X - Ray Flares and Their Connection With Prompt Emission in GRBs

We use a wavelet technique to investigate the time variations in the light curves from a sample of GRBs detected by Fermi and Swift. We focus primarily on the behavior of the flaring region of Swift-XRT light curves in order to explore connections between variability time scales and pulse parameters (such as rise and decay times, widths, strengths, and separation distributions) and spectral lags. Tight correlations between some of these temporal features suggest a common origin for the production of X-ray flares and the prompt emission.Comment: 7th Huntsville Gamma-Ray Burst Symposium, GRB 2013: paper 15 in eConf Proceedings C130414

Gamma-Ray Bursts: Temporal Scales and the Bulk Lorentz Factor

For a sample of Swift and Fermi GRBs, we show that the minimum variability timescale and the spectral lag of the prompt emission is related to the bulk Lorentz factor in a complex manner: For small $\Gamma$'s, the variability timescale exhibits a shallow (plateau) region. For large $\Gamma$'s, the variability timescale declines steeply as a function of $\Gamma$ ($\delta T\propto\Gamma^{-4.05\pm0.64}$). Evidence is also presented for an intriguing correlation between the peak times, t$_p$, of the afterglow emission and the prompt emission variability timescale.Comment: Accepted for publication in Ap

Ab-initio density functional studies of stepped TaC surfaces

We report on density functional total energy calculations of the step formation and interaction energies for vicinal TaC(001) surfaces. Our calculations show that double and triple-height steps are favored over single-height steps for a given vicinal orientation, which is in agreement with recent experimental observations. We provide a description of steps in terms of atomic displacements and charge localization and predict an experimentally observable rumpled structure of the step-edges, where the Ta atoms undergo larger displacements compared to the C atoms.Comment: 4 pages, 4 figure

Phase Diagram of the Attractive Hubbard Model with Inhomogeneous Interactions

The phase diagram of the attractive Hubbard model with spatially inhomogeneous interactions is obtained using a single site dynamical mean field theory like approach. The model is characterized by three parameters: the interaction strength, the active fraction (fraction of sites with the attractive interaction), and electron filling. The calculations indicate that in a parameter regime with intermediate values of interaction strength (compared to the bare bandwidth of the electrons), and intermediate values of the active fraction, "non-BCS" superconductivity is obtained. The results of this work are likely to be relevant to many systems with spatially inhomogeneous superconductivity such as strongly correlated oxides, systems with negative U centers, and, in future, cold atom optical lattices.Comment: 9 pages, 7 figures, to appear in Physical Review

Angular distribution of power from an undulator and a wiggler on a 6-GeV storage ring

There are two fundamental reasons to have a full knowledge of the angular distribution of power from an insertion device: 1. To evaluate the heat-load distribution on the first optical element in a beamline. 2. To estimate the total radiated power which will impinge on the walls of an insertion device. This is important to ensure needed cooling of the insertion device walls. The photodesorption is another closely related phenomenon determined by the exposure of the insertion device walls to the radiated power and of consequence to the successful operation of the storage ring. In this paper, we will primarily focus on undulators, but also consider situations as the value of K increases to the wiggler regime. These calculations are very involved and cumbersome and we shall only present some specific results related to the 6-GeV insertion devices

A new approach to high-current operation of the Advanced Photon Source.

It is shown that the operation of the Advanced Photon Source (APS) storage ring at 6 GeV will (1) deliver higher brilliance at x-ray energies used by a majority of users due to natural reduction in electron beam emittance at lower storage ring energy, and (2) lower the total power produced by insertion devices thus permitting stored currents up to 300 mA with minimal changes in accelerator or beamline hardware. While higher brilliance x-ray beams can be realized from the APS undulators by only increasing the stored current for the present modes of operation, this however leads to serious high heat load concerns. This report includes detailed analyses of radiation brilliance, undulator tunability, power, power density, and total and coherent flux as a function of x-ray energy from various harmonics of undulator-A, for operation at 6.0, 6.5 and 7.0 GeV with 100, 140, 200 and 300 mA currents. A discussion of a smaller period (2.7 cm) undulator's spectral performance is also presented. It is shown that the APS can be immediately operated at 6 GeV with 200-300 mA current to benefit user science in the x-ray energy range below 35 keV. This may not require any hardware change either in the storage ring or the beamlines, making this switch without any interruptions to user research programs. Finally, the suggested 6 GeV operation with 300 mA should be considered a temporary step towards higher brilliance operation. A need for re-optimization of the APS technical operations, defined by storage-ring energy, current, undulator period, and power, are also addressed in this report

Estimation of total radiative power from the 6-GeV ring

Here we make an estimation of the total power radiated from a positron trajectory through bending magnets, undulators, and wigglers. Placement of insertion devices is also described

Undulators on a 6-GeV ring general considerations

In, designing undulators on a 6-GeV storage ring the primary consideration will be the power that such devices will deliver. The beam line design should be capable of handling large powers that such undulators will deliver. Specifically, in a beam line in the front end we have masks (fixed and movable) followed by various optical components. Many thermal designs are now being developed to improve on the capability of various components to handle a greater beat load than ever been possible before. For example, designs for rotating optics and liquid metal cooled optics are actively pursued in the MST division