4,675 research outputs found
The central engines of radio-quiet quasars
Two rival hypotheses have been proposed for the origin of the compact radio
flux observed in radio-quiet quasars (RQQs). It has been suggested that the
radio emission in these objects, typically some two or three orders of
magnitude less powerful than in radio-loud quasars (RLQs), represents either
emission from a circumnuclear starburst or is produced by radio jets with bulk
kinetic powers 10^3 times lower than those of RLQs with similar luminosity
ratios in other wavebands. We describe the results of high resolution
(parsec-scale) radio-imaging observations of a sample of 12 RQQs using the Very
Long Baseline Array (VLBA). We find strong evidence for jet-producing central
engines in 8 members of our sample.Comment: 7 pages, 8 figures. Accepted for publication in MNRA
The Atacama Large Millimeter/submillimeter Array: Overview & status
The Atacama Large Millimeter/submillimeter Array (ALMA) is an international
radio telescope under construction in the Atacama Desert of northern Chile.
ALMA will be situated on a high-altitude site at 5000 m elevation which
provides excellent atmospheric transmission over the instrument wavelength
range of 0.3 to 3 mm. ALMA will be comprised of two key observing components:
an array of up to sixty-four 12-m diameter antennas arranged in a multiple
configurations ranging in size from 0.15 to ~14 km, and a set of four 12-m and
twelve 7-m antennas operating in closely-packed configurations ~50m in diameter
(known as the Atacama Compact Array, or ACA), providing both interferometric
and total-power astronomical information. High-sensitivity dual-polarization 8
GHz-bandwidth spectral-line and continuum measurements between all antennas
will be available from two flexible digital correlators
Scheduling aircraft landings - the static case
This is the publisher version of the article, obtained from the link below.In this paper, we consider the problem of scheduling aircraft (plane) landings at an airport. This problem is one of deciding a landing time for each plane such that each plane lands within a predetermined time window and that separation criteria between the landing of a plane and the landing of all successive planes are respected. We present a mixed-integer zero–one formulation of the problem for the single runway case and extend it to the multiple runway case. We strengthen the linear programming relaxations of these formulations by introducing additional constraints. Throughout, we discuss how our formulations can be used to model a number of issues (choice of objective function, precedence restrictions, restricting the number of landings in a given time period, runway workload balancing) commonly encountered in practice. The problem is solved optimally using linear programming-based tree search. We also present an effective heuristic algorithm for the problem. Computational results for both the heuristic and the optimal algorithm are presented for a number of test problems involving up to 50 planes and four runways.J.E.Beasley. would like to acknowledge the financial support of the Commonwealth Scientific and Industrial Research Organization, Australia
Evidence for a black hole in a radio-quiet quasar nucleus
We present the first milli-arcsecond resolution radio images of a radio-quiet
quasar, detecting a high brightness temperature core with data from the VLBA.
On maps made with lower-frequency data from MERLIN and the VLA jets appear to
emanate from the core in opposite directions, which correspond to
radio-emission on arcsecond scales seen with the VLA at higher frequencies.
These provide strong evidence for a black-hole--based jet-producing central
engine, rather than a starburst, being responsible for the compact radio
emission in this radio-quiet quasar.Comment: 10 pages including 1 postscript figure; uses aaspp4.sty. Accepted for
publication in Ap. J. Lett. Also available from
http://www-astro.physics.ox.ac.uk/research/preprints
The radio remnant of SN1993J: an instrumental explanation for the evolving complex structure
We present simulated images of Supernova 1993J at 8.4 GHz using Very Long
Baseline Interferometry (VLBI) techniques. A spherically symmetric source model
is convolved with realistic uv-plane distributions, together with standard
imaging procedures, to assess the extent of instrumental effects on the
recovered brightness distribution. In order to facilitate direct comparisons
between the simulations and published VLBI images of SN1993J, the observed
uv-coverage is determined from actual VLBI observations made in the years
following its discovery.
The underlying source model only exhibits radial variation in its density
profile, with no azimuthal dependence and, even though this model is
morphologically simple, the simulated VLBI observations qualitatively reproduce
many of the azimuthal features of the reported VLBI observations, such as
appearance and evolution of complex azimuthal structure and apparent rotation
of the shell. We demonstrate that such features are inexorably coupled to the
uv-plane sampling.
The brightness contrast between the peaks and the surrounding shell material
are not as prominent in the simulations (which of course assume no antenna- or
baseline-based amplitude or phase errors, meaning no self-calibration
procedures will have incorporated any such features in models). It is
conclusive that incomplete uv-plane sampling has a drastic effect on the final
images for observations of this nature. Difference imaging reveals residual
emission up to the 8 sigma level. Extreme care should be taken when using
interferometric observations to directly infer the structure of objects such as
supernovae.Comment: 14 pages, 10 figures, 2 tables, accepted for publication in MNRA
Low-speed aerodynamic characteristics of a 13-percent-thick airfoil section designed for general aviation applications
Wind-tunnel tests were conducted to determine the low-speed section characteristics of a 13 percent-thick airfoil designed for general aviation applications. The results were compared with NACA 12 percent-thick sections and with the 17 percent-thick NASA airfoil. The tests were conducted ovar a Mach number range from 0.10 to 0.35. Chord Reynolds numbers varied from about 2,000,000 to 9,000,000
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