344 research outputs found
Radio observations of active galactic nuclei with mm-VLBI
Over the past few decades, our knowledge of jets produced by active galactic
nuclei (AGN) has greatly progressed thanks to the development of
very-long-baseline interferometry (VLBI). Nevertheless, the crucial mechanisms
involved in the formation of the plasma flow, as well as those driving its
exceptional radiative output up to TeV energies, remain to be clarified. Most
likely, these physical processes take place at short separations from the
supermassive black hole, on scales which are inaccessible to VLBI observations
at centimeter wavelengths. Due to their high synchrotron opacity, the dense and
highly magnetized regions in the vicinity of the central engine can only be
penetrated when observing at shorter wavelengths, in the millimeter and
sub-millimeter regimes. While this was recognized already in the early days of
VLBI, it was not until the very recent years that sensitive VLBI imaging at
high frequencies has become possible. Ongoing technical development and wide
band observing now provide adequate imaging fidelity to carry out more detailed
analyses.
In this article we overview some open questions concerning the physics of AGN
jets, and we discuss the impact of mm-VLBI studies. Among the rich set of
results produced so far in this frequency regime, we particularly focus on
studies performed at 43 GHz (7 mm) and at 86 GHz (3 mm). Some of the first
findings at 230 GHz (1 mm) obtained with the Event Horizon Telescope are also
presented.Comment: Published in The Astronomy & Astrophysics Review. Open access:
https://link.springer.com/article/10.1007/s00159-017-0105-
S5 0716+714 : GeV variability study
The GeV observations by Fermi-LAT give us the opportunity to characterize the
high-energy emission (100 MeV - 300 GeV) variability properties of the BL Lac
object S5 0716+714. In this study, we performed flux and spectral analysis of
more than 3 year long (August 2008 to April 2012) Fermi-LAT data of the source.
During this period, the source exhibits two different modes of flux variability
with characteristic timescales of ~75 and ~140 days, respectively. We also
notice that the flux variations are characterized by a weak spectral hardening.
The GeV spectrum of the source shows a clear deviation from a simple power law,
and is better explained by a broken power law. Similar to other bright Fermi
blazars, the break energy does not vary with the source flux during the
different activity states. We discuss several possible scenarios to explain the
observed spectral break.Comment: 21 pages, 10 figures, Accepted for publication in Advances in Space
Research journa
Connection between inner jet kinematics and broadband flux variability in the BL Lac object S5 0716+714
We present a high-frequency very long baseline interferometry (VLBI)
kinematical study of the BL Lac object S5 0716+714 over the time period of
September 2008 to October 2010. The aim of the study is to investigate the
relation of the jet kinematics to the observed broadband flux variability. We
find significant non-radial motions in the jet outflow of the source. In the
radial direction, the highest measured apparent speed is \sim37 c, which is
exceptionally high, especially for a BL Lac object. Patterns in the jet flow
reveal a roughly stationary feature \sim0.15 mas downstream of the core. The
long-term fits to the component trajectories reveal acceleration in the sub-mas
region of the jet. The measured brightness temperature, T_{B}, follows a
continuous trend of decline with distance, T_B \propto
r_{jet}^{-(2.36\pm0.41)}, which suggests a gradient in Doppler factor along the
jet axis. Our analysis suggest that a moving disturbance (or a shock wave) from
the base of the jet produces the high-energy (optical to \gamma-ray) variations
upstream of the 7 mm core, and then later causes an outburst in the core.
Repetitive optical/\gamma-ray flares and the curved trajectories of the
associated components suggest that the shock front propagates along a bent
trajectory or helical path. Sharper \gamma-ray flares could be related to the
passage of moving disturbances through the stationary feature. Our analysis
suggests that the \gamma-ray and radio emission regions have different Doppler
factors.Comment: 12 pages, 8 figures, accepted for publication in A&
The magnetic field structure in CTA 102 from high-resolution mm-VLBI observations during the flaring state in 2016-2017
CONTEXT: Investigating the magnetic field structure in the innermost regions of relativistic jets is fundamental to understanding the crucial physical processes giving rise to jet formation, as well as to their extraordinary radiation output up to γ-ray energies.
AIMS: We study the magnetic field structure of the quasar CTA 102 with 3 and 7 mm VLBI polarimetric observations, reaching an unprecedented resolution (∼50 μas). We also investigate the variability and physical processes occurring in the source during the observing period, which coincides with a very active state of the source over the entire electromagnetic spectrum.
METHODS: We perform the Faraday rotation analysis using 3 and 7 mm data and we compare the obtained rotation measure (RM) map with the polarization evolution in 7 mm VLBA images. We study the kinematics and variability at 7 mm and infer the physical parameters associated with variability. From the analysis of γ-ray and X-ray data, we compute a minimum Doppler factor value required to explain the observed high-energy emission.
RESULTS: Faraday rotation analysis shows a gradient in RM with a maximum value of ∼6 × 104⁴ rad m⁻² and intrinsic electric vector position angles (EVPAs) oriented around the centroid of the core, suggesting the presence of large-scale helical magnetic fields. Such a magnetic field structure is also visible in 7 mm images when a new superluminal component is crossing the core region. The 7 mm EVPA orientation is different when the component is exiting the core or crossing a stationary feature at ∼0.1 mas. The interaction between the superluminal component and a recollimation shock at ∼0.1 mas could have triggered the multi-wavelength flares. The variability Doppler factor associated with such an interaction is large enough to explain the high-energy emission and the remarkable optical flare occurred very close in time.Accepted manuscrip
Evidence for a large-scale helical magnetic field in the quasar 3C 454.3
Most current theoretical models link the launching of relativistic jets from active galactic nuclei to the presence of twisted magnetic fields close to the supermassive black hole. While these models predict a large-scale, ordered, helical magnetic field near the central engine, it is not clear if, and to what extent, this order is preserved further downstream in the jet. Here, we present compelling evidence that suggests that the radio emission from the jet of the quasar 3C 454.3 exhibits multiple signatures of a large-scale, ordered helical magnetic field component at a distance of hundreds of parsecs from the launching point. Our results provide observational support for magnetic jet launching models and indicate that the ordered helical field component may remain stable over a large distance down the jet
86 GHz polarimetry of OVV1633+382 after a major mm flare
The 18 mag QSO 1633+382 (4C38.41, z=1.807) showed a very pronounced outburst
in 2001/2002. With a peak amplitude of more than 9 Jy at 90GHz, this flare was
brighter than any known previous flare in this source (data available since
1980).During onset, the mm-flare was particulary fast, with an increase of more
than 2 Jy at 230 GHz in less than 8 days. Since January 2002, the mm-flux of
1633+382 is decaying. During this decline, however, local flux variations with
amplitudes of 1-3 Jy were seen, indicative of underlying and more rapid source
activity on time scales of 1-2 months. After the main peak occurring in
2001.99, the 90 GHz flux showed secondary maxima at approximately half year
intervals in 2002.3, 2002.7 and 2003.13. This kind of periodicity might be
explained via the lighthouse model (Camenzind and Krockenberger 1992), which is
based on the magnetic accelerator of Blanford and Payne (1982). At present the
millimeter flux is nearly back to its quiescent level of 2-2.5 Jy, which the
source had before the flare began. Our VLBA Polarimetry monitoring started June
2002 during the onset of the flare. At cm wavelength, the flare is only
marginally detected which implies very high opacity of the source.Comment: 4 pages. 5 figures. Proceedings of the 7th European VLBI Network
Symposium held in Toledo, Spain on October 12-15, 2004. Editors: R.
Bachiller, F. Colomer, J.-F. Desmurs, P. de Vicente (Observatorio Astronomico
Nacional), p. 85-88. Needs evn2004.cl
Jet outflow and gamma-ray emission correlations in S5 0716+714
Using millimeter-very long baseline interferometry (VLBI) observations of the
BL Lac object S5 0716+714 from August 2008 to September 2013, we investigate
variations in the core flux density and orientation of the sub-parsec scale jet
i.e. position angle. The gamma-ray data obtained by the Fermi-LAT (Large Area
Telescope) are used to investigate the high-energy flux variations over the
same time period. For the first time in any blazar, we report a significant
correlation between the gamma-ray flux variations and the position angle (PA)
variations in the VLBI jet. The cross-correlation analysis also indicates a
positive correlation such that the mm-VLBI core flux density variations are
delayed with respect to the gamma-ray flux by 8232 days. This suggests
that the high-energy emission is coming from a region located
(3.81.9) parsecs upstream of the mm-VLBI core (closer to the central
black hole). These results imply that the observed inner jet morphology has a
strong connection with the observed gamma-ray flares.Comment: 5 pages, 6 figures, accepted for publication in A&A Letter
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