1,053 research outputs found
Emergence of a confined state in a weakly bent wire
In this paper we use a simple straightforward technique to investigate the
emergence of a bound state in a weakly bent wire. We show that the bend behaves
like an infinitely shallow potential well, and in the limit of small bending
angle and low energy the bend can be presented by a simple 1D delta function
potential.Comment: 4 pages, 3 Postscript figures (uses Revtex); added references and
rewritte
Scatterer that leaves "footprints" but no "fingerprints"
We calculate the exact transmission coefficient of a quantum wire in the
presence of a single point defect at the wire's cut-off frequencies. We show
that while the conductance pattern (i.e., the scattering) is strongly affected
by the presence of the defect, the pattern is totally independent of the
defect's characteristics (i.e., the defect that caused the scattering cannot be
identified from that pattern).Comment: 4 pages, 3 figure
Afterglow Observations Shed New Light on the Nature of X-ray Flashes
X-ray flashes (XRFs) and X-ray rich gamma-ray bursts (XRGRBs) share many
observational characteristics with long duration GRBs, but the reason for which
their prompt emission peaks at lower photon energies, , is still under
debate. Although many different models have been invoked in order to explain
the lower values, their implications for the afterglow emission were not
considered in most cases, mainly because observations of XRF afterglows have
become available only recently. Here we examine the predictions of the various
XRF models for the afterglow emission, and test them against the observations
of XRF 030723 and XRGRB 041006, the events with the best monitored afterglow
light curves in their respective class. We show that most existing XRF models
are hard to reconcile with the observed afterglow light curves, which are very
flat at early times. Such light curves are, however, naturally produced by a
roughly uniform jet with relatively sharp edges that is viewed off-axis (i.e.
from outside of the jet aperture). This type of model self consistently
accommodates both the observed prompt emission and the afterglow light curves
of XRGRB 041006 and XRF 030723, implying viewing angles from the
jet axis of and ,
respectively, where deg is the jet half-opening angle. This
suggests that GRBs, XRGRBs and XRFs are intrinsically similar relativistic jets
viewed from different angles, corresponding to
of less than 1, between 1 and a few, and more than a few, respectively, where
is the Lorentz factor. Future observations with Swift could help test
this unification scheme in which GRBs, XRGRBs and XRFs share the same basic
physics and differ only by their orientation relative to our line of sight.Comment: some references added, small typos corrected, and the important role
of HETE II emphasize
The Prompt Gamma-Ray and Afterglow Energies of Short-Duration Gamma-Ray Bursts
I present an analysis of the gamma-ray and afterglow energies of the complete
sample of 17 short duration GRBs with prompt X-ray follow-up. I find that 80%
of the bursts exhibit a linear correlation between their gamma-ray fluence and
the afterglow X-ray flux normalized to t=1 d, a proxy for the kinetic energy of
the blast wave ($F_{X,1}~F_{gamma}^1.01). An even tighter correlation is
evident between E_{gamma,iso} and L_{X,1} for the subset of 13 bursts with
measured or constrained redshifts. The remaining 20% of the bursts have values
of F_{X,1}/F_{gamma} that are suppressed by about three orders of magnitude,
likely because of low circumburst densities (Nakar 2007). These results have
several important implications: (i) The X-ray luminosity is generally a robust
proxy for the blast wave kinetic energy, indicating nu_X>nu_c and hence a
circumburst density n>0.05 cm^{-3}; (ii) most short GRBs have a narrow range of
gamma-ray efficiency, with ~0.85 and a spread of 0.14 dex; and
(iii) the isotropic-equivalent energies span 10^{48}-10^{52} erg. Furthermore,
I find tentative evidence for jet collimation in the two bursts with the
highest E_{gamma,iso}, perhaps indicative of the same inverse correlation that
leads to a narrow distribution of true energies in long GRBs. I find no clear
evidence for a relation between the overall energy release and host galaxy
type, but a positive correlation with duration may be present, albeit with a
large scatter. Finally, I note that the outlier fraction of 20% is similar to
the proposed fraction of short GRBs from dynamically-formed neutron star
binaries in globular clusters. This scenario may naturally explain the
bimodality of the F_{X,1}/F_{gamma} distribution and the low circumburst
densities without invoking speculative kick velocities of several hundred km/s.Comment: Submitted to ApJ; 9 pages, 2 figures, 1 tabl
Cell adhesion molecules and hyaluronic acid as markers of inflammation, fibrosis and response to antiviral therapy in chronic hepatitis C patients.
OBJECTIVE: Cell adhesion molecules (intracellular adhesion molecule-1 (ICAM-1), vascular cell adhesion molecule-1 (VCAM-1)) and hyaluronic acid, markers of inflammation and fibrosis were monitored in hepatitis C patients to determine whether changes in plasma levels, during antiviral treatment, can predict long-term response to therapy. METHODS: In 55 patients with chronic hepatitis C virus (HCV), 33 treated with interferon (IFN) and 22 treated with IFN + ribavirin, sera was collected prior to treatment, at 3 + 6 months of therapy and 6 months post-treatment. Levels of ICAM-1, VCAM-1 and hyaluronic acid were correlated with alanine aminotransferase levels, HCV-RNA-polymerase chain reaction status and histological fibrosis scoring. RESULTS: A decrease in ICAM-1 levels at 3 and 6 months of therapy, compared with pretreatment levels, was observed in responders to IFN + ribavirin therapy but this decrease in ICAM-1 levels was not evident following cessation of treatment. Hyaluronic acid levels, in both treatment groups, did not differ significantly between responders and non-responders. Hyaluronic acid levels did correlate, significantly, with degree of fibrosis whereas VCAM-1 levels were marginally increased only in patients with moderate (grade III) fibrosis. CONCLUSIONS: Monitoring of VCAM-1 and hyaluronic acid, during antiviral therapy, does not differentiate between responders and non-responders. A decrease in ICAM-1 levels during IFN + ribavirin treatment is associated with response to therapy, and its efficacy in predicting long-term response should be further substantiated
An Off-Axis Model for GRB 031203
The low luminosity radio emission of the unusually faint GRB 031203 has been
argued to support the idea of a class of intrinsically sub-energetic gamma-ray
bursts (GRBs), currently comprising two members. While low energy GRBs probably
exist, we show that the collective prompt and multiwavelength observations of
the afterglow of GRB 031203 do not necessarily require a sub-energetic nature
for that event. In fact, the data are more consistent with a typical, powerful
GRB seen at an angle of about twice the opening angle of the central jet. The
(redshift corrected) peak energy, E_p, of GRB 031203 then becomes ~ 2 MeV,
similar to many other GRBs.Comment: 4 pages, 3 figures; ApJL in pres
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