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 $\nu_a$ and derive an accurate expression for $\nu_a$. We show that the image is rather homogeneous for $\nu<\nu_a$, 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: $E_{52}=0.53$, $\epsilon_e=0.57$, $\epsilon_B=0.0082$, $n_1=5.3$. 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 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&

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