14,731 research outputs found
Fluid thermal actuator
Operational characteristics of actuator for spacecraft thermal control syste
Bi-Directional Relativistic Jets of the Radio Galaxy 1946+708: Constraints on the Hubble Constant
We present measurements of bi-directional motions in the jets of the radio
galaxy 1946+708 at z=0.101. This is a Compact Symmetric Object with striking
S-symmetry. Sensitive 15 GHz observations reveal a compact component at the
center of symmetry with a strongly inverted spectrum, that we identify as the
core. From five 4.9 GHz observations spread over 4 years we have determined the
velocities of four compact jet components. If simple kinematic models can be
applied then the inclination of the source and the bulk jet velocity can be
directly determined for any assumed value of the Hubble constant. Conversely,
the measurements already place constraints on the Hubble constant, and we show
how further observations of 1946+708 can yield an increasingly accurate
determination of H_0.Comment: in press at ApJ Letters, 12 page LaTex document includes 5 postscript
figure
Monitoring the Bi-Directional Relativistic Jets of the Radio Galaxy 1946+708
We report on a multi-frequency, multi-epoch campaign of Very Long Baseline
Interferometry observations of the radio galaxy 1946+708 using the VLBA and a
Global VLBI array. From these high-resolution observations we deduce the
kinematic age of the radio source to be 4000 years, comparable with the
ages of other Compact Symmetric Objects (CSOs). Ejections of pairs of jet
components appears to take place on time scales of 10 years and these
components in the jet travel outward at intrinsic velocities between 0.6 and
0.9 c. From the constraint that jet components cannot have intrinsic velocities
faster than light, we derive H_0 > 57 km s^-1 Mpc^-1 from the fastest pair of
components launched from the core. We provide strong evidence for the ejection
of a new pair of components in ~1997. From the trajectories of the jet
components we deduce that the jet is most likely to be helically confined,
rather than purely ballistic in nature.Comment: 20 pages, 8 figures, accepted to Ap
Finite strain in Elastic problems. II
This article does not have an abstract
The two sided parsec scale structure of the Low Luminosity Active Galactic Nucleus in NGC 4278
We present new Very Long Baseline Interferometry observations of the LINER
galaxy NGC 4278. The observations were taken with the Very Long Baseline Array
(VLBA) and a single antenna of the Very Large Array (VLA) at 5 GHz and 8.4 GHz
and have a linear resolution of <0.1 pc. Our radio data reveal a two sided
structure, with symmetric S-shaped jets emerging from a flat spectrum core. We
fit the jet brightness with gaussian components, which we identify from a
previous observation taken five years before. By comparing the positions of the
components in the two epochs, we measure motions between 0.45 +/- 0.14 and 3.76
+/- 0.65 mas, corresponding to apparent velocities < 0.2c, and to ages in the
range 8.3 - 65.8 years. Assuming that the radio morphology is intrinsically
symmetric and its appearance is governed by Doppler beaming effects, we find
that NGC4278 has mildly relativistic jets (beta ~ 0.75), closely aligned to the
line-of-sight (2 degrees < theta < 4 degrees). Alternatively, the source could
be oriented at a larger angle and asymmetries could be related to the jet
interaction with the surrounding medium. We also present new simultaneous VLA
observations between 1.4 and 43 GHz, and a 5 GHz light curve between 1972 and
2003. The radio spectrum can be fit by a relatively steep power-law (alpha =
0.54). We find significant variability at 5 GHz. All these arguments indicate
that the radiation from NGC 4278 is emitted via the synchrotron process by
relativistic particles accelerated by a supermassive black hole. Despite a much
lower power, this is the same process that takes place in ordinary radio loud
AGNs.Comment: 29 pages, 9 figures, ApJ accepte
Kinematic Age Estimates for 4 Compact Symmetric Objects from the Pearson-Readhead Survey
Based on multi-epoch observations at 15 and 43 GHz with the Very Long
Baseline Array (VLBA) we detect significant angular expansions between the two
hot spots of 4 Compact Symmetric Objects (CSOs). From these relative motions we
derive kinematic ages of between 300 and 1200 years for the radio emission.
These ages lend support to the idea that CSOs are produced in a recent phase of
activity. These observations also allow us to study the evolution of the hot
spots dynamically in individual sources. In all 4 sources the hot spots are
separating along the source axis, but in 1031+567 the tip of one hot spot
appears to be moving almost orthogonally to the source axis. Jet components,
seen in 3 of the 4 sources observed, are found to be moving relativistically
outward from the central engines towards the more slowly moving hot spots.Comment: in press at ApJ for v. 541 Oct. 1, 2000, 23 page LaTeX document
includes 6 postscript figure
The incidence and risk factors for new onset atrial fibrillation in the PROSPER study
Aims Atrial fibrillation/flutter (AF) is the most common arrhythmia in older people. It associates with reduced exercise capacity, increased risk of stroke, and mortality. We aimed to determine retrospectively whether pravastatin reduces the incidence of AF and whether any electrocardiographic measures or clinical conditions might be risk factors for its development. Methods and results The PROspective Study of Pravastatin in the Elderly at Risk (PROSPER) was a randomized, double-blind controlled trial that recruited 5804 individuals aged 70-82 years with a history of, or risk factors for, vascular disease. A total of 2891 were allocated to pravastatin and 2913 to placebo; mean follow-up was 3.2 years. Electrocardiograms (ECGs), which were recorded at baseline, annually thereafter, and at run-out, were processed by computer and reviewed manually. In all, 264 of 2912 (9.1%) of the placebo group and 283 of 2888 (9.8%) of the pravastatin-treated group developed AF [hazard ratio 1.08 (0.92,1.28), P = 0.35)]. Multivariate analysis showed that PR and QTc intervals, age, left ventricular hypertrophy, and ST-T abnormalities were related to development of AF after adjustment for many variables including alcohol consumption, which itself was univariately predictive of developing AF. Previous myocardial infarction on the ECG was not a risk factor. A history of vascular disease was strongly linked with developing AF but not diabetes and hypertension. Conclusion Pravastatin does not reduce the incidence of AF in older people at risk of vascular disease, at least in the short-medium term. Risk factors for AF include older age, prolongation of PR or QTc intervals, left ventricular hypertrophy, and ST-T abnormalities on the EC
Performance of formulae based estimates of glomerular filtration rate for carboplatin dosing in stage 1 seminoma
<b>Background:</b> Single cycle carboplatin, dosed by glomerular filtration rate (GFR), is standard adjuvant therapy for stage 1 seminoma. Accurate measurement of GFR is essential for correct dosing. Isotopic methods remain the gold standard for the determination of GFR. Formulae to estimate GFR have improved the assessment of renal function in non-oncological settings. We assessed the utility of these formulae for carboplatin dosing.<p></p>
<b>Methods:</b> We studied consecutive subjects receiving adjuvant carboplatin for stage 1 seminoma at our institution between 2007 and 2012. Subjects underwent 51Cr-ethylene diamine tetra-acetic acid (EDTA) measurement of GFR with carboplatin dose calculated using the Calvert formula. Theoretical carboplatin doses were calculated from estimated GFR using Chronic Kidney Disease-Epidemiology (CKD-EPI), Management of Diet in Renal Disease (MDRD) and CockcroftâGault (CG) formulae with additional correction for actual body surface area (BSA). Carboplatin doses calculated by formulae were compared with dose calculated by isotopic GFR; a difference <10% was considered acceptable.<p></p>
<b>Results:</b> 115 patients were identified. Mean isotopic GFR was 96.9 ml/min/1.73 m2. CG and CKD-EPI tended to overestimate GFR whereas MDRD tended to underestimate GFR. The CKD-EPI formula had greatest accuracy. The CKD-EPI formula, corrected for actual BSA, performed best; 45.9% of patients received within 10% of correct carboplatin dose. Patients predicted as underdosed (13.5%) by CKD-EPI were more likely to be obese (p = 0.013); there were no predictors of the 40.5% receiving an excess dose.<p></p>
<b>Conclusions:</b> Our data support further evaluation of the CKD-EPI formula in this patient population but clinically significant variances in carboplatin dosing occur using non-isotopic methods of GFR estimation. Isotopic determination of GFR should remain the recommended standard for carboplatin dosing when accuracy is essential.<p></p>
A VLBA search for binary black holes in active galactic nuclei with double-peaked optical emission line spectra
We have examined a subset of 11 active galactic nuclei (AGN) drawn from a
sample of 87 objects that possess double-peaked optical emission line spectra,
as put forward by Wang et al. (2009a) and are detectable in the FIRST survey at
radio wavelengths. The double-peaked nature of the optical emission line
spectra has been suggested as evidence for the existence of binary black holes
in these AGN, although this interpretation is controversial. We make a simple
suggestion, that direct evidence of binary black holes in these objects could
be searched for in the form of dual sources of compact radio emission
associated with the AGN. To explore this idea, we have used the Very Long
Baseline Array to observe these 11 objects from the Wang et al. (2009a) sample.
Of the 11 objects, we detect compact radio emission from two, SDSS
J151709+335324 and SDSS J160024+264035. Both objects show single components of
compact radio emission. The morphology of SDSS J151709+335324 is consistent
with a recent comprehensive multi-wavelength study of this object by Rosario et
al. (2010). Assuming that the entire sample consists of binary black holes, we
would expect of order one double radio core to be detected, based on radio
wavelength detection rates from FIRST and VLBI surveys. We have not detected
any double cores, thus this work does not substantially support the idea that
AGN with double-peaked optical emission lines contain binary black holes.
However, the study of larger samples should be undertaken to provide a more
secure statistical result, given the estimated detection rates.Comment: 14 pages, 3 figures. To appear in A
Interferometric Phase Calibration Sources in the Declination Range 0deg to -30deg
We present a catalog of 321 compact radio sources in the declination range
0deg > delta > -30deg. The positions of these sources have been measured with a
two-dimensional rms accuracy of 35 milliarcseconds using the NRAO Very Large
Array. Each source has a peak flux density >50 mJy at 8.4 GHz. We intend for
this catalog to be used mainly for selection of phase calibration sources for
radio interferometers, although compact radio sources have other scientific
uses.Comment: 9 pages. To appear in ApJS. Catalog (Table 3) is abbreviated in
printed version. Complete catalog available at
ftp://ftp.aoc.nrao.edu/pub/staff/jwrobel/WPW2003_ApJS.tx
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