1,286 research outputs found
Universal conductance reduction in a quantum wire
Even a single point defect in a quantum wire causes a conductance reduction.
In this paper we prove (without any approximations) that for any point impurity
this conductance reduction in all the sub-bands is exactly 2e^2/h. Moreover, it
is shown that in the case of a surface defect, not only is the conductance
minimum independent of the defect characteristics, but the transmission matrix
also converges to universal (defect-independent) values. We also discuss
particle confinement between two arbitrarily weak point defects.Comment: 4 pages, 4 figures (Revtex
High Energy Emission from the Prompt Gamma-Ray Burst
We study the synchrotron and synchrotron self-Compton (SSC) emission from
internal shocks that are responsible for the prompt gamma-ray emission in GRBs,
and consider the relation between these two components, taking into account the
high energy (HE) cutoff due to pair production and Thomson scattering. We find
that in order for the peak energy of the synchrotron to be E_p\sim 300 keV with
a variability time t_v>1 ms, a Lorentz factor \Gamma<350 is needed, implying no
HE emission above \sim 30 MeV and the synchrotron component would dominate at
all energies. If we want both E_p\sim 300 keV and prompt HE emission up to 2
GeV, as detected by EGRET for GRB 940217, we need \Gamma\sim 600 and t_v\sim
0.1 ms, which might be resolved by super AGILE. If such prompt HE emission is
common in GRBs, as may be tested by GLAST, then for t_v\gtrsim 1 ms we need
\Gamma\gtrsim 350, which implies E_p\lesssim 100 keV. Therefore if X-ray
flashes are GRBs with high values of t_v and \Gamma, they should produce
\gtrsim 1 GeV emission. For an electron power law index p>2, the SSC component
dominates the emission above 100 MeV. Future observations by GLAST may help
determine the value of p and whether the HE emission is consistent with a
single power law (one component--the synchrotron, dominates) or has a break
where the \nuF_\nu slope turns from negative to positive, implying that the SSC
component becomes dominant above \sim 100 MeV. The HE emission is expected to
show similar variability and time structure to that of the soft gamma-ray
emission. Finally, we find that in order to see delayed HE emission from the
prompt GRB due to pair production with the cosmic IR background, extremely
small intergalactic magnetic fields (\lessim 10^{-22} G) are required.Comment: 11 pages, 1 figur
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
Gamma-Ray Burst afterglow scaling coefficients for general density profile
Gamma-ray burst (GRB) afterglows are well described by synchrotron emission
originating from the interaction between a relativistic blast wave and the
external medium surrounding the GRB progenitor. We introduce a code to
reconstruct spectra and light curves from arbitrary fluid configurations,
making it especially suited to study the effects of fluid flows beyond those
that can be described using analytical approximations. As a check and first
application of our code we use it to fit the scaling coefficients of
theoretical models of afterglow spectra. We extend earlier results of other
authors to general circumburst density profiles. We rederive the physical
parameters of GRB 970508 and compare with other authorsComment: 11 pages, 5 figures. Revised edition removes references to unphysical
chromatic break and adds appendix on hot region directly behind shoc
Letter of the guest editors of the special issue on “Music as a multimodal experience”.
This editorial introduces a collection of papers brought together under the heading of “Music as a Multimodal Experience.” This issue aims to bring to the foreground recent developments in our understanding of music as a multimodal experience
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
Synchrotron Self Absorption in GRB Afterglow
GRB afterglow is reasonably described by synchrotron emission from
relativistic blast waves at cosmological distances. We perform detailed
calculations taking into account the effect of synchrotron self absorption. We
consider emission from the whole region behind the shock front, and use the
Blandford McKee self similar solution to describe the fluid behind the shock.
We calculate the spectra and the observed image of a GRB afterglow near the
self absorption frequency and derive an accurate expression for
. We show that the image is rather homogeneous for , as
opposed to the bright ring at the outer edge and dim center, which appear at
higher frequencies. We compare the spectra we obtain to radio observations of
GRB970508. We combine the calculations of the spectra near the self absorption
frequency with other parts of the spectra and obtain revised estimates for the
physical parameters of the burst: , ,
, . These estimates are different by up to two
orders of magnitude than the estimates based on an approximate spectrum.Comment: 19 page latex file including 6 figures and 1 tabl
Detailed study of the GRB 030329 radio afterglow deep into the non-relativistic phase
We explore the physics behind one of the brightest radio afterglows ever, GRB
030329, at late times when the jet is non-relativistic. We determine the
physical parameters of the blast wave and its surroundings, in particular the
index of the electron energy distribution, the energy of the blast wave, and
the density (structure) of the circumburst medium. We then compare our results
with those from image size measurements. We observed the GRB 030329 radio
afterglow with the Westerbork Synthesis Radio Telescope and the Giant Metrewave
Radio Telescope at frequencies from 325 MHz to 8.4 GHz, spanning a time range
of 268-1128 days after the burst. We modeled all the available radio data and
derived the physical parameters. The index of the electron energy distribution
is p=2.1, the circumburst medium is homogeneous, and the transition to the
non-relativistic phase happens at t_NR ~ 80 days. The energy of the blast wave
and density of the surrounding medium are comparable to previous findings. Our
findings indicate that the blast wave is roughly spherical at t_NR, and they
agree with the implications from the VLBI studies of image size evolution. It
is not clear from the presented dataset whether we have seen emission from the
counter jet or not. We predict that the Low Frequency Array will be able to
observe the afterglow of GRB 030329 and many other radio afterglows,
constraining the physics of the blast wave during its non-relativistic phase
even further.Comment: 9 pages, 2 figures; accepted for publication in Astronomy &
Astrophysics after minor revisions; small changes in GMRT fluxes at 1280 MH
The Internationalization Of Indian SMEs In B-to-B Markets
Purpose – The study aims to examine the effect of market orientation on the international performance of Indian SMEs in the business-to-business market; assess the potential moderating effect of environmental uncertainty, along with market orientation, on SMEs’ performance; and discuss implications to guide SME owners/managers in their efforts to successfully expand internationally in turbulent markets. Design/methodology/approach – Surveys were sent to a random cross-sectional industry sample of SMEs located in India. The data consisted of 150 responses. Findings – Market orientation and international orientation are positively related to export performance and the relationship between the market orientation and international performance of Indian SMEs is moderated by market turbulence. Research limitations/implications – The data were collected at a single point in time, and therefore do not allow the determination of cause and effect or the impact of changes over time. Data were collected with a reliance on self-reports for all of the research variables. Practical implications – The positive relationship between market orientation, international orientation, and the degree of internationalization of the firm would suggest that the upper management of Indian SMEs should place a priority on promoting behaviors consistent with a positive international orientation, especially in turbulent markets. Originality/value – Studies focused on market orientation have been done primarily in the context of advanced economies. The significant economic contribution of SMEs is well understood, but their business practices in emerging economies have not been studied extensively. This study extends the literature concerning factors that impact business success in an important emerging market such as India
The origin of the late rebrightening in GRB 080503
GRB 080503, detected by Swift, belongs to the class of bursts whose prompt
phase consists of an initial short spike followed by a longer soft tail. It did
not show any transition to a regular afterglow at the end of the prompt
emission but exhibited a surprising rebrightening after one day. We aim to
explain this rebrightening with two different scenarios - refreshed shocks or a
density clump in the circumburst medium - and two models for the origin of the
afterglow, the standard one where it comes from the forward shock, and an
alternative one where it results from a long-lived reverse shock. We computed
afterglow light curves either using a single-zone approximation for the shocked
region or a detailed multizone method that more accurately accounts for the
compression of the material. We find that in several of the considered cases
the detailed model must be used to obtain a reliable description of the shock
dynamics. The density clump scenario is not favored. We confirm previous
results that the presence of the clump has little effect on the forward shock
emission, except if the microphysics parameters evolve when the shock enters
the clump. Moreover, we find that the rebrightening from the reverse shock is
also too weak when it is calculated with the multi-zone method. On the other
hand, in the refreshed-shock scenario both the forward and reverse shock models
provide satisfactory fits of the data under some additional conditions on the
distribution of the Lorentz factor in the ejecta and the beaming angle of the
relativistic outflow.Comment: 8 pages, 4 figures, accepted for publication in A&
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