2,027 research outputs found
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
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
CONSTITUTIVE mRNA AND PROTEIN PRODUCTION OF MACROPHAGE COLONY-STIMULATING FACTOR BUT NOT OF OTHER CYTOKINES BY SYNOVIAL FIBROBLASTS FROM RHEUMATOID ARTHRITIS AND OSTEOARTHRITIS PATIENTS
This study analyses the mRNA and protein production and their regulation of macrophage colony-stimulating factor (M-CSF), granulocyte-macrophage colony-stimulating factor (GM-CSF), IL-8 and IL-6 by synovial fibroblasts obtained from patients with RA and OA. M-CSF was found to be produced constitutively as opposed to other cytokines. Stimulation of the cells with IL-1β caused a marked increase of GM-CSF, IL-8, IL-6 and as well as of M-CSF mRNA levels. In parallel, a time-dependent increase of M-CSF, GM-CSF, IL-8 and IL-6 protein production was observed. Among the cytokine mRNAs examined only that of M-CSF exhibited a pronounced stability in unstimulated synovial fibroblasts, whereas the other cytokines displayed short mRNA half-lives of 1-2 h. Induction by IL-1β markedly prolonged IL-8, IL-6 and GM-CSF mRNA half-lives to >8 h which indicates increased mRNA stability. These findings suggest that among the cytokines that are produced in the inflamed synovium M-CSF may be particularly important for sustaining long-term influx, activation and survival of mononuclear phagocytes. GM-CSF, IL-8 and IL-6, by contrast, may be more involved in more acute cellular response
The approach to criticality in sandpiles
A popular theory of self-organized criticality relates the critical behavior
of driven dissipative systems to that of systems with conservation. In
particular, this theory predicts that the stationary density of the abelian
sandpile model should be equal to the threshold density of the corresponding
fixed-energy sandpile. This "density conjecture" has been proved for the
underlying graph Z. We show (by simulation or by proof) that the density
conjecture is false when the underlying graph is any of Z^2, the complete graph
K_n, the Cayley tree, the ladder graph, the bracelet graph, or the flower
graph. Driven dissipative sandpiles continue to evolve even after a constant
fraction of the sand has been lost at the sink. These results cast doubt on the
validity of using fixed-energy sandpiles to explore the critical behavior of
the abelian sandpile model at stationarity.Comment: 30 pages, 8 figures, long version of arXiv:0912.320
The VLBA Calibrator Search for the BeSSeL Survey
We present the results of a survey of radio continuum sources near the
Galactic plane using the Very Long Baseline Array (VLBA). Our observations are
designed to identify compact extragalactic sources of milliarcsecond size that
can be used for parallax measurements in the Bar and Spiral Structure Legacy
Survey. We selected point sources from the NVSS and CORNISH catalogs with flux
densities above 30 mJy and within 1.5\degr of known maser targets. Of the
1529 sources observed, 199 were detected. For sources detected on 3 or more
baselines, we determined accurate positions and evaluated their quality as
potential calibrators. Most of the 1330 sources that were not detected with the
VLBA are probably of extragalactic origin.Comment: 27 pages, 3 figures, 3 tables. Table 3 is available on the homepage
of the BeSSeL survey:
http://www.mpifr-bonn.mpg.de/staff/abrunthaler/BeSSeL/index.shtm
Characterisation of Long Baseline Calibrators at 2.3 GHz
We present a detailed multi-epoch analysis of 31 potential southern
hemisphere radio calibrators that were originally observed as part of a program
to maintain the International Celestial Reference Frame (ICRF). At radio
wavelengths, the primary calibrators are Active Galactic Nuclei (AGN), powerful
radio emitters which exist at the centre of most galaxies. These are known to
vary at all wavelengths at which they have been observed. By determining the
amount of radio source structure and variability of these AGN, we determine
their suitability as phase calibrators for long baseline radio interferometry
at 2.3 GHz. For this purpose, we have used a set of complementary metrics to
classify these 31 southern sources into five categories pertaining to their
suitability as VLBI calibrators. We find that all of the sources in our sample
would be good interferometric calibrators and almost ninety per cent would be
very good calibrators.Comment: 9 pages, 7 Figures, accepted MNRA
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
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