10,894 research outputs found
Intrinsic Differences in the Inner Jets of High- and Low-Optically Polarized Radio Quasars
A significant fraction of compact radio-loud quasars display most of the
characteristics of relativistically beamed, high-optical polarization blazars,
yet are weakly polarized in the optical regime. We have used the VLBA at 22 and
43 GHz to look for differences in the parsec-scale magnetic field structures of
18 high- and low-optically polarized, compact radio-loud quasars (HPQs and
LPRQs, respectively). We find a strong correlation between the polarization
level of the unresolved parsec-scale radio core at 43 GHz and overall optical
polarization of the source, which suggests a common (possibly co-spatial)
origin for the emission at these two wavelengths. The magnetic fields of the
polarized 43 GHz radio cores are aligned roughly transverse to the jet axis.
Similar orientations are seen in the optical, suggesting that the polarized
flux at both wavelengths is due to one or more strong transverse shocks located
very close to the base of the jet. In LPRQs, these shocks appear to be weak
near the core, and gradually increase in strength down the jet. The LPRQs in
our sample tend to have less luminous radio cores than the HPQs, and jet
components with magnetic fields predominantly parallel to the flow. The
components in HPQ jets, on the other hand, tend to have transverse alignments.
These differences cannot be accounted for by a simple model in which HPQs and
LPRQs are the same type of object, seen at different angles to the line of
sight. A more likely scenario is that LPRQs represent a quiescent phase of
blazar activity, in which the inner jet flow does not undergo strong shocks.Comment: 29 pages, includes 25 figures and 6 tables. Uses emulateapj5.sty.
Accepted for publication in the Astrophysical Journal. A version with better
quality figures (785Kb, gzipped) can be found at
http://sgra.jpl.nasa.gov/html_lister/LPQ
Chandra Discovery of 10 New X-Ray Jets Associated With FR II Radio Core-Selected AGNs in the MOJAVE Sample
The Chandra X-ray observatory has proven to be a vital tool for studying
high-energy emission processes in jets associated with Active Galactic Nuclei
(AGN).We have compiled a sample of 27 AGN selected from the radio flux-limited
MOJAVE (Monitoring of Jets in AGN with VLBA Experiments) sample of highly
relativistically beamed jets to look for correlations between X-ray and radio
emission on kiloparsec scales. The sample consists of all MOJAVE quasars which
have over 100 mJy of extended radio emission at 1.4 GHz and a radio structure
of at least 3" in size. Previous Chandra observations have revealed X-ray jets
in 11 of 14 members of the sample, and we have carried out new observations of
the remaining 13 sources. Of the latter, 10 have Xray jets, bringing the
overall detection rate to ~ 78%. Our selection criteria, which is based on
highly compact, relativistically beamed jet emission and large extended radio
flux, thus provides an effective method of discovering new X-ray jets
associated with AGN. The detected X-ray jet morphologies are generally well
correlated with the radio emission, except for those displaying sharp bends in
the radio band. The X-ray emission mechanism for these powerful FR II
(Fanaroff-Riley type II) jets can be interpreted as inverse Compton scattering
off of cosmic microwave background (IC/CMB) photons by the electrons in the
relativistic jets. We derive viewing angles for the jets, assuming a
non-bending, non-decelerating model, by using superluminal parsec scale speeds
along with parameters derived from the inverse Compton X-ray model. We use
these angles to calculate best fit Doppler and bulk Lorentz factors for the
jets, as well as their possible ranges, which leads to extreme values for the
bulk Lorentz factor in some cases. When both the non-bending and
non-decelerating assumptions are relaxed [abridged]Comment: 38 Pages, 4 Figures, 5 Tables, accepted for publication in Ap
2D Bayesian automated tilted-ring fitting of disk galaxies in large HI galaxy surveys: 2DBAT
We present a novel algorithm based on a Bayesian method for 2D tilted-ring
analysis of disk galaxy velocity fields. Compared to the conventional
algorithms based on a chi-squared minimisation procedure, this new
Bayesian-based algorithm suffers less from local minima of the model parameters
even with highly multi-modal posterior distributions. Moreover, the Bayesian
analysis, implemented via Markov Chain Monte Carlo (MCMC) sampling, only
requires broad ranges of posterior distributions of the parameters, which makes
the fitting procedure fully automated. This feature will be essential when
performing kinematic analysis on the large number of resolved galaxies expected
to be detected in neutral hydrogen (HI) surveys with the Square Kilometre Array
(SKA) and its pathfinders. The so-called '2D Bayesian Automated Tilted-ring
fitter' (2DBAT) implements Bayesian fits of 2D tilted-ring models in order to
derive rotation curves of galaxies. We explore 2DBAT performance on (a)
artificial HI data cubes built based on representative rotation curves of
intermediate-mass and massive spiral galaxies, and (b) Australia Telescope
Compact Array (ATCA) HI data from the Local Volume HI Survey (LVHIS). We find
that 2DBAT works best for well-resolved galaxies with intermediate inclinations
(20 deg < i < 70 deg), complementing three-dimensional techniques better suited
to modelling inclined galaxies.Comment: Accepted for publication in MNRAS; 46 pages, 33 figure
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Machine dependence and reproducibility for coupled climate simulations: the HadGEM3-GC3.1 CMIP Preindustrial simulation
When the same weather or climate simulation is run on different high-performance computing (HPC) platforms, model outputs may not be identical for a given initial condition. While the role of HPC platforms in delivering better climate projections is to some extent discussed in the literature, attention is mainly focused on scalability and performance rather than on the impact of machine-dependent processes on the numerical solution.
Here we investigate the behaviour of the Preindustrial (PI) simulation prepared by the UK Met Office for the forthcoming CMIP6 (Coupled Model Intercomparison Project Phase 6) under different computing environments.
Discrepancies between the means of key climate variables were analysed at different timescales, from decadal to centennial. We found that for the two simulations to be statistically indistinguishable, a 200-year averaging period must be used for the analysis of the results. Thus, constant-forcing climate simulations using the HadGEM3-GC3.1 model are reproducible on different HPC platforms provided that a sufficiently long duration of simulation is used.
In regions where El Niño–Southern Oscillation (ENSO) teleconnection patterns were detected, we found large sea surface temperature and sea ice concentration differences on centennial timescales. This indicates that a 100-year constant-forcing climate simulation may not be long enough to adequately capture the internal variability of the HadGEM3-GC3.1 model, despite this being the minimum simulation length recommended by CMIP6 protocols for many MIP (Model Intercomparison Project) experiments.
On the basis of our findings, we recommend a minimum simulation length of 200 years whenever possible
Ytterbium-doped tantalum pentoxide waveguide lasers
We have demonstrated a Yb:Ta2O5 waveguide laser fabricated by RF magnetron sputtering on oxidised silicon. The waveguide laser was end-pumped with a laser diode at 977 nm and lasing was observed between 1015 and 1020 nm. The launched pump power threshold and slope efficiency were measured to be ~25 mW and 1.78 %, respectively
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