12,531 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
First Principles Study of Work Functions of Double Wall Carbon Nanotubes
Using first-principles density functional calculations, we investigated work
functions (WFs) of thin double-walled nanotubes (DWNTs) with outer tube
diameters ranging from 1nm to 1.5nm. The results indicate that work function
change within this diameter range can be up to 0.5 eV, even for DWNTs with same
outer diameter. This is in contrast with single-walled nanotubes (SWNTs) which
show negligible WF change for diameters larger than 1nm. We explain the WF
change and related charge redistribution in DWNTs using charge equilibration
model (CEM). The predicted work function variation of DWNTs indicates a
potential difficulty in their nanoelectronic device applications.Comment: 11 pages, 3 figures, to appear as rapid communication on Physical
Review
QCD corrections to polarization of J/\psi and \Upsilon at Fermilab Tevatron and CERN LHC
In this work, we present more detail of the calculation on the NLO QCD
corrections to polarization of direct J/psi production via color singlet at
Tevatron and LHC, as well as the results for Upsilon for the first time. Our
results show that the J/psi polarization status drastically changes from
transverse polarization dominant at LO into longitudinal polarization dominant
in the whole range of the transverse momentum of J/psi when the NLO
corrections are counted. For Upsilon production, the p_t distribution of the
polarization status behaves almost the same as that for J/psi except that the
NLO result is transverse polarization at small p_t range. Although the
theoretical evaluation predicts a larger longitudinal polarization than the
measured value at Tevatron, it may provide a solution towards the previous
large discrepancy for J/psi and Upsilon polarization between theoretical
predication and experimental measurement, and suggests that the next important
step is to calculate the NLO corrections to hadronproduction of color octet
state J/psi^(8) and Upsilon^(8). Our calculations are performed in two ways,
namely we do and do not analytically sum over the polarizations, and then check
them with each other.Comment: 12 pages, 12 figures, two columns, use revtex4; to appear in PR
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