3,351 research outputs found
A Search for Hard X-Ray Emission from Globular Clusters - Constraints from BATSE
We have monitored a sample of 27 nearby globular clusters in the hard X-ray
band (20-120 keV) for approximately 1400 days using the BATSE instrument on
board the Compton Gamma-Ray Observatory. Globular clusters may contain a large
number of compact objects (e.g., pulsars or X-ray binaries containing neutron
stars) which can produce hard X-ray emission. Our search provides a sensitive
(~50 mCrab) monitor for hard X-ray transient events on time scales of >1 day
and a means for observing persistent hard X-ray emission. We have discovered no
transient events from any of the clusters and no persistent emission. Our
observations include a sensitive search of four nearby clusters containing dim
X-ray sources: 47 Tucanae, NGC 5139, NGC 6397, and NGC 6752. The non-detection
in these clusters implies a lower limit for the recurrence time of transients
of 2 to 6 years for events with luminosities >10^36 erg s^-1 (20-120 keV) and
~20 years if the sources in these clusters are taken collectively. This
suggests that the dim X-ray sources in these clusters are not transients
similar to Aql~X-1. We also place upper limits on the persistent emission in
the range 2-10*10^34 erg s^-1 (2 sigma, 20-120 keV) for these four clusters.
For 47 Tuc the upper limit is more sensitive than previous measurements by a
factor of 3. We find a model dependent upper limit of 19 isolated millisecond
pulsars (MSPs) producing gamma-rays in 47 Tuc, compared to the 11 observed
radio MSPs in this cluster.Comment: 20 pages; accepted, ApJ; uu encoded tar file; 7 figure
Effective Bandgap Shrinkage in GaAs
Electrical measurements of the equilibrium np product (n2ie) in heavily doped nâ and pâGaAs were performed. The n2ieDproduct (where D is the diffusivity) was measured by fitting the collector currentâvoltage characteristic of a homojunction bipolar transistor to an ideal diode equation modified to account for transport in thin base transistors.The n2ie product was then extracted from n2ieD by utilizing diffusivity results obtained with the zeroâfield timeâofâflight technique. Our results show significant effective bandâgap shrinkage in heavily doped pâGaAs, and very little effective bandâgap shrinkage in heavily doped nâGaAs. At extremely heavy dopings, an effective bandâgap widening is observed for both nâ and pâGaAs and is attributed to the effects of degeneracy
Unusual temperature dependence of band dispersion in Ba(Fe(1-x)Ru(x))2As2 and its consequences for antiferromagnetic ordering
We have performed detailed studies of the temperature evolution of the
electronic structure in Ba(Fe(1-x)Ru(x))2As2 using Angle Resolved Photoemission
Spectroscopy (ARPES). Surprisingly, we find that the binding energy of both
hole and electron bands changes significantly with temperature in pure and Ru
substituted samples. The hole and electron pockets are well nested at low
temperature in unsubstituted (BaFe2As2) samples, which likely drives the spin
density wave (SDW) and resulting antiferromagnetic order. Upon warming, this
nesting is degraded as the hole pocket shrinks and the electron pocket expands.
Our results demonstrate that the temperature dependent nesting may play an
important role in driving the antiferromagnetic/paramagnetic phase transition.Comment: 5 pages, 6 figure
Thermal velocity limits to diffusive electron transport in thinâbase np+n GaAs bipolar transistors
We present experimental evidence that minority electron transport across a thin, quasineutral p+ GaAs region is limited by the thermal velocity of the electrons rather than by conventional diffusive transport. A set of GaAs homojunction np+n transistors with base widths of 4000, 2000, 1000, and 500 Ă
was fabricated and characterized. The diffusive modelpredicts that the dc collector current of the 500âĂ
base width transistors should be eight times larger than the collector current of transistors with a 4000âĂ
âwide base. The experimental results, however, show only a factor of ~3.5 increase in collector current. The measured collector current versus base width characteristic agrees well with theoretical treatments of thinâbase transport. These new results present evidence of quasiballistic electron transport in p+ GaAs and have important implications for GaAs transistor design
Neurodevelopmental Outcomes at Two Years of Age for Premature Infants Diagnosed With Neonatal Obstructive Sleep Apnea
STUDY OBJECTIVES:
Neurocognitive deficits have been shown in school-aged children with sleep apnea. The effect of obstructive sleep apnea (OSA) on the neurodevelopmental outcome of preterm infants is unknown.
METHODS:
A retrospective chart review was performed for all preterm infants ( 1 event/h. Regression analyses were performed to find a relationship between PSG parameters and cognitive, language, and motor scores.
RESULTS:
Fifteen patients (males: n = 10) were eligible for the study. Median postmenstrual age at the time of the PSG was 41 weeks (37-46). Median AHI for the cohort was 17.4 events/h (2.2-41.3). Median cognitive, language, and motor scores were 90 (65-125), 89 (65-121), and 91 (61-112), respectively. Mean end-tidal CO2 (median 47 mm Hg [25-60]) negatively correlated with cognitive scores (P = .01) but did not significantly correlate with language or motor scores. AHI was not associated with cognitive, language, or motor scores.
CONCLUSIONS:
The median score for cognitive, language, and motor scores for preterm infants with neonatal OSA were within one standard deviation of the published norm. Mean end-tidal CO2, independent of AHI, may serve as a biomarker for predicting poor cognitive outcome in preterm infants with neonatal OSA
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