16,625 research outputs found
A comprehensive analysis of Swift/XRT data: I. Apparent spectral evolution of GRB X-ray tails
An early steep decay component following the prompt GRBs is commonly observed
in {\em Swift} XRT light curves, which is regarded as the tail emission of the
prompt gamma-rays. Prompted by the observed strong spectral evolution in the
tails of GRBs 060218 and 060614, we present a systematic time-resolved spectral
analysis for the {\em Swift} GRB tails detected between 2005 February and 2007
January. We select a sample of 44 tails that are bright enough to perform
time-resolved spectral analyses. Among them 11 tails are smooth and without
superimposing significant flares, and their spectra have no significant
temporal evolution. We suggest that these tails are dominated by the curvature
effect of the prompt gamma-rays due to delay of propagation of photons from
large angles with respect to the line of sight . More interestingly, 33 tails
show clear hard-to-soft spectral evolution, with 16 of them being smooth tails
directly following the prompt GRBs,while the others being superimposed with
large flares. We focus on the 16 clean, smooth tails and consider three toy
models to interpret the spectral evolution. The curvature effect of a
structured jet and a model invoking superposition of the curvature effect tail
and a putative underlying soft emission component cannot explain all the data.
The third model, which invokes an evolving exponential spectrum, seems to
reproduce both the lightcurve and the spectral evolution of all the bursts,
including GRBs 060218 and 060614. More detailed physical models are called for
to understand the apparent evolution effect.Comment: 13 pages in emulateapj style,6 figures, 1 table, expanded version,
matched to published version, ApJ, 2007, in press. This is the first paper of
a series. Paper II see arXiv:0705.1373 (ApJ,2007, in press
The gain and carrier density in semiconductor lasers under steady-state and transient conditions
The carrier distribution functions in a semiconductor crystal in the presence of a strong optical field are obtained. These are used to derive expressions for the gain dependence on the carrier density and on the optical intensity-the gain suppression effect. A general expression for high-order nonlinear gain coefficients is obtained. This formalism is used to describe the carrier and power dynamics in semiconductor lasers above and below threshold in the static and transient regimes
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