113,793 research outputs found
GRB990123: The Case for Saturated Comptonization
The recent simultaneous detection of optical, X-ray and gamma-ray photons
from GRB990123 during the burst provides the first broadband multi-wavelength
characterization of the burst spectrum and evolution. Here we show that a
direct correlation exists between the time-varying gamma-ray spectral shape and
the prompt optical emission. This combined with the unique signatures of the
time-resolved spectra of GRB990123 convincingly supports earlier predictions of
the saturated Comptonization model. Contrary to other suggestions, we find that
the entire continuum from optical to gamma-rays can be generated from a single
source of leptons (electrons and pairs). The optical flux only appears to lag
the gamma-ray flux due to the high initial Thomson depth of the plasma. Once
the plasma has completely thinned out, the late time afterglow behavior of our
model is the same as in standard models based on the Blandford-McKee (1976)
solution.Comment: 10 pages, including 3 figures and 1 table, submitted to The
Astrophysical Journal Letter
Mathematical and computer modeling of electro-optic systems using a generic modeling approach
The conventional approach to modelling electro-optic sensor systems is to develop separate models for individual systems or classes of system, depending on the detector technology employed in the sensor and the application. However, this ignores commonality in design and in components of these systems. A generic approach is presented for modelling a variety of sensor systems operating in the infrared waveband that also allows systems to be modelled with different levels of detail and at different stages of the product lifecycle. The provision of different model types (parametric and image-flow descriptions) within the generic framework can allow valuable insights to be gained
Photometry of two unusual A supergiant systems in the Small Magellanic Cloud
We present multiwavelength broadband photometry and V, I time resolved
photometry for two variable bright stars in the SMC, OGLE004336.91-732637.7
(SMC-SC3) and OGLE004633.76-731204.3 (SMC-SC4). The light curves span 12 years
and show long-term periodicities (SMC-SC3) and modulated eclipses (SMC-SC4)
that are discussed in terms of wide-orbit intermediate mass interacting
binaries and associated envelopes. SMC-SC3 shows a primary period of 238.1 days
along with a complicated waveform suggesting ellipsoidal variablity influenced
by an eccentric orbit. This star also shows a secondary variability with an
unstable periodicity that has a mean value of 15.3 days. We suggest this could
be associated with nonradial pulsations.Comment: To be published in Publications of the Astronomical Society of the
Pacific (PASP)
The Energy Dependence of the Aperiodic Variability for Cygnus X-1, GX 339-4, GRS 1758-258, & 1E 1740.7-2942
Using the data from the Rossi X-ray Timing Explorer (RXTE), we report the
different energy dependence of the variability of the four persistent hard
X-ray sources in the low-hard state: Cygnus X-1, GX 339-4, GRS 1758-258 and 1E
1740.7-2942. Cygnus X-1 is found to have a flatter power density spectrum (PDS)
shape at higher energies. The other three sources have energy independent PDS
shapes. The energy dependence of the overall variability (the integrated rms
amplitude) varies from source to source and from observation to observation.
1E~1740.7-2942, for example, has a variability generally increasing with energy
while GX 339-4 has a decreasing variability. A general trend is found in the
four sources that the integrated rms amplitude anti-correlates with the X-ray
flux. We compare these distinct energy dependent behaviors with several
emission models. None of the models can fully explain all the features that we
have found.Comment: 18 pages, 6 figures. Accepted for publication in Ap
Quantum simulation of correlated-hopping models with fermions in optical lattices
By using a modulated magnetic field in a Feshbach resonance for ultracold
fermionic atoms in optical lattices, we show that it is possible to engineer a
class of models usually referred to as correlated-hopping models. These models
differ from the Hubbard model in exhibiting additional density-dependent
interaction terms that affect the hopping processes. In addition to the
spin-SU(2) symmetry, they also possess a charge-SU(2) symmetry, which opens the
possibility of investigating the -pairing mechanism for superconductivity
introduced by Yang for the Hubbard model. We discuss the known solution of the
model in 1D (where states have been found in the degenerate manifold of
the ground state) and show that, away from the integrable point, quantum Monte
Carlo simulations at half filling predict the emergence of a phase with
coexisting incommensurate spin and charge order.Comment: 10 pages, 9 figure
A Thermal-Nonthermal Inverse Compton Model for Cyg X-1
Using Monte Carlo methods to simulate the inverse Compton scattering of soft
photons, we model the spectrum of the Galactic black hole candidate Cyg X-1,
which shows evidence of a nonthermal tail extending beyond a few hundred keV.
We assume an ad hoc sphere of leptons, whose energy distribution consists of a
Maxwellian plus a high energy power-law tail, and inject 0.5 keV blackbody
photons. The spectral data is used to constrain the nonthermal plasma fraction
and the power-law index assuming a reasonable Maxwellian temperature and
Thomson depth. A small but non-negligible fraction of nonthermal leptons is
needed to explain the power-law tail.Comment: 5 pages, 2 PostScript figure, uses aipproc.sty, to appear in
Proceedings of Fourth Compton Symposiu
Different regimes of Forster energy transfer between an epitaxial quantum well and a proximal monolayer of semiconductor nanocrystals
We calculate the rate of non-radiative, Forster-type energy transfer (ET)
from an excited epitaxial quantum well (QW) to a proximal monolayer of
semiconductor nanocrystal quantum dots (QDs). Different electron-hole
configurations in the QW are considered as a function of temperature and
excited electron-hole density. A comparison of the theoretically determined ET
rate and QW radiative recombination rate shows that, depending on the specific
conditions, the ET rate is comparable to or even greater than the radiative
recombination rate. Such efficient Forster ET is promising for the
implementation of ET-pumped, nanocrystal QD-based light emitting devices.Comment: 14 pages, 4 figure
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