1,311 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
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
The Celestial Reference Frame at 24 and 43 GHz. II. Imaging
We have measured the sub-milli-arcsecond structure of 274 extragalactic
sources at 24 and 43 GHz in order to assess their astrometric suitability for
use in a high frequency celestial reference frame (CRF). Ten sessions of
observations with the Very Long Baseline Array have been conducted over the
course of 5 years, with a total of 1339 images produced for the 274
sources. There are several quantities that can be used to characterize the
impact of intrinsic source structure on astrometric observations including the
source flux density, the flux density variability, the source structure index,
the source compactness, and the compactness variability. A detailed analysis of
these imaging quantities shows that (1) our selection of compact sources from
8.4 GHz catalogs yielded sources with flux densities, averaged over the
sessions in which each source was observed, of about 1 Jy at both 24 and 43
GHz, (2) on average the source flux densities at 24 GHz varied by 20%-25%
relative to their mean values, with variations in the session-to-session flux
density scale being less than 10%, (3) sources were found to be more compact
with less intrinsic structure at higher frequencies, and (4) variations of the
core radio emission relative to the total flux density of the source are less
than 8% on average at 24 GHz. We conclude that the reduction in the effects due
to source structure gained by observing at higher frequencies will result in an
improved CRF and a pool of high-quality fiducial reference points for use in
spacecraft navigation over the next decade.Comment: 63 pages, 18 figures, 6 tables, accepted by the Astronomical Journa
Growth Rates and Explosions in Sandpiles
We study the abelian sandpile growth model, where n particles are added at
the origin on a stable background configuration in Z^d. Any site with at least
2d particles then topples by sending one particle to each neighbor. We find
that with constant background height h <= 2d-2, the diameter of the set of
sites that topple has order n^{1/d}. This was previously known only for h<d.
Our proof uses a strong form of the least action principle for sandpiles, and a
novel method of background modification.
We can extend this diameter bound to certain backgrounds in which an
arbitrarily high fraction of sites have height 2d-1. On the other hand, we show
that if the background height 2d-2 is augmented by 1 at an arbitrarily small
fraction of sites chosen independently at random, then adding finitely many
particles creates an explosion (a sandpile that never stabilizes).Comment: 19 pages, 4 figures, to appear in Journal of Statistical Physics. v2
corrects the proof of the outer bound of Theorem 4.1 of arXiv:0704.068
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
Chaos in Sandpile Models
We have investigated the "weak chaos" exponent to see if it can be considered
as a classification parameter of different sandpile models. Simulation results
show that "weak chaos" exponent may be one of the characteristic exponents of
the attractor of \textit{deterministic} models. We have shown that the
(abelian) BTW sandpile model and the (non abelian) Zhang model posses different
"weak chaos" exponents, so they may belong to different universality classes.
We have also shown that \textit{stochasticity} destroys "weak chaos" exponents'
effectiveness so it slows down the divergence of nearby configurations. Finally
we show that getting off the critical point destroys this behavior of
deterministic models.Comment: 5 pages, 6 figure
Symbolic Manipulators Affect Mathematical Mindsets
Symbolic calculators like Mathematica are becoming more commonplace among
upper level physics students. The presence of such a powerful calculator can
couple strongly to the type of mathematical reasoning students employ. It does
not merely offer a convenient way to perform the computations students would
have otherwise wanted to do by hand. This paper presents examples from the work
of upper level physics majors where Mathematica plays an active role in
focusing and sustaining their thought around calculation. These students still
engage in powerful mathematical reasoning while they calculate but struggle
because of the narrowed breadth of their thinking. Their reasoning is drawn
into local attractors where they look to calculation schemes to resolve
questions instead of, for example, mapping the mathematics to the physical
system at hand. We model the influence of Mathematica as an integral part of
the constant feedback that occurs in how students frame, and hence focus, their
work
Prevalence of non-O157:H7 shiga toxin-producing Escherichia coli in diarrheal stool samples from Nebraska.
We determined the prevalence of Shiga toxin-producing Escherichia coli (STEC) in diarrheal stool samples from Nebraska by three methods: cefixime-tellurite sorbitol MacConkey (CT- SMAC) culture, enterohemorrhagic E. coli (EHEC) enzyme immunoassay, and stx1,2 polymerase chain reaction (PCR). Fourteen (4.2%) of 335 specimens were positive by at least one method (CT-SMAC culture [6 of 14], EHEC enzyme immunoassay [13 of 14], stx1,2 PCR [14 of 14]). Six contained serogroup O157, while non-O157 were as prevalent as O157 serogroups
Limiting shapes for deterministic centrally seeded growth models
We study the rotor router model and two deterministic sandpile models. For
the rotor router model in , Levine and Peres proved that the
limiting shape of the growth cluster is a sphere. For the other two models,
only bounds in dimension 2 are known. A unified approach for these models with
a new parameter (the initial number of particles at each site), allows to
prove a number of new limiting shape results in any dimension .
For the rotor router model, the limiting shape is a sphere for all values of
. For one of the sandpile models, and (the maximal value), the
limiting shape is a cube. For both sandpile models, the limiting shape is a
sphere in the limit . Finally, we prove that the rotor router
shape contains a diamond.Comment: 18 pages, 3 figures, some errors corrected and more explanation
added, to appear in Journal of Statistical Physic
Long-term results of a multicenter SAKK trial on high-dose ifosfamide and doxorubicin in advanced or metastatic gynecologic sarcomas
Background: Dose intensive chemotherapy has not been tested prospectively for the treatment of gynecologic sarcomas. We investigated the antitumor activity and toxicity of high-dose ifosfamide and doxorubicin, in the context of a multidisciplinary strategy for the treatment of advanced and metastatic, not pretreated, gynecologic sarcomas. Patients and methods: Thirty-nine patients were enrolled onto a phase I-II multicenter trial of ifosfamide, 10 g/m2 as a continuous infusion over 5 days, plus doxorubicin intravenously, 25 mg/m2/day for 3 days with Mesna and granulocyte-colony-stimulating factor every 21 days. Salvage therapy was allowed after chemotherapy. Results: Among the 37 evaluable patients, the tumor was locally advanced (n = 11), with concomitant distant metastases (n = 5) or with distant metastases only (n = 21). After a median of three (range 1-7) chemotherapy cycles, six patients experienced a complete response and 12 a partial response for an overall response rate of 49% (95% CI 32% to 66%). The response rate was higher in poorly differentiated tumors (62%) compared with moderately well differentiated ones (18%), but was not different according to histology subtypes. Eleven patients had salvage therapy, either immediately following chemotherapy (n = 7) or at time of progression (n = 4). With a median follow-up time of 5 years, the median overall survival was 30.5 months. Hematological toxicity was as expected neutropenia, thrombopenia and anemia ≥grade 3 at 50%, 34% and 33% of cycles respectively. No toxic death occurred. Conclusions: High-dose ifosfamide plus doxorubicin is an active regimen for all subtypes of gynecological sarcomas. Its toxicity was manageable in a multicentric setting. The prolonged survival might be due to the multidisciplinary strategy that was possible in one-third of the patient
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