2,066 research outputs found
Highly efficient, dual state emission from an organic semiconductor
We report highly efficient, simultaneous fluorescence and phosphorescence
(74% yield) at room temperature from a single molecule ensemble of (BzP)PB
dispersed into a polymer host. The slow phosphorescence (208 ms lifetime) is
very efficient (50%) at room temperature and only possible because the
non-radiative rate for the triplet state is extremely low. The ability of an
organic molecule to function as an efficient dual state emitter at room
temperature is unusual and opens new fields of applications including the use
as broadband down-conversion emitters, optical sensors and attenuators, exciton
probes, and spin-independent intermediates for F\"orster resonant energy
transfer
A Late-Time Flattening of Afterglow Light Curves
We present a sample of radio afterglow light curves with measured decay
slopes which show evidence for a flattening at late times compared to optical
and X-ray decay indices. The simplest origin for this behavior is that the
change in slope is due to a jet-like outflow making a transition to
sub-relativistic expansion. This can explain the late-time radio light curves
for many but not all of the bursts in the sample. We investigate several
possible modifications to the standard fireball model which can flatten
late-time light curves. Changes to the shock microphysics which govern particle
acceleration, or energy injection to the shock (either radially or azimuthally)
can reproduce the observed behavior. Distinguishing between these different
possibilities will require simultaneous optical/radio monitoring of afterglows
at late times.Comment: ApJ, submitte
Automatic wheeze detection based on auditory modelling
Automatic wheeze detection has several potential benefits compared with reliance on human auscultation: it is experience independent, an automated historical record can easily be kept, and it allows quantification of wheeze severity. Previous attempts to detect wheezes automatically have had partial success but have not been reliable enough to become widely accepted as a useful tool. In this paper an improved algorithm for automatic wheeze detection based on auditory modelling is developed, called the frequency- and duration-dependent threshold algorithm. The mean frequency and duration of each wheeze component are obtained automatically. The detected wheezes are marked on a spectrogram. In the new algorithm, the concept of a frequency- and duration-dependent threshold for wheeze detection is introduced. Another departure from previous work is that the threshold is based not on global power but on power corresponding to a particular frequency range. The algorithm has been tested on 36 subjects, 11 of whom exhibited characteristics of wheeze. The results show a marked improvement in the accuracy of wheeze detection when compared with previous algorithms
The afterglows of gamma-ray bursts
Gamma-ray burst astronomy has undergone a revolution in the last three years, spurred by the discovery of fading long-wavelength counterparts. We now know that at least the long duration GRBs lie at cosmological distances with estimated electromagnetic energy release of 10^51–10^53 erg, making these the brightest explosions in the Universe. In this article we review the current observational state, beginning with the statistics of X-ray, optical, and radio afterglow detections. We then discuss the insights these observations have given to the progenitor population, the energetics of the GRB events, and the physics of the afterglow emission. We focus particular attention on the evidence linking GRBs to the explosion of massive stars. Throughout, we identify remaining puzzles and uncertainties, and emphasize promising observational tools for addressing them. The imminent launch of HETE-2 and the increasingly sophisticated and coordinated ground-based and space-based observations have primed this field for fantastic growth
The afterglows of gamma-ray bursts
Gamma-ray burst astronomy has undergone a revolution in the last three years, spurred by the discovery of fading long-wavelength counterparts. We now know that at least the long-duration GRBs lie at cosmological distances with estimated electromagnetic energy release of 10^51–10^53 erg, making these the brightest explosions in the Universe. In this article we review the current observational state, beginning with the statistics of X-ray, optical, and radio afterglow detections. We then discuss the insights these observations have given to the progenitor population, the energetics of the GRB events, and the physics of the afterglow emission. We focus particular attention on the evidence linking GRBs to the explosion of massive stars. Throughout, we identify remaining puzzles and uncertainties, and emphasize promising observational tools for addressing them. The imminent launch of HETE-2 and the increasingly sophisticated and coordinated ground-based and space-based observations have primed this field for fantastic growth
Effect of Vacuum on the Performance of the Flame Ionization Detector Used for Vacuum-Outlet Gas Chromatography
Vacuum-outlet operation of short fused-silica open tubular columns for gas chromatography provides benefits to analysis speed by increasing the optimum velocity while minimizing the loss in resolution. Vacuum-outlet operation of a column with a gas chromatographic detector necessitates that the detector also be under vacuum. Simple modifications were made to a gas chromatograph-flame ionization detector (GC-FID) for vacuum-inlet and vacuum-outlet operation. The vacuum-inlet system was operated in the splitless mode to allow for efficient sample loading prior to operation at reduced pressures. The goal of FID operation at 100 torr or less was achieved by using oxygen in place of air and optimizing the gas flows under vacuum to maintain a stable flame at pressures as low as 46 torr. An outlet pressure of 85 torr with optimized gas flows allowed for routine operation of the FID without solvent flame-out. It was discovered that the sensitivity is enhanced compared to atmospheric operation over a range of outlet pressures from approximately 200 to 400 torr; however, operation of the FID at the lowest possible pressures decreases the analytical sensitivity due to both the outlet pressure and the absence of helium makeup gas
The afterglows of gamma-ray bursts
Gamma-ray burst astronomy has undergone a revolution in the last three years, spurred by the discovery of fading long-wavelength counterparts. We now know that at least the long duration GRBs lie at cosmological distances with estimated electromagnetic energy release of 10^51–10^53 erg, making these the brightest explosions in the Universe. In this article we review the current observational state, beginning with the statistics of X-ray, optical, and radio afterglow detections. We then discuss the insights these observations have given to the progenitor population, the energetics of the GRB events, and the physics of the afterglow emission. We focus particular attention on the evidence linking GRBs to the explosion of massive stars. Throughout, we identify remaining puzzles and uncertainties, and emphasize promising observational tools for addressing them. The imminent launch of HETE-2 and the increasingly sophisticated and coordinated ground-based and space-based observations have primed this field for fantastic growth. This overview is a combined write-up of talks given at this conference and in NASA's Goddard Space Flight Center
Cosmological Uses of Gamma-Ray Bursts
Studies of the cosmic gamma-ray bursts (GRBs) and their host galaxies are
starting to provide interesting or even unique new insights in observational
cosmology. GRBs represent a new way of identifying a population of star-forming
galaxies at cosmological redshifts. GRB hosts are broadly similar to the normal
field galaxy populations at comparable redshifts and magnitudes, and indicate
at most a mild luminosity evolution out to z ~ 1.5 - 2. GRB optical afterglows
seen in absorption provide a powerful new probe of the ISM in dense, central
regions of their host galaxies, complementary to the traditional studies using
QSO absorbers. Some GRB hosts are heavily obscured, and provide a new way to
select a population of cosmological sub-mm sources, and a novel constraint on
the total obscured fraction of star formation over the history of the universe.
Finally, detection of GRB afterglows at z > 6 may provide a unique way to probe
the primordial star formation, massive IMF, early IGM, and chemical enrichment
at the end of the cosmic reionization era.Comment: An invited review, to appear in: "Gamma-Ray Bursts in the Afterglow
Era: 3rd Workshop", ASPCS, in press; LaTeX file, 8 pages, 1 eps figure, style
files include
The Cosmic Gamma-Ray Bursts
Cosmic gamma-ray bursts are one of the great frontiers of astrophysics today.
They are a playground of relativists and observers alike. They may teach us
about the death of stars and the birth of black holes, the physics in extreme
conditions, and help us probe star formation in the distant and obscured
universe. In this review we summarise some of the remarkable progress in this
field over the past few years. While the nature of the GRB progenitors is still
unsettled, it now appears likely that at least some bursts originate in
explosions of very massive stars, or at least occur in or near the regions of
massive star formation. The physics of the burst afterglows is reasonably well
understood, and has been tested and confirmed very well by the observations.
Bursts are found to be beamed, but with a broad range of jet opening angles;
the mean gamma-ray energies after the beaming corrections are ~ 10^51 erg.
Bursts are associated with faint ~ 25 mag) galaxies at cosmological
redshifts, with ~ 1. The host galaxies span a range of luminosities and
morphologies, but appear to be broadly typical for the normal, actively
star-forming galaxy populations at comparable redshifts and magnitudes. Some of
the challenges for the future include: the nature of the short bursts and
possibly other types of bursts and transients; use of GRBs to probe the
obscured star formation in the universe, and possibly as probes of the very
early universe; and their detection as sources of high-energy particles and
gravitational waves.Comment: An invited review, to appear in: Proc. IX Marcel Grossmann Meeting,
eds. V. Gurzadyan, R. Jantzen, and R. Ruffini, Singapore: World Scientific,
in press (2001); Latex file, 33 pages, 22 eps figures, style files include
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