2,809 research outputs found
Bendings of radio jets in BL Lacertae objects I: EVN and MERLIN observations
Several blazars, and BL Lac objects in particular, show a misalignment
between the jet orientation on parsec and kiloparsec scales. Some authors (i.e.
Conway & Murphy, 1993) have attempted to explain this behaviour invoking
helical jets for misalignment angles around 90\degr, showing how in this case
there are interesting implications for the understanding of the medium into
which the jet is expanding. By comparing sensitive VLA observations (Cassaro et
al., 1999) with images available in the literature for the BL Lac objects from
the 1-Jy Sample (Stickel et al., 1991), it is clear that there is a wide range
of misalignments between the initial jet direction and the kpc-scale jet, when
detected. We have carried out VLBI observations of these BL Lac objects, in
order to investigate the spatial evolution of the radio jets from few tens to
hundreds of mas, and to search for helical jets in this class of sources. We
present here the first dataset obtained from EVN+MERLIN observations at 5 GHz
for seven objects. From these observations we never have a clear detection of
helical jets, we only have a possible signature of their presence in 2 objects.
In only one of the sources with a misalignment angle around 90\degr the
presence of helical jets can be ruled out. This implies that it is not possible
to invoke helical jets to explain the morphology of all the sources showing a
misalignment of about 90\degr between the parsec and the kiloparsec scale
jets.Comment: 12 pages, 9 figures, latex, accepted by Astronomy & Astrophysic
Current Results from the RRFID Kinematic Survey: Apparent Speeds from the First Five Years of Data
We present current results from our ongoing project to study the parsec-scale
relativistic jet kinematics of sources in the U.S. Naval Observatory's Radio
Reference Frame Image Database (RRFID). The RRFID consists of snapshot
observations using the VLBA plus up to 9 additional antennas at 8 and 2 GHz.
The Image Database currently contains about 3000 images of 450 sources from
1994 to 2004, with some sources having images at 20 epochs or more. We have now
completed analysis of the 8 GHz images for all sources observed at 3 or more
epochs from 1994 to 1998. The completed analysis comprises 966 images of 87
sources, or an average of 11 epochs per source. Apparent jet speeds have been
measured for these sources, and the resulting speed distribution has been
compared with results obtained by other large VLBI surveys. The measured
apparent speed distribution agrees with those found by the 2 cm survey and
Caltech-Jodrell Bank (CJ) survey; however, when a source-by-source comparison
is done with the 2 cm survey results, significant disagreement is found in the
apparent speed measurements for a number of sources. This disagreement can be
traced in most cases to either an insufficient time baseline for the current
RRFID results, or to apparent component mis-identification in the 2 cm survey
results caused by insufficient time sampling. These results emphasize the need
for long time baselines and dense time sampling for multi-epoch monitoring of
relativistic jets.Comment: 4 pages, To be published in the Proceedings of the 7th European VLBI
Network Symposiu
Driving sandpiles to criticality and beyond
A popular theory of self-organized criticality relates driven dissipative
systems to systems with conservation. This theory predicts that the stationary
density of the abelian sandpile model equals the threshold density of the
fixed-energy sandpile. We refute this prediction for a wide variety of
underlying graphs, including the square grid. Driven dissipative sandpiles
continue to evolve even after reaching criticality. This result casts doubt on
the validity of using fixed-energy sandpiles to explore the critical behavior
of the abelian sandpile model at stationarity.Comment: v4 adds referenc
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