1,284 research outputs found
Helical Magnetic Fields Associated with the Relativistic Jets of Four BL Lac Objects
Evidence has been mounting that many of the transverse jet B fields observed
in BL Lac objects on parsec scales represent the dominant vvroidal compoent of
the intrinsic jet B fields. Such fields could come about, for example, as a
result of the "winding up" of an initial "seed" field with a significant
longitudinal component by the rotation of the central accreting object. If this
is the case, this should give rise to gradients in the rotation measure (RM)
across the jets, due to the systematic change in the line-of-sight component of
the jet B field. We present evidence for transverse RM gradients in four BL Lac
objects, strengthening arguments that the jets of these objects do indeed have
toroidal or helical B fields. This underlines the view of the jets as
fundamentally electromagnetic structures, and suggests that they may well carry
non-zero currents. It also provides a natural means to collimate the jets.Comment: 6 pages, 2 figures, accepted for publication in MNRAS Letter
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
Changes in the trajectory of the radio jet in 0735+178?
We present multi-epoch 8.4 and 43 GHz Very Long Baseline Array images of the
BL Lac object 0735+178. The images confirm the presence of a twisted jet with
two sharp apparent bends of 90 within two milliarcseconds of the
core, resembling a helix in projection. The observed twisted geometry could be
the result of precession of the jet inlet, but is more likely produced by
pressure gradients in the external medium through which the jet propagates.
Quasi-stationary components are observed at the locations of the 90
bends, possibly produced by differential Doppler boosting. Identification of
components across epochs, since the earliest VLBI observations of this source
in 1979.2, proves difficult due to the sometimes large time gaps between
observations. One possible identification suggests the existence of
superluminal components following non--ballistic trajectories with velocities
up to . However, in images obtained after mid-1995,
components show a remarkable tendency to cluster near several jet positions,
suggesting a different scenario in which components have remained nearly
stationary in time at least since mid-1995. Comparison with the earlier
published data, covering more than 19 years of observations, suggests a
striking qualitative change in the jet trajectory sometime between mid-1992 and
mid-1995, with the twisted jet structure with stationary components becoming
apparent only at the later epochs. This would require a re-evaluation of the
physical parameters estimated for 0735+178, such as the observing viewing
angle, the plasma bulk Lorentz factor, and those deduced from these.Comment: 18 pages, 5 figures, accepted for publication in MNRA
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