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By combining OSSE, SMM and TGRS observations of the galactic center region, Purcell et al. (1997) and Cheng et al. (1997) produced the first maps of galactic positron annihilation. That data-set has been augmented with additional data, both recent and archival, and re-analyzed to improve the spectral fitting. The improved spectral fitting has enabled the first maps of positronium continuum emission and the most extensive maps of 511 keV line emission. Bulge and disk combinations have been compared with the 511 keV line data, demonstrating that extended bulges are favored over a GC point source for every disk model tested. This result is independent of whether OSSE-only, OSSE/SMM, or OSSE/SMM/TGRS data-sets are used. The estimated bulge to disk ratio (and to a lesser extent the total flux) is shown to be dependent upon the assumption of bulge shape. A positive latitude enhancement is shown to have an effect upon the B/D ratio, but this effect is secondary to the choice of bulge shape., Comment: 10 pages, to appear in "Proceedings of the 5th COMPTON Symposium"

The Josephson quasiparticle (JQP) cycle in a voltage-biased superconducting single-electron transistor (SSET) combines coherent Cooper pair tunneling with incoherent quasiparticle decay. We have measured the influence of current flow through an independently-biased SSET on the JQP cycle when the two SSET's have a strong mutual capacitive coupling. We find, among other effects, that the JQP current in one SSET is enhanced by the presence of a quasiparticle current in the other SSET. A simplified model of the coupled-SSET system is presented which reproduces the enhancement effect.

We present a new algorithm, called Multiresolution Regularized Expectation Maximization (MREM), for the reconstruction of gamma-ray intensity maps from COMPTEL data. The algorithm is based on the iterative Richardson-Lucy scheme to which we added a wavelet thresholding step in order to eliminate image-noise in the reconstruction. The wavelet thresholding explicitly accounts for spatial correlations in the data, and adapts the angular resolution locally, depending on the significance of the signal in the data. We compare the performance of MREM to that of the maximum entropy and the Richardson-Lucy algorithms by means of Monte-Carlo simulations of COMPTEL 1.809 MeV gamma-ray line observations. The simulations demonstrate that the maximum entropy and Richardson-Lucy algorithms provide virtually identical reconstructions which are heavily disturbed by image noise. MREM largely suppresses this noise in the reconstructions, showing only the significant structures that are present in the data. Application of MREM to COMPTEL 1.8 MeV gamma-ray line data results in a 1.809 MeV sky map that is much smoother than the maximum entropy or Richardson-Lucy reconstructions presented previously. The essential features of this map are (1) an asymmetric galactic ridge emission reaching from l=45 deg to l=240 deg, (2) a bright localised emission feature in the Cygnus region around (l,b)=(80 deg,0 deg), (3) two emission spots at l=317 deg and l=332 deg situated in the galactic plane, and (4) an extended emission region around (l,b)=(160 deg, 0 deg). Comparison of the MREM map to the simulated reconstructions demonstrates that the 1.809 MeV emission is confined to the galactic plane., Comment: 13 pages, 7 figures, A&A, in press

Deconvolving COMPTEL gamma-ray data into images presents a major methodological challenge. We developed a new algorithm called Multiresolution Regularized Expectation Maximization (MREM), which explicitly accounts for spatial correlations in the image by using wavelets (Dixon et al. 1998). We demonstrate that MREM largely suppresses image noise in the reconstruction, showing only significant structures that are present in the data. Application to 1.8 MeV data results in a sky map that is much smoother than the maximum entropy reconstructions presented previously, but it shows the same characteristic emission features which have been established earlier., Comment: 4 pages, 1 figure, Proc. of the 3rd INTEGRAL workshop, Taormina

Image processing is an increasingly important aspect for analysis of data from X and $\gamma$-ray astrophysics missions. In this paper, I review a method proposed by Kebede (L. W. Kebede 1994, ApJ, 423, 878), and point out an error in the derivation of this method. It turns out that this error is not debilitating -- the method still ``works'' in some sense -- but as published is rather sub-optimal, as demonstrated both on theoretical grounds and via a set of examples., Comment: 10 pages, 13 figures. Submitted to ApJ

We present the multilevel fabrication and measurement of a Coulomb-blockade device displaying tunable negative differential resistance (NDR). Applications for devices displaying NDR include amplification, logic, and memory circuits. Our device consists of two Al/Al$_{x}$O$_{y}$ islands that are strongly coupled by an overlap capacitor. Our measurements agree excellently with a model based on the orthodox theory of single-electron transport., Comment: 3 pages, 3 figures; submitted to APL

We present quantitative statistical evidence for a $\gamma$-ray emission halo surrounding the Galaxy. Maps of the emission are derived. EGRET data were analyzed in a wavelet-based non-parametric hypothesis testing framework, using a model of expected diffuse (Galactic + isotropic) emission as a null hypothesis. The results show a statistically significant large scale halo surrounding the center of the Milky Way as seen from Earth. The halo flux at high latitudes is somewhat smaller than the isotropic gamma-ray flux at the same energy, though of the same order (O(10^(-7)--10^(-6)) ph/cm^2/s/sr above 1 GeV)., Comment: Final version accepted for publication in New Astronomy. Some additional results/discussion included, along with entirely revised figures. 19 pages, 15 figures, AASTeX. Better quality figs (PS and JPEG) are available at http://tigre.ucr.edu/halo/paper.html

The region near the Galactic center may have experienced recurrent episodes of injection of energy in excess of $\sim$ 10$^{55}$ ergs due to repeated starbursts involving more than $\sim$ 10$^4$ supernovae. This hypothesis can be tested by measurements of $\gamma$-ray lines produced by the decay of radioactive isotopes and positron annihilation, or by searches for pulsars produced during starbursts. Recent OSSE observations of 511 keV emission extending above the Galactic center led to the suggestion of a starburst driven fountain from the Galactic center. We present EGRET observations that might support this picture., Comment: 5 pages, 1 embedded Postscript figure. To appear in the Proceedings of the Fourth Compton Symposium

We present progress toward using scanned OSSE observations for mapping and sky survey work. To this end, we have developed a technique for detecting pointlike sources of unknown number and location, given that they appear in a background which is relatively featureless or which can be modeled. The technique, based on the newly developed concept and mean field annealing, is described, with sample reconstructions of data from the OSSE Virgo Survey. The results demonstrate the capability of reconstructing source information without any a priori information about the number and/or location of pointlike sources in the field-of-view., Comment: 15 pages, 4 Postscript figs, AASTeX, uses psfig and epsf. To appear in the Astrophysical Journal, vol. 484

An effective field approximation, similar to the atomic Thomas-Fermi approach, is proposed for studying non-Abelian gauge theories which includes finite-volume effects. As applications of the formalism the equation of state for an SU(2) gauge theory with massless fermions is obtained. The extensions to realistic situations are briefly discussed., Comment: 43 pages, 8 figures, plain TeX; requires epsf and phyzzx

Non-deterministic chaos is a new dynamical paradigm where a non-deterministic system is influenced by random perturbations to produce the appearance of complexity. The non-determinism is envisioned to occur only at a single point in phase space, where many trajectories intersect. In the presence of external random perturbations (noise), whenever the phase space trajectory approaches the singularity, it will jump in an unpredictable way to a different solution. This behavior, while similar in appearance to deterministic chaos, has rather different implications for prediction and control., Comment: 6 pages, REVTeX, 7 PostScript figures available upon request. This paper is an extensively revised and expanded version (read, "it's different") of an earlier paper with the same title

A method for detecting possible non-deterministic dynamics underlying a time series is introduced. Non-deterministic dynamics may arise due to the failure of the Lipschitz condition in the equations of motion. At a singular point, the phase space trajectory of the system may jump from one solution to another. This discontinous change implies divergence of the second derivative of the solution whenever it passes near the singular point. A time series can be examined for such divergences, which may indicate non-determinism in the dynamical system. Examples with both simulated and actual data are given., Comment: 4 pages, RevTeX, 3 PostScript figures available upon request

Non-deterministic chaos is a form of low-dimensional dynamics which is characterized by the existence of a countable set of {\em sensitive decision points} (SDP's). Away from these points, the dynamics is well-behaved. Near these points, however, perturbations (e.g., thermal noise) may cause the outgoing trajectory to be chosen randomly. An example of a non-deterministic chaotic system is given, and a statistical method of analyzing the resultant dynamics is developed., Comment: 3 pages, RevTex, one PostScript figure available upon request