Potential Barrier Classification by Short-Time Measurement

We investigate the short-time dynamics of a delta-function potential barrier on an initially confined wave-packet. There are mainly two conclusions: A) At short times the probability density of the first particles that passed through the barrier is unaffected by it. B) When the barrier is absorptive (i.e., its potential is imaginary) it affects the transmitted wave function at shorter times than a real potential barrier. Therefore, it is possible to distinguish between an imaginary and a real potential barrier by measuring its effect at short times only on the transmitting wavefunction.Comment: 6 pages, 5 figure

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

The structure and dynamics of GRB jets

Original paper can be found at: http://www.astroscu.unam.mx/~rmaa/rmaa.html Copyright Universidad Nacional Autonoma de MexicoThere are several lines of evidence which suggest that the relativistic outflows in gamma-ray bursts (GRBs) are collimated into narrow jets. The jet structure has important implications for the true energy release and the event rate of GRBs, and can constrain the mechanism responsible for the acceleration and collimation of the jet. Nevertheless, the jet structure and its dynamics as it sweeps up the external medium and decelerates, are not well understood. In this review I discuss our current understanding of GRB jets, stressing their structure and dynamics.Peer reviewe

The Most Probable Cause for the High Gamma-Ray Polarization in GRB 021206

The exciting detection of a very high degree of linear polarization, $P=(80\pm 20)$, in the prompt gamma-ray emission of GRB 021206, supports synchrotron emission as being the dominant radiation mechanism. There were also claims that this implies a magnetic field ordered on large scales within the ejecta, that must therefore be produced at the source, which in turn was used as an argument in favor magnetic fields playing an active role in the production of GRB jets. However, an alternative explanation was suggested which also works with a magnetic field that is generated in the internal shocks and does not originate at the source: a very narrow jet, of opening angle $\theta_j\sim 1/\gamma$, where $\gamma\gtrsim 100$ is the Lorentz factor during the GRB, viewed at $\theta_j<\theta_{obs}\lesssim\theta_j+1/\gamma$. We calculate $P$ for these two scenarios, and find that it is significantly easier to produce $P\gtrsim 50$ with an ordered field. More specifically, we obtain $P\sim(43-61)$ for an ordered transverse magnetic field, $B_{ord}$, whereas a random field within the plane of the shock, $B_\perp$, produces $P\lesssim(38-54)$ for a single pulse in the GRB light curve, but the integrated emission over many pulses (as measured in GRB 021206) is a factor of $\sim 2$ lower. A magnetic field normal to the shock front, $B_\parallel$, produces $P\sim(35-62)$ for the emission integrated over many pulses. However, polarization measurements from GRB afterglows suggest a more isotropic shock-produced field configuration that would reduce $P$ by a factor $\sim 2-3$. Therefore, an ordered magnetic field, $B_{ord}$, that originates at the source, can produce the observed polarization most naturally, while $B_\parallel$ is less likely, and $B_\perp$ is the least likely of the above.Comment: 4 pages, 4 figures; an improved assumption on the distribution of cooled electrons simplifies the expressions for the polarization. ApJL in pres