Design and real-time implementation of robust FACTS controller for damping inter-area oscillation

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Study of 5 and 10 mm thick CZT strip detectors

We report progress in the study of 5 and 10 mm thick CZT strip detectors featuring orthogonal coplanar anode contacts. This novel anode geometry combines the advantages of pixel detectors with those of double-sided strip detectors. Like pixel detectors, these are electron-only devices that perform well as hard x-ray and y-ray spectrometers and imagers even in the thicker configurations required for reasonable detection efficiency at 1 MeV. Like double-sided strip detectors in an N x N configuration, these detectors require only 2N readout channels to form N2 “pixels”. Unlike doublesided strip detectors, all signal contacts for spectroscopy and 3- d imaging are formed on one detector surface. Polymer flip chip bonding to a ceramic substrate is employed resulting in a rugged and compact detector assembly. Prototype detector modules 5 mm thick have been fabricated and tested. Prototype modules, 10 mm thick, are currently in procurement. Measurements confirm these devices are efficient detectors throughout their volume. Sub-millimeter position resolution and energy resolution (FWHM) better than 3% at 662 keV and 15% at 60 keV throughout the detector volume are demonstrated. Options for processing the signals from the non-collecting anode strip contacts are discussed. Results from tests of one prototype circuit are presented. We also report on detector simulation studies aimed at defining an optimum geometry for the anode contacts and at determining optimum operating conditions and the requirements of the signal processing electronics

Identication-robust moment-based tests for Markov switching in autoregressive models

This paper develops tests of the null hypothesis of linearity in the context of autoregressive models with Markov-switching means and variances. These tests are robust to the identification failures that plague conventional likelihood-based inference methods. The approach exploits the moments of normal mixtures implied by the regime-switching process and uses Monte Carlo test techniques to deal with the presence of an autoregressive component in the model specification. The proposed tests have very respectable power in comparison with the optimal tests for Markov-switching parameters of Carrasco et al. (2014), and they are also quite attractive owing to their computational simplicity. The new tests are illustrated with an empirical application to an autoregressive model of USA output growth

Energy and position resolution of a CdZnTe gamma-ray detector with orthogonal coplanar anodes

We report on the simulation, construction and performance of prototype CZT imaging detectors employing orthogonal coplanar anodes. These detectors employ a novel electrode geometry with non-collecting anode strips in 1D and collecting anode pixels, interconnected in rows, in the orthogonal dimensions. These detectors retain the spectroscopic and detection efficiency advantages of single carried charge sensing devices as well as the principal advantage of conventional strip detectors with orthogonal anode and cathode strips, i.e. an N X N array of imagin pixels are realized with only 2N electronic channels. Charge signals induced on the various electrodes of a prototype detector with 8 X 8 unit cells are in good agreement with the simulations. The position resolution is about 1 mm in the direction perpendicular to the pixel lines while it is of the order of 100 micrometers in the other direction. Energy resolutions of 0.9 percent at 662 keV, 2.6 percent at 122 keV and 5.7 percent at 60 keV have been obtained at room temperature

Hot DAVs : a probable new class of pulsating white dwarf stars

We have discovered a pulsating DA white dwarf at the lower end of the temperature range 45 000–30 000 K where a few helium atmosphere white dwarfs are known. There are now three such pulsators known, suggesting that a new class of theoretically predicted pulsating white dwarf stars exists. We name them the hot DAV stars. From high-speed photometric observations with the ULTRACAM photometer on the 4.2-m William Herschel Telescope, we show that the hydrogen atmosphere white dwarf star WD1017−138 pulsates in at least one mode with a frequency of 1.62 mHz (a period of 624 s). The amplitude of that mode was near 1 mmag at a 10σ confidence level on one night of observation and an 8.4σ confidence level on a second night. The combined data have a confidence level of 11.8σ. This supports the two other detections of hot DAV stars previously reported. From three Very Large Telescope Ultraviolet and Visual Echelle Spectrograph spectra we confirm also that WD1017−138 is a hydrogen atmosphere white dwarf with no trace of helium or metals with Teff = 32 600 K, log g = 7.8 (cgs) and M = 0.55 M⊙. The existence of pulsations in these DA white dwarfs at the cool edge of the 45 000–30 000 K temperature range supports the thin hydrogen layer model for the deficit of helium atmosphere white dwarfs in this range. DA white dwarfs with thick hydrogen layers do not have the superadiabatic, chemically inhomogeneous (μ-gradient) zone that drives pulsation in this temperature range. The potential for higher amplitude hot DAV stars exists; their discovery would open the possibility of a direct test of the explanation for the deficit of helium atmosphere white dwarfs at these temperatures by asteroseismic probing of the atmospheric layers of the hot DAV stars. A search for pulsation in a further 22 candidates with ULTRACAM on the European Southern Observatory New Technology Telescope gave null results for pulsation at precisions in the range 0.5–3 mmag, suggesting that the pulsation amplitudes in such stars are relatively low, hence near the detection limit with the ground-based telescopes used in the surve

Spectroscopy with random and displaced random ensembles

Due to the time reversal invariance of the angular momentum operator J^2, the average energies and variances at fixed J for random two-body Hamiltonians exhibit odd-even-J staggering, that may be especially strong for J=0. It is shown that upon ensemble averaging over random runs, this behaviour is reflected in the yrast states. Displaced (attractive) random ensembles lead to rotational spectra with strongly enhanced BE2 transitions for a certain class of model spaces. It is explained how to generalize these results to other forms of collectivity.Comment: 4 pages, 4 figure

The Nature of the Halo Population of NGC 5128 Resolved with NICMOS on the Hubble Space Telescope

We present the first infrared color-magnitude diagram (CMD) for the halo of a giant elliptical galaxy. The CMD for the stars in the halo of NGC 5128 (Centaurus A) was constructed from HST NICMOS observations of the WFPC2 CHIP-3 field of Soria et al. (1996) to a 50% completeness magnitude limit of [F160W]=23.8. This field is located at a distance of 08'50" (~9 kpc) south of the center of the galaxy. The luminosity function (LF) shows a marked discontinuity at [F160W]=20.0. This is 1-2 mag above the tip of the red giant branch (TRGB) expected for an old population (~12 Gyr) at the distance modulus of NGC 5128. We propose that the majority of stars above the TRGB have intermediate ages (~2 Gyr), in agreement with the WFPC2 observations of Soria et al. (1996). Five stars with magnitudes brighter than the LF discontinuity are most probably due to Galactic contamination. The weighted average of the mean giant branch color above our 50% completeness limit is [F110W]-[F160W]=1.22+-0.08 with a dispersion of 0.19 mag. From our artificial-star experiments we determine that the observed spread in color is real, suggesting a real spread in metallicity. We estimate the lower and upper bounds of the stellar metallicity range by comparisons with observations of Galactic star clusters and theoretical isochrones. Assuming an old population, we find that, in the halo field of NGC 5128 we surveyed, stars have metallicities ranging from roughly 1% of solar at the blue end of the color spread to roughly solar at the red end, with a mean of [Fe/H]=-0.76 and a dispersion of 0.44 dex.Comment: Accepted for publication in AJ, 23 pages of text, 13 figures, uses aastex v5.