303 research outputs found
Magnetic field structure of the extended 3C 380 jet
An earlier study of the complex jet of 3C 380 by Papageorgiou et al. revealed total intensity and polarization structure associated with a bright knot K1 about 0.7 arcsec from the core that was reminiscent of that expected for a conical shock wave. In this new study, 1.42, 1.66 and 4.99 GHz total intensity, polarization and Faraday rotation images are presented and analysed. These images were derived from observations with the Very Long Baseline Array plus one antenna of the Very Large Array, obtained in 2006 March. These new images confirm the overall magnetic field structure of the knot K1 indicated in the earlier observations. In addition, a clear Faraday rotation gradient has been detected across the jet, extending roughly from 10 to 30 mas (70–200 pc) along the jet from the core (a radial distance of approximately two beamwidths). The gradient spans roughly 3.5 beamwidths in the transverse direction, and the difference in the rotation measures on either side of the jet is 4–5σ, demonstrating that the detection of the gradient is firm. We interpret this transverse Faraday rotation gradient as reflecting systematic variation of the line-of-sight component of a helical or toroidal magnetic field (B) associated with the jet of 3C 380. These results provide evidence that the helical field arising due to the joint action of the rotation of the central black hole and its accretion disc and the jet outflow can survive to distances of hundreds of parsecs from the central engine
The parsec-scale radio jet of BL Lac
Polarization-sensitive lambda= 6 cm global VLBI images of BL Lac for data taken at three epochs are presented and analysed, together with an additional epoch for which only total intensity data are available. Previous lambda= 6 cm and lambda= 3.6 cm VLBI polarization observations of BL Lac have shown that the jet components in this source tend to have magnetic field transverse to the jet direction. On the whole, these images confirm this tendency, though there is evidence for possibly oblique or longitudinal magnetic fields in some features at some epochs. The flux densities of individual features moving from the core can initially drop rapidly, but the rate of flux decrease slows with time, so that many features are, in fact, remarkably long-lived. This has made it possible to trace the movements of some components over more than a decade. The trajectories of individual components show oscillations whose amplitude appears to grow with distance from the VLBI core
Analysing the Transverse Structure of the Relativistic Jets of AGN
This paper describes a method of fitting total intensity and polarization profiles in VLBI images of astrophysical jets to profiles predicted by a theoretical model. As an example, the method is used to fit profiles of the jet in the Active Galactic Nucleus Mrk501 with profiles predicted by a model in which a cylindrical jet of synchrotron plasma is threaded by a magnetic field with helical and disordered components. This fitting yields model Stokes Q profiles that agree with the observed profiles to within the 1-2 \sigma uncertainties; the I model and observed profiles are overall not in such good agreement, with the model I profiles being generally more symmetrical than the observed profiles. Consistent fitting results are obtained for profiles derived from 6cm VLBI images at two distances from the core, and also for profiles obtained for different wavelengths at a single location in the VLBI jet. The most striking success of the model is its ability to reproduce the spine-sheath polarization structure observed across the jet. Using the derived viewing angle in the jet rest frame, \delta' approximately 83 degrees, together with a superluminal speed reported in the literature, \beta apparent = 3.3, yields a solution for the viewing angle and velocity of the jet in the observer's frame \delta degrees and \beta approximately 0.96. Although these results for Mrk501 must be considered tentative, the combined analysis of polarization profiles and apparent component speeds holds promise as a means of further elucidating the magnetic field structures and other parameters of parsec-scale AGN jets
The Gaussian Plasma Lens in Astrophysics. Refraction
We consider the geometrical optics for refraction of a distant radio source
by an interstellar plasma lens, with application to a lens with a Gaussian
electron column density profile. The refractive properties of the lens are
specified completely by a dimensionless parameter, alpha, which is a function
of the wavelength of observation, the lens' electron column density, the
lens-observer distance, and the transverse diameter of the lens. Relative
motion of the observer and lens produces modulations in the source's light
curve. Plasma lenses are diverging so the light curve displays a minimum, when
the lens is on-axis, surrounded by enhancements above the unlensed flux
density. Lensing can also produce caustics, multiple imaging, and angular
position wander of the background source. If caustics are formed, the
separation of the outer caustics can constrain alpha, while the separation of
the inner caustics can constrain the size of the lens. We apply our analysis to
0954+654, a source for which we can identify caustics in its light curve, and
1741-038, for which polarization observations were obtained during and after
the scattering event. We find general agreement between modelled and observed
light curves at 2.25 GHz, but poor agreement at 8.1 GHz. The discrepancies may
result from a combination of lens substructure or anisotropic shape, a lens
that only grazes the source, or unresolved source substructure. Our analysis
places the following constraints on the lenses: Toward 0954+654 (1741-038) the
lens was 0.38 AU (0.065 AU) in diameter, with a peak column density of 0.24 pc
cm^{-3} (1E-4 pc cm^{-3}) and an electron density of 1E5 cm^{-3} (300 cm^{-3}).
The angular wander caused by the lens was 250 mas (0.4 mas) at 2.25 GHz. For
1741-038, we place an upper limit of 100 mG on the lens' magnetic field.Comment: 26 pages, LaTeX2e using AASTeX macro aaspp4, 11 PostScript figures;
to be published in Ap
Space very long baseline interferometry observations of polarization in the jet of 3C 380
A comparison between low-frequency space very long baseline interferometry (VLBI) and high-frequency ground-based VLBI images can, in principle, be used to detect small variations in rotation measure (RM) on fine angular scales inaccessible to ground arrays alone. This paper reports an attempt to perform such a comparison using the jet in the quasar 3C 380. Observations made with the VSOP antenna HALCA together with a ground array at wavelength 1.6 GHz provide total intensity and polarization images of comparable resolution to those from the ground array alone at 5 GHz. The results provide an image showing derotated magnetic vector position angle of somewhat higher resolution than that available earlier. The results show variations in an RM around component A of the order of 10 rad m(-2) that could not have been detected with the ground array alone. It is concluded that satellite VLBI observations provide a promising means to study the distribution of matter and magnetic fields around parsec-scale jets. The ground observations used here follow the steady outward drift of component A, which has approximately doubled its distance from the core since the first observations in 1982. They also reveal total intensity and polarization structure associated with a bright knot 0.7 arcsec from the core which is reminiscent of that expected for a conical shock wave
The parsec-scale jet of PKS 1749+096
PKS 1749+096 is a BL Lac object showing weak extended jet emission to the
northeast of the compact VLBI core on parsec scales. We aim at better
understanding the jet kinematics and variability of this source and finding
clues that may applicable to other BL Lac objects. The jet was studied with
multi-epoch multi-frequency high-resolution VLBI observations. The jet is
characterized by a one-sided curved morphology at all epochs and all
frequencies. The VLBI core, located at the southern end of the jet, was
identified based on its spectral properties. The equipartition magnetic field
of the core was investigated, through which we derived a Doppler factor of 5,
largely consistent with that derived from kinematics (component C5). The study
of the detailed jet kinematics at 22 and 15 GHz, spanning a period of more than
10 years, indicates the possible existence of a bimodal distribution of the jet
apparent speed. Ballistic and non-ballistic components are found to coexist in
the jet. Superluminal motions in the range of 5-21 c were measured in 11
distinct components. We estimated the physical jet parameters with the minimum
Lorentz factor of 10.2 and Doppler factors in the range of 10.2-20.4 (component
C5). The coincidence in time of the component's ejection and flares supports
the idea that, at least in PKS 1749+096, ejection of new jet components is
connected with major outbursts in flux density. For the best-traced component
(C5) we found that the flux density decays rapidly as it travels downstream the
jet, accompanied by a steepening of its spectra, which argues in favor of a
contribution of inverse Compton cooling. These properties make PKS 1749+096 a
suitable target for an intensive monitoring to decipher the variability
phenomenon of BL Lac objects.Comment: Replaced to match published versio
The kinematics in the pc-scale jets of AGN The case of S5 1803+784
We present a kinematic analysis of jet component motion in the VLBI jet of
the BL Lac object S5 1803+784, which does not reveal long-term outward motion
for most of the components. Understanding the complex kinematic phenomena can
possibly provide insights into the differences between quasars and BL Lac
objects. The blazar S5 1803+784 has been studied with VLBI at =1.6, 2.3,
5, 8.4, and 15 GHz between 1993.88 and 2005.68 in 26 observing runs. We
(re)analyzed the data and present Gaussian model-fits. We collected the already
published kinematic information for this source from the literature and
re-identified the components according to the new scenario presented in this
paper. Altogether, 94 epochs of observations have been investigated. A careful
study of the long-term kinematics reveals a new picture for component motion in
S5 1803+784. In contrast to previously discussed motion scenarios, we find that
the jet structure within 12 mas of the core can most easily be described by the
coexistence of several bright jet features that remain on the long-term at
roughly constant core separations (in addition to the already known {\it
stationary} jet component 1.4 mas) and one faint component moving with
an apparent superluminal speed ( 19c, based on 3 epochs). While most of
the components maintain long-term roughly constant distances from the core, we
observe significant, smooth changes in their position angles. We report on an
evolution of the whole jet ridge line with time over the almost 12 years of
observations. The width of the jet changes periodically with a period of
8 to 9 years. We find a correlation between changes in the position angle and
maxima in the total flux-density. We present evidence for a geometric origin of
the phenomena and discuss possible models.Comment: The manuscript will be published by A&
- …