23 research outputs found
Unusual Radio Properties of the BL Lac Object 0820+225
We present the results of simultaneous VLBA polarisation observations of the
BL Lacertae object 0820+225 at 5, 8, and 15 GHz, together with earlier images
at 5 GHz. This source has an unusually rich total intensity and polarisation
structure compared to other objects with comparable redshifts. The magnetic
field in the inner part of the complex and highly twisted VLBI jet is
transverse, as is typical of BL Lacertae objects, but becomes roughly
longitudinal further from the core, possibly due to shear. Although the
integrated rotation measure of 0820+225 is modest, the rotation-measure
distribution on parsec scales is non-uniform, and clearly shows regions where
the rotation measure is substantially higher than the integrated value.Comment: 8 pages, 8 figures, accepted for publication in Monthly Notices of
the Royal Astronomical Societ
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
Circular polarization of AGNs on the parsec VLBI scales
Faraday effects possibly plays the major role in generation of circular polarization observed on Very Long Base Interferometry scales. Multi-frequency circular polarization measurementrs can become the desired breakthrough in understanding the active galactic nuclei jet physics and the only possibility to estimate some of their vital parameters. We review the possible mechanisms of circular polarization generation and their connection to the jet parameters. We throw a glimpse on the methods of data reduction and finally discuss our current observational progress and its possible interpretation
Do BL Lacs lack the circular polarization?
We present the first preliminary evidence of difference in the circular polarized emission between quasars and BL Lacs. BL Lac objects seem to lack the sources with high circular polarization degrees. It cannot be due to selection effects, so some intrinsic effects are involved. These possible differences can be attributed to jet composition, energy of particles or magnetic field properties. The first one can be the intriguing possibility of different central engine mechanisms of these objects. However, the small detection rate for BL Lac objects and therefore a small sample size still prevents us from making the firm conclusion, so further observations of BL Lacs are highly desired
The Point of Origin of the Radio Radiation from the Unresolved Cores of Radio-Loud Quasars
Locating the exact point of origin of the core radiation in active galactic
nuclei (AGN) would represent important progress in our understanding of
physical processes in the central engine of these objects. However, due to our
inability to resolve the region containing both the central compact object and
the jet base, this has so far been difficult. Here, using an analysis in which
the lack of resolution does not play a significant role, we demonstrate that it
may be impossible even in most radio loud sources for more than a small
percentage of the core radiation at radio wavelengths to come from the jet
base. We find for 3C279 that percent of the core flux at 15 GHz must
come from a separate, reasonably stable, region that is not part of the jet
base, and that then likely radiates at least quasi-isotropically and is
centered on the black hole. The long-term stability of this component also
suggests that it may originate in a region that extends over many Schwarzschild
radii.Comment: 7 pages with 3 figures, accepted for publication in Astrophysics and
Space Scienc
Correlations between the peak flux density and the position angle of inner-jet in three blazars
We aim to investigate the relation between the long-term flux density and the
position angle (PA) evolution of inner-jet in blazars. We have carried out the
elliptic Gaussian model-fit to the `core' of 50 blazars from 15 GHz VLBA data,
and analyzed the variability properties of three blazars from the model-fit
results. Diverse correlations between the long-term peak flux density and the
PA evolution of the major axis of the `core' have been found in 20% of
the 50 sources. Of them, three typical blazars have been analyzed, which also
show quasi-periodic flux variations of a few years (T). The correlation between
the peak flux density and the PA of inner-jet is positive for S5~0716+714, and
negative for S4~1807+698. The two sources cannot be explained with the
ballistic jet models, the non-ballistic models have been analyzed to explain
the two sub-luminal blazars. A correlation between the peak flux density and
the PA (with a T/4 time lag) of inner-jet is found in [HB89]~1823+568, this
correlation can be explained with a ballistic precession jet model. All the
explanations are based mainly on the geometric beaming effect; physical flux
density variations from the jet base would be considered for more complicated
situations in future, which could account for the no or less significance of
the correlation between the peak flux density and the PA of inner-jet in the
majority blazars of our sample.Comment: 6 pages, 7 figures, accepted for publication in Astrophysics and
Space Scienc
Eigenmodes and growth rates of relativistic current filamentation instability in a collisional plasma
I theoretically found eigenmodes and growth rates of relativistic current
filamentation instability in collisional regimes, deriving a generalized
dispersion relation from self-consistent beam-Maxwell equations. For
symmetrically counterstreaming, fully relativistic electron currents, the
collisional coupling between electrons and ions creates the unstable modes of
growing oscillation and wave, which stand out for long-wavelength
perturbations. In the stronger collisional regime, the growing oscillatory mode
tends to be dominant for all wavelengths. In the collisionless limit, those
modes vanish, while maintaining another purely growing mode that exactly
coincides with a standard relativistic Weibel mode. It is also shown that the
effects of electron-electron collisions and thermal spread lower the growth
rate of the relativistic Weibel instability. The present mechanisms of
filamentation dynamics are essential for transport of homogeneous electron beam
produced by the interaction of high power laser pulses with plasma.Comment: 44 pages, 12 figures. Accepted for publication in Phys. Rev.
The VLBI Jets of BL Lac Objects
Very Long Baseline Interferometry (VLBI) can be used to
construct images of the radio emission of active galactic nuclei on scales
of parsecs. Although BL Lacertac objects distinguished from other
core-dominated AGN primarily in the absence of strong optical line
emission - have "core-jet" structures similar to those seen in other
types of compact extragalactic radio sources, their parsec-scale structure
shows a number of distinctive features, such as the dominance
of transverse magnetic fields. This B-field structure may be due to
the presence of relativistic shocks: alternatively, it may reflect the
dominance of the toroidal comportent of an underlying helical B field.
The superluminal speeds observed in the VLBI jets of BL Lac objects
are somewhat lower than those in quasars, suggesting that the typical
Lorentz factors for the jet flows are smaller