for publication by Astronomy &
Astrophysic
Sub-milliarcsecond imaging of a bright flare and ejection event in the extragalactic jet 3C 111
ContextFlares in radio-loud active galactic nuclei are thought to be associated with the injection of fresh plasma into the compact jet base. Such flares are usually strongest and appear earlier at shorter radio wavelengths. Hence, very long baseline interferometry (VLBI) at millimeter(mm)-wavelengths is the best-suited technique for studying the earliest structural changes of compact jets associated with emission flares. AimsWe study the morphological changes of the parsec-scale jet in the nearby (z = 0.049) γ-ray bright radio galaxy 3C 111 following a flare that developed into a major radio outburst in 2007. Methods\ua0We analyse three successive observations of 3C 111 at 86 GHz with the Global mm-VLBI Array (GMVA) between 2007 and 2008 which yield a very high angular resolution of ∼45  μas. In addition, we make use of single-dish radio flux density measurements from the F-GAMMA and POLAMI programmes, archival single-dish and VLBI data. ResultsWe resolve the flare into multiple plasma components with a distinct morphology resembling a bend in an otherwise remarkably straight jet. The flare-associated features move with apparent velocities of ∼4.0c to ∼4.5c and can be traced also at lower frequencies in later epochs. Near the base of the jet, we find two bright features with high brightness temperatures up to ∼1011 K, which we associate with the core and a stationary feature in the jet. ConclusionsThe flare led to multiple new jet components indicative of a dynamic modulation during the ejection. We interpret the bend-like feature as a direct result of the outburst which makes it possible to trace the transverse structure of the jet. In this scenario, the components follow different paths in the jet stream consistent with expectations for a spine-sheath structure, which is not seen during intermediate levels of activity. The possibility of coordinated multiwavelength observations during a future bright radio flare in 3C 111 makes this source an excellent target for probing the radio-γ-ray connection
F-GAMMA: On the phenomenological classification of continuum radio spectra variability patterns of Fermi blazars
The F-GAMMA program is a coordinated effort to investigate the physics of
Active Galactic Nuclei (AGNs) via multi-frequency monitoring of Fermi blazars.
In the current study we show and discuss the evolution of broad-band radio
spectra, which are measured at ten frequencies between 2.64 and 142 GHz using
the Effelsberg 100-m and the IRAM 30-m telescopes. It is shown that any of the
78 sources studied can be classified in terms of their variability
characteristics in merely 5 types of variability. It is argued that these can
be attributed to only two classes of variability mechanisms. The first four
types are dominated by spectral evolution and can be described by a simple
two-component system composed of: (a) a steep quiescent spectral component from
a large scale jet and (b) a time evolving flare component following the
"Shock-in-Jet" evolutionary path. The fifth type is characterised by an
achromatic change of the broad band spectrum, which could be attributed to a
different mechanism, likely involving differential Doppler boosting caused by
geometrical effects. Here we present the classification, the assumed physical
scenario and the results of calculations that have been performed for the
spectral evolution of flares.Comment: Proceedings of the conference: "The Central Kiloparsec in Galactic
Nucleic: Astronomy at High Angular Resolution 2011", August 29 - September 2,
2011, Bad Honnef, German
- …