424 research outputs found
Surprising Evolution of the Parsec-scale Faraday Rotation Gradients in the Jet of the BL Lac Object B1803+784
Several multi-frequency polarization studies have shown the presence of
systematic Faraday Rotation gradients across the parsec-scale jets of Active
Galactic Nuclei (AGN), taken to be due to the systematic variation of the
line-of-sight component of a helical magnetic (B) field across the jet. Other
studies have confirmed the presence and sense of these gradients in several
sources, thus providing evidence that these gradients persist over time and
over large distances from the core. However, we find surprising new evidence
for a reversal in the direction of the Faraday Rotation gradient across the jet
of B1803+784, for which multi-frequency polarization observations are available
at four epochs. At our three epochs and the epoch of Zavala & Taylor (2003), we
observe transverse Rotation Measure (RM) gradients across the jet, consistent
with the presence of a helical magnetic field wrapped around the jet. However,
we also observe a "flip" in the direction of the gradient between June 2000 and
August 2002. Although the origins of this phenomena are not entirely clear,
possibly explanations include (i) the sense of rotation of the central
supermassive black hole and accretion disc has remained the same, but the
dominant magnetic pole facing the Earth has changed from North to South; (ii) a
change in the direction of the azimuthal B field component as a result of
torsional oscillations of the jet; and (iii) a change in the relative
contributions to the observed rotation measures of the "inner" and "outer"
helical fields in a magnetic-tower model. Although we cannot entirely rule out
the possibility that the observed changes in the RM distribution are associated
instead with changes in the thermal-electron distribution in the vicinity of
the jet, we argue that this explanation is unlikely.Comment: 21 pages, 10 figures. Accepted for publication in MNRA
Three dimensional magnetic field structure of six parsec-scale active galactic nuclei jets
The parsec-scale Faraday rotation measure (RM) distribution of six "blazars"
is investigated using multi-frequency (4.6--43 GHz) polarization observations
taken on 2006 July 2 with the VLBA. Analysis of the RM provides the direction
of the line-of-sight (LoS) magnetic field component, as well as the intrinsic
2-D polarization distribution on the plane of the sky. Our results show that
the magnitude of the core RM increases systematically with frequency, and is
well described by a power-law, where |RM_{core}| \propto \nu^a. Our measured
values of vary from 0.9 to 3.8, providing information on the assumed
power-law fall-off in the electron density with distance from the central
engine for each source. RM gradients were detected across the jets of three
sources, supporting the presence of helical magnetic fields in a sheath or
boundary layer surrounding their jets. We find a bi-modal distribution of the
intrinsic jet polarization orientation; either aligned or orthogonal to the jet
direction. A helical magnetic field geometry can neatly explain both the
bi-model distribution of the jet polarization orientation and the ordered
polarization structure detected on these scales. In half the sources, we find
that the core RM changes sign with distance from the central engine. We provide
an explanation for this by considering a boundary layer of Faraday rotating
material threaded by a helical magnetic field, where bends in the relativistic
jet or accelerating/decelerating flows give rise to changes in the dominant LoS
components of the magnetic field, which in turn gives rise to different signs
of the RM. (abridged)Comment: 29 pages, 26 figures, accepted for publication in MNRAS, v2 -> proof
corrections: references update
Magnetic Field Geometry in "Red" and "Blue" BL Lacs
We compare the systematics of the magnetic field geometry in the "red"
low-energy peaked BL Lacs (LBLs) and "blue" high-energy peaked BL Lacs (HBLs)
using VLBI polarimetric images. The LBLs are primarily "radio--selected" BL
Lacs and the HBLs are primarily "X-ray selected". In contrast to the LBLs,
which show predominantly transverse jet magnetic fields, the HBLs show
predominantly longitudinal fields. Thus, while the SED peaks of core-dominated
quasars, LBLs and HBLs form a sequence of increasing frequency, the magnetic
field geometry does not follow an analogous sequence. We briefly investigate
possible connections between the observed parsec-scale magnetic field
structures and circular polarization measurements in the literature on various
spatial scales.Comment: 12 pages, 5 figures, Proceedings of the Amsterdam workshop on
"Circular polarisation from relativistic jet sources", to be published in
Astrophysics and Space Science, eds. Rob Fender & J-P Macquar
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
Investigating the Effects of Finite Resolution on Observed Transverse Jet Profiles
Both the emission properties and evolution of Active Galactic Nuclei (AGN)
radio jets are dependent on the magnetic fields that thread them. Faraday
Rotation gradients are a very important way of investigating these magnetic
fields, and can provide information on the orientation and structure of the
magnetic field in the immediate vicinity of the jet; for example, a toroidal or
helical field component should give rise to a systematic gradient in the
observed Faraday rotation across the jet, as well as characteristic intensity
and polarization profiles. However, real observed radio images have finite
resolution, usually expressed via convolution with a Gaussian beam whose size
corresponds to the central lobe of the point source response function. This
will tend to blur transverse structure in the jet profile, raising the question
of how well resolved a jet must be in the transverse direction in order to
reliably detect transverse structure associated with a helical jet magnetic
field. We present results of simulated intensity, polarization and Faraday
rotation images designed to directly and empirically investigate the effect of
finite resolution on observed transverse jet structures
A new method for estimating frequency-dependent core shifts in active galactic nucleus jets
We discuss the opacity in the core regions of active galactic nuclei observed
with Very Long Baseline Interferometry (VLBI), and describe a new method for
deriving the frequency-dependent shifts of the VLBI core from the
frequency-dependent time lags of flares observed with single-dish observations.
Application of the method to the core shifts of the quasar 3C 345 shows a very
good agreement between the core shifts directly measured from VLBI observations
and derived from flares in the total flux density using the proposed method.
The frequency-dependent time lags of flares can be used to derive physical
parameters of the jets, such as distance from the VLBI core to the base of the
jet and the magnetic fields in the core region. Our estimates for 3C 345
indicate core magnetic fields ~0.1 G and magnetic field at 1 pc ~0.4 G.Comment: 8 pages, 6 figures, accepted for publication in MNRA
Radio Circular Polarization Produced in Helical Magnetic Fields in Eight Active Galactic Nuclei
Homan & Lister (2006) have recently published circular-polarization (CP)
detections for 34 objects in the MOJAVE sample - a set of bright, compact AGN
being monitored by the Very Long Baseline Array at 15 GHz. We report the
detection of 15-GHz parsec-scale CP in two more AGN (3C345 and 2231+114), and
confirm the MOJAVE detection of CP in 1633+382. It is generally believed that
the most likely mechanism for the generation of this CP is Faraday conversion
of linear polarization to CP. A helical jet magnetic-field (B-field) geometry
can facilitate this process - linearly polarized emission from the far side of
the jet is converted to CP as it passes through the magnetised plasma at the
front side of the jet on its way toward the observer. In this case, the sign of
the generated CP is essentially determined by the pitch angle and helicity of
the helical B field. We have determined the pitch-angle regimes and helicities
of the helical jet B fields in 8 AGN for which parsec-scale CP has been
detected, and used them to predict the expected CP signs for these AGN if the
CP is generated via conversion in these helical fields. We have obtained the
intriguing result that our predictions agree with the observed signs in all
eight cases, provided that the longitudinal B-field components in the jets
correspond to South magnetic poles. This clearly non-random pattern
demonstrates that the observed CP in AGN is directly associated with the
presence of helical jet B fields. These results suggest that helical B fields
are ubiquitous in AGN jets.Comment: 24 pages, 6 figures, accepted for publication in Monthly Notices of
the Royal Astronomical Society (MNRAS
The milliarcsecond-scale jet of PKS 0735+178 during quiescence
We present polarimetric 5 GHz to 43 GHz VLBI observations of the BL Lacertae
object PKS 0735+178, spanning March 1996 to May 2000. Comparison with previous
and later observations suggests that the overall kinematic and structural
properties of the jet are greatly influenced by its activity. Time intervals of
enhanced activity, as reported before 1993 and after 2000 by other studies, are
followed by highly superluminal motion along a rectilinear jet. In contrast the
less active state in which we performed our observations, shows subluminal or
slow superluminal jet features propagating through a twisted jet with two sharp
bends of about 90 deg. within the innermost three-milliarcsecond jet structure.
Proper motion estimates from the data presented here allow us to constrain the
jet viewing angle to values < 9 deg., and the bulk Lorentz factor to be between
2 and 4.Comment: 11 pages, 12 figures. Accepted for publication in A&
- …