16,626 research outputs found
OSSE observations of active galactic nuclei
The Oriented Scintillation Spectrometer Experiment (OSSE) was designed to address a broad range of scientific objectives through gamma ray observations in the 0.05 to 10 MeV energy range. A significant number of these observations shall be directed to the study of Active Galactic Nuclei (AGN). The characteristics of the OSSE instrument and the current observation plans are discussed. Examples of the scientific issues which OSSE expects to address are provided
The temperatures of dust-enshrouded AGNs
A high density of massive dark objects (MDOs), probably supermassive black
holes, in the centres of nearby galaxies has been inferred from recent
observations. There are various indications that much of the accretion
responsible for producing these objects took place in dust-enshrouded active
galactic nuclei (AGNs). If so, then measurements of the intensity of background
radiation and the source counts in the far-infrared and submillimetre wavebands
constrain the temperature of dust in these AGNs. An additional constraint comes
from the hard X-ray background, if this is produced by accretion. One
possibility is that the dust shrouds surrounding the accreting AGNs are cold,
about 30 K. In this event, the dusty AGNs could be some subset of the
population of luminous distant sources discovered at 850 microns using the
SCUBA array on the JCMT, as proposed by Almaini et al. (1999). An alternative
is that the dust shrouds surrounding the accreting AGNs are much hotter (> 60
K). These values are closer to the dust temperatures of a number of
well-studied low-redshift ultraluminous galaxies that are thought to derive
their power from accretion. If the local MDO density is close to the maximum
permitted, then cold sources cannot produce this density without the
submillimetre background being overproduced if they accrete at high radiative
efficiency, and thus a hot population is required. If the dust-enshrouded
accretion occurred at similar redshifts to that taking place in unobscured
optical quasars, then a significant fraction of the far-infrared background
radiation measured by COBE at 140 microns, but very little of the
submilllimetre background at 850 microns, may have been produced by hot
dust-enshrouded AGNs which may have already been seen in recent X-ray surveys.Comment: MNRAS in pres
Open Inflation Without False Vacua
We show that within the framework of a definite proposal for the initial
conditions for the universe, the Hartle-Hawking `no boundary' proposal, open
inflation is generic and does not require any special properties of the
inflaton potential. In the simplest inflationary models, the semiclassical
approximation to the Euclidean path integral and a minimal anthropic condition
lead to . This number may be increased in models with
more fields or extra dimensions.Comment: 10 pages, compressed and RevTex file with one postscript figure,
openlet1.p
Order of acquisition in learning perceptual categories: a laboratory analogue of the age-of-acquisition effect?
In the age-of-acquisition (AoA) effect, an advantage for recognition and production is found for items learned early in life, as compared with items learned later. In this laboratory analogue, participants learned to categorize novel random checkerboard stimuli. Some stimuli were presented from the onset of training; others were introduced later. At test, when early and late stimuli had equal cumulative frequency, early stimuli were classified significantly more quickly. Because stimuli were randomly assigned to be introduced either early or late, we can conclude that early stimuli were categorized more quickly because of their order of acquisition. This finding suggests that age- or order-of-acquisition effects are a general property of any learning system
Preliminary flight assessment of the X-29A advanced technology demonstrator
Several new technologies integrated on the X-29A advanced technology demonstrator are being evaluated for the next generation of fighter aircraft. Some of the most noteworthy ones are the forward-swept wing, digital fly-by-wire flight control system, close-coupled wing-canard configuration, aeroelastically tailored composite wing skins, three-surface pitch control configuration, and a highly unstable airframe. The expansion of the aircraft 1-g and maneuver flight envelopes was recently completed over a two-year period in 84 flights. Overall flight results confirmed the viability of the aircraft design, and good agreement with preflight predictions was obtained. The individual technologies' operational workability and performance were confirmed. This paper deals with the flight test results and the preliminary evaluation of the X-29A design and technologies. A summary of the primary technical findings in structural static loads, structural dynamic characteristics, flight control system characteristics, aerodynamic stability and control, and aerodynamic performance is presented
The electron-phonon processes of the nitrogen-vacancy center in diamond
Applications of negatively charged nitrogen-vacancy center in diamond exploit
the center's unique optical and spin properties, which at ambient temperature,
are predominately governed by electron-phonon interactions. Here, we
investigate these interactions at ambient and elevated temperatures by
observing the motional narrowing of the center's excited state spin resonances.
We determine that the center's Jahn-Teller dynamics are much slower than
currently believed and identify the vital role of symmetric phonon modes. Our
results have pronounced implications for center's diverse applications
(including quantum technology) and for understanding its fundamental
properties.Comment: 5 pages, 4 figure
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