12,156 research outputs found
Gamma-ray Novae: Rare or Nearby?
Classical Novae were revealed as a surprise source of gamma-rays in Fermi LAT
observations. During the first 8 years since the LAT was launched, 6 novae in
total have been detected to > 5 sigma in gamma-rays, in contrast to the 69
discovered optically in the same period. We attempt to resolve this discrepancy
by assuming all novae are gamma-ray emitters, and assigning peak one-day fluxes
based on a flat distribution of the known emitters to a simulated population.
To determine optical parameters, the spatial distribution and magnitudes of
bulge and disc novae in M31 are scaled to the Milky Way, which we approximate
as a disc with a 20 kpc radius and elliptical bulge with semi major axis 3 kpc
and axis ratios 2:1 in the xy plane. We approximate Galactic reddening using a
double exponential disc with vertical and radial scale heights of r_d = 5 kpc
and z_d = 0.2 kpc, and demonstrate that even such a rudimentary model can
easily reproduce the observed fraction of gamma-ray novae, implying that these
apparently rare sources are in fact nearby and not intrinsically rare. We
conclude that classical novae with m_R < 12 and within ~8 kpc are likely to be
discovered in gamma-rays using the Fermi LAT.Comment: Accepted by MNRAS, 10 pages, 7 figure
Acoustically excited heated jets. 3: Mean flow data
This is Part 3 of a report on the excitability of heated jets under the influence of acoustic excitation. The effects of upstream internal acoustic excitation on jet mixing were described in Part 1. Part 2 described the effects of external excitation on flow mixing. Part 3 contains quantitative results from the measurements of mean Mach number and temperature and consists of radial profiles and centerline distributions measured at selected jet operating conditions for internally excited and unexcited jets. The mean flow data are presented in both graphical and tabulated forms. For the sake of completeness, this part contains temperature probe calibration curves also
Acoustically excited heated jets. 1: Internal excitation
The effects of relatively strong upstream acoustic excitation on the mixing of heated jets with the surrounding air are investigated. To determine the extent of the available information on experiments and theories dealing with acoustically excited heated jets, an extensive literature survey was carried out. The experimental program consisted of flow visualization and flowfield velocity and temperature measurements for a broad range of jet operating and flow excitation conditions. A 50.8-mm-diam nozzle was used for this purpose. Parallel to the experimental study, an existing theoretical model of excited jets was refined to include the region downstream of the jet potential core. Excellent agreement was found between theory and experiment in moderately heated jets. However, the theory has not yet been confirmed for highly heated jets. It was found that the sensitivity of heated jets to upstream acoustic excitation varies strongly with the jet operating conditions and that the threshold excitation level increases with increasing jet temperature. Furthermore, the preferential Strouhal number is found not to change significantly with a change of the jet operating conditions. Finally, the effects of the nozzle exit boundary layer thickness appear to be similar for both heated and unheated jets at low Mach numbers
Acoustically excited heated jets. 2: In search of a better understanding
The second part of a three-part report on the effects of acoustic excitation on jet mixing includes the results of an experimental investigation directed at resolving the question of poor excitability of some of the heated jets. The theoretical predictions discussed in Part 1 are examined to find explanations for the observed discrepancies between the measured and the predicted results. Additional testing was performed by studying the self excitation of the shock containing hot jets and also by exciting the jet by sound radiated through source tubes located externally around the periphery of the jet. The effects of nozzle-exit boundary layer conditions on jet excitability was also investigated. It is concluded that high-speed, heated jet mixing rates and consequently also the jet excitability strongly depends on nozzle exit boundary layer conditions
Nuclear imaging in the diagnosis of primary aldosteronism.
PURPOSE OF REVIEW: Primary aldosteronism is increasingly recognized as a common secondary cause of hypertension. Successful demonstration of a unilateral cause (e.g. a classical 'Conn's adenoma') offers the potential for curative adrenalectomy. Adrenal vein sampling (AVS), in conjunction with cross-sectional imaging, remains the 'gold standard' for distinguishing unilateral and bilateral disease, but is technically demanding and frequently unsuccessful or inconclusive. As such, alternative strategies for lateralization, including nuclear medicine techniques, are being developed and brought into clinical practice. RECENT FINDINGS: Metomidate, a potent ligand of CYP11B1 and CYP11B2, can be C11H3-labelled as a PET tracer and has been shown to offer a rapid noninvasive alternative to AVS for localizing unilateral aldosterone-producing adenomas. SUMMARY: Increasing experience with 11C-metomidate PET-CT supports its use as an adjunct to AVS when this has failed, is ambiguous, or cannot be undertaken.A.S.P. and M.G. are supported by the National Institute
for Health Research Cambridge Biomedical Research
Centre. M.J.B. is a National Institute of Health Research
Senior Investigator.This is the final published version. It first appeared at http://journals.lww.com/co-endocrinology/Fulltext/2015/06000/Nuclear_imaging_in_the_diagnosis_of_primary.3.aspx
Lewis Research Center spin rig and its use in vibration analysis of rotating systems
The Lewis Research Center spin rig was constructed to provide experimental evaluation of analysis methods developed under the NASA Engine Structural Dynamics Program. Rotors up to 51 cm (20 in.) in diameter can be spun to 16,000 rpm in vacuum by an air motor. Vibration forcing functions are provided by shakers that apply oscillatory axial forces or transverse moments to the shaft, by a natural whirling of the shaft, and by an air jet. Blade vibration is detected by strain gages and optical blade-tip motion sensors. A variety of analogy and digital processing equipment is used to display and analyze the signals. Results obtained from two rotors are discussed. A 56-blade compressor disk was used to check proper operation of the entire spin rig system. A special two-blade rotor was designed and used to hold flat and twisted plates at various setting and sweep angles. Accurate Southwell coefficients have been obtained for several modes of a flat plate oriented parallel to the plane of rotation
Pilot interaction with automated airborne decision making systems
The use of advanced software engineering methods (e.g., from artificial intelligence) to aid aircraft crews in procedure selection and execution is investigated. Human problem solving in dynamic environments as effected by the human's level of knowledge of system operations is examined. Progress on the development of full scale simulation facilities is also discussed
The Shortest Period Detached Binary White Dwarf System
We identify SDSS J010657.39-100003.3 (hereafter J0106-1000) as the shortest
period detached binary white dwarf (WD) system currently known. We targeted
J0106-1000 as part of our radial velocity program to search for companions
around known extremely low-mass (ELM, ~ 0.2 Msol) WDs using the 6.5m MMT. We
detect peak-to-peak radial velocity variations of 740 km/s with an orbital
period of 39.1 min. The mass function and optical photometry rule out a
main-sequence star companion. Follow-up high-speed photometric observations
obtained at the McDonald 2.1m telescope reveal ellipsoidal variations from the
distorted primary but no eclipses. This is the first example of a tidally
distorted WD. Modeling the lightcurve, we constrain the inclination angle of
the system to be 67 +- 13 deg. J0106-1000 contains a pair of WDs (0.17 Msol
primary + 0.43 Msol invisible secondary) at a separation of 0.32 Rsol. The two
WDs will merge in 37 Myr and most likely form a core He-burning single subdwarf
star. J0106-1000 is the shortest timescale merger system currently known. The
gravitational wave strain from J0106-1000 is at the detection limit of the
Laser Interferometer Space Antenna (LISA). However, accurate ephemeris and
orbital period measurements may enable LISA to detect J0106-1000 above the
Galactic background noise.Comment: MNRAS Letters, in pres
Are trainee clinical psychologists fit for delivering positive behavioural support on qualification?
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