arXiv subjects

J. Miller

Publications and source records attributed to J. Miller.

104 records · Page 6Linked to original sources

arXiv2006

XMM-Newton and INTEGRAL spectroscopy of the microquasar GRO J1655-40 during its 2005 outburst

We report on two simultaneous XMM-Newton and INTEGRAL observations of the microquasar GRO J1655-40 during the 2005 outburst when the source was in its high-soft state. The 0.3-200 keV spectra are complex with an overall continuum which may be modeled using an absorbed blackbody together with a weak, steep, power-law component. In addition, there is evidence for the presence of both a relativistically broadened Fe K line and a highly photo-ionized absorber. The photo-ionized absorber is responsible for strong K absorption lines of Fe XXV and Fe XXVI in the EPIC pn spectra and blue-shifted (v = -540 +/- 120 km/s) Ne X and Fe XXIV features in the RGS spectra. The parameters of the photo-ionized absorber were different during the two observations with the ionization parameter, logxi, decreasing from 3.60 +/- 0.04 to 3.30 +/- 0.04 erg cm/s and the column density decreasing from (5.2 +/- 1.0) x 10^22 atom cm-2 to (1.5 +/- 1.0) x 10^22 atom cm-2 during the first and second observations as the 0.5-200 keV GRO J1655-40 luminosity decreased by around a half. At 90% confidence the INTEGRAL data show no evidence of a break in the power-law component up to energies of 380 keV and 90 keV for the first and second observations, respectively.

astro-ph↗

arXiv2004

INTEGRAL/SPI Limits on Electron-Positron Annihilation Radiation from the Galactic Plane

The center of our Galaxy is a known strong source of electron-positron 511-keV annihilation radiation. Thus far, however, there have been no reliable detections of annihilation radiation outside of the central radian of our Galaxy. One of the primary objectives of the INTEGRAL (INTErnational Gamma-RAy Astrophysics Laboratory) mission, launched in Oct. 2002, is the detailed study of this radiation. The Spectrometer on INTEGRAL (SPI) is a high resolution coded-aperture gamma-ray telescope with an unprecedented combination of sensitivity, angular resolution and energy resolution. We report results from the first 10 months of observation. During this period a significant fraction of the observing time was spent in or near the Galactic Plane. No positive annihilation flux was detected outside of the central region (|l| > 40 deg) of our Galaxy. In this paper we describe the observations and data analysis methods and give limits on the 511-keV flux.

astro-ph↗

arXiv2004

Synthetic Spectral Analysis of the Hot Component in the S-Type Symbiotic Variable EG Andromeda

We have applied grids of NLTE high gravity model atmospheres and optically thick accretion disk models for the first time to archival IUE and FUSE spectra of the S-type symbiotic variable EG And taken at superior spectroscopic conjunction when Rayleigh scattering should be minimal and the hot component is viewed in front of the red giant. For EG And's widely accepted, published hot component mass, orbital inclination and distance from the Hipparcos parallax, we find that hot, high gravity, NLTE photosphere model fits to the IUE spectra yield distances from the best-fitting models which agree with the Hipparcos parallax distance but at temperatures substantially lower than the modified Zanstra temperatures. NLTE fits to an archival FUSE spectrum taken at the same orbital phase as the IUE spectra yield the same temperature as the IUE temperature (50,000K). However, for the same hot component mass, inclination and parallax-derived distance, accretion disk models at moderately high inclinations, $\sim 60-75^{\circ}$ with accretion rates $\dot{M} = 1\times 10^{-8}$ to $1\times 10^{-9} M_{\sun}/yr$ for white dwarf masses $M_{wd} = 0.4 M_{\sun}$ yield distances grossly smaller than the distance from the Hipparcos parallax. Therefore, we rule out an accretion disk as the dominant source of the FUV flux. Our findings support a hot bare white dwarf as the dominant source of FUV flux.

astro-ph↗

arXiv2003

Symbolic computation of exact solutions expressible in hyperbolic and elliptic functions for nonlinear PDEs

Algorithms are presented for the tanh- and sech-methods, which lead to closed-form solutions of nonlinear ordinary and partial differential equations (ODEs and PDEs). New algorithms are given to find exact polynomial solutions of ODEs and PDEs in terms of Jacobi's elliptic functions. For systems with parameters, the algorithms determine the conditions on the parameters so that the differential equations admit polynomial solutions in tanh, sech, combinations thereof, Jacobi's sn or cn functions. Examples illustrate key steps of the algorithms. The new algorithms are implemented in Mathematica. The package DDESpecialSolutions.m can be used to automatically compute new special solutions of nonlinear PDEs. Use of the package, implementation issues, scope, limitations, and future extensions of the software are addressed. A survey is given of related algorithms and symbolic software to compute exact solutions of nonlinear differential equations.

nlin.SI↗

arXiv2003

NGC 4261 and NGC 4697: rejuvenated elliptical galaxies

Chandra images present evidence for a non-uniform spatial distribution of discrete X-ray sources in the elliptical galaxy NGC 4261. This non-uniform distribution is inconsistent with the optical morphology of NGC 4261 at greater than a 99.9% confidence level. Similar evidence is seen in one more elliptical galaxy (NGC 4697; 98% confidence level) out of five cases we investigated. NGC 4261 and NGC 4697 have old stellar populations (9-15 Gyrs) and fine structure parameters of 1 and 0 respectively, suggesting no recent merging activity. On the basis of simulations of galaxy interactions, we propose that the X-ray sources responsible for the non-uniform distribution are associated with young stellar populations, related to the rejuvenating fall-back of material in tidal tails onto a relaxed merger remnant, or shock induced star-formation along the tidal tails.

astro-ph↗

arXiv2002

Optical Identification of Multiple Faint X-ray Sources in the Globular Cluster NGC 6752: Evidence for Numerous Cataclysmic Variables

We report on the Chandra ACIS-S3 imaging observation of the globular cluster NGC 6752. We detect 6 X-ray sources within the 10.5" core radius and 13 more within the 115" half-mass radius down to a limiting luminosity of Lx approx 10^{30} erg/s for cluster sources. We reanalyze archival data from the Hubble Space Telescope and the Australia Telescope Compact Array and make 12 optical identifications and one radio identification. Based on X-ray and optical properties of the identifications, we find 10 likely cataclysmic variables (CVs), 1-3 likely RS CVn or BY Dra systems, and 1 or 2 possible background objects. Of the 7 sources for which no optical identifications were made, we expect that ~2-4 are background objects and that the rest are either CVs or some or all of the five millisecond pulsars whose radio positions are not yet accurately known. These and other Chandra results on globular clusters indicate that the dozens of CVs per cluster expected by theoretical arguments are being found. The findings to date also suggest that the ratio of CVs to other types of X-ray sources is remarkably similar in clusters of very different structural parameters.

astro-ph↗

arXiv2001

Emergence of highly-designable protein-backbone conformations in an off-lattice model

Despite the variety of protein sizes, shapes, and backbone configurations found in nature, the design of novel protein folds remains an open problem. Within simple lattice models it has been shown that all structures are not equally suitable for design. Rather, certain structures are distinguished by unusually high designability: the number of amino-acid sequences for which they represent the unique ground state; sequences associated with such structures possess both robustness to mutation and thermodynamic stability. Here we report that highly designable backbone conformations also emerge in a realistic off-lattice model. The highly designable conformations of a chain of 23 amino acids are identified, and found to be remarkably insensitive to model parameters. While some of these conformations correspond closely to known natural protein folds, such as the zinc finger and the helix-turn-helix motifs, others do not resemble known folds and may be candidates for novel fold design.

cond-mat.soft↗

arXiv2000

Performance characteristics of the new Keck Observatory echelle spectrograph and imager

The Echelle Spectrograph and Imager (ESI) is a multipurpose instrument which has been delivered by the Instrument Development Laboratory of Lick Observatory for use at the Cassegrain focus of the Keck II telescope. ESI saw first light on August 29, 1999. The optical performance of the instrument has been measured using artificial calibration sources and starlight. Measurements of the average image FWHM in echelle mode are 22 microns (0.22 arcseconds), 16 to 18 microns (0.16 to 0.18 arcseconds) in broad band imaging mode, and comparable in the low-dispersion prismatic mode (LDP). Images on the sky, under best seeing conditions show FWHM sizes of 34 microns (0.34 arcseconds). Maximum efficiencies are measured to be 30% for echelle and anticipated to be greater than 38% for low dispersion prismatic mode including atmospheric, telescope and detector losses. In this paper we describe the instrument and its specifications. We discuss the testing that led to the above conclusions.

astro-ph↗

arXiv1999

Chern numbers and localization by non-Hermitean operators

In Hermitean quantum mechanics, extended current-carrying states are distinguished from localized ones by their non-zero Chern number. We generalize the notion of Chern number to non-Hermitean localization problems such as tiltedflux lines in type-II superconductors with line defects and passive scalar transport in random vorticity fields. We find that the usualpoint eigenvalue degeneracies that occur in the Hermitean case are now loops bounding a branch cut sheet with the same quantized total Berry flux, and hence Chern number, as the original point. If a loop cuts the integration surface non-quantized contributions to the total flux result.

cond-mat.mes-hall↗

arXiv1997

Inclusive Dielectron Cross Sections in p+p and p+d Interactions at Beam Energies from 1.04 to 4.88 GeV

Measurements of dielectron production in p+p and p+d collisions with beam kinetic energies from 1.04 to 4.88 GeV are presented. The differential cross section is presented as a function of invariant pair mass, transverse momentum, and rapidity. The shapes of the mass spectra and their evolution with beam energy provide information about the relative importance of the various dielectron production mechanisms in this energy regime. The p+d to p+p ratio of the dielectron yield is also presented as a function of invariant pair mass, transverse momentum, and rapidity. The shapes of the transverse momentum and rapidity spectra from the p+d and p+p systems are found to be similar to one another for each of the beam energies studied. The beam energy dependence of the integrated cross sections is also presented.

nucl-ex↗

arXiv1997

Quantum state manipulation of trapped atomic ions

A single laser-cooled and trapped 9Be+ ion is used to investigate methods of coherent quantum-state synthesis and quantum logic. We create and characterize nonclassical states of motion including "Schroedinger-cat" states. A fundamental quantum logic gate is realized which uses two states of the quantized ion motion and two ion internal states as qubits. We explore some of the applications for, and problems in realizing, quantum computation based on multiple trapped ions.

quant-ph↗

arXiv1997

Dielectron Cross Section Measurements in Nucleus-Nucleus Reactions at 1.0 A GeV

We present measured dielectron production cross sections for Ca+Ca, C+C, He+Ca, and d+Ca reactions at 1.0 A GeV. Statistical uncertainties and systematic effects are smaller than in previous DLS nucleus-nucleus data. For pair mass < 0.35 GeV/c2 : 1) the Ca+Ca cross section is larger than the previous DLS measurement and current model results, 2) the mass spectra suggest large contributions from pi0 and eta Dalitz decays, and 3) dsigma/dM is proportional to ApAt. For M > 0.5 GeV/c2 the Ca+Ca to C+C cross section ratio is significantly larger than the ratio of ApAt values.

nucl-ex↗

arXiv1994

Dilepton production from p-p to Ca-Ca at the Bevalac

The DLS collaboration has recently completed a high statistics study of dilepton production at the Bevalac. In particular, we have measured dielectrons (e+e-) from p-p and p-d collisions to understand the basic dilepton production mechanisms in the energy range from 1.05 - 4.9 GeV. These data can be used to determine the basic processes which contribute to nucleon-nucleon dilepton production such as hadronic bremsstrahlung, vector meson processes, and hadronic Dalitz decay. The data show that a simple elastic bremsstrahlung calculation is insufficient to explain the data. Theoretical models are compared with the data. A new high statistics study of Ca-Ca at 1.05 A GeV has been made to study the collectivity of A-A collisions.

nucl-ex↗

arXiv1993

Weak Disorder Expansion for the Anderson Model on a Tree

We show how certain properties of the Anderson model on a tree are related to the solutions of a non-linear integral equation. Whether the wave function is extended or localized, for example, corresponds to whether or not the equation has a complex solution. We show how the equation can be solved in a weak disorder expansion. We find that, for small disorder strength $λ$, there is an energy $E_c(λ)$ above which the density of states and the conducting properties vanish to all orders in perturbation theory. We compute perturbatively the position of the line $E_c(λ)$ which begins, in the limit of zero disorder, at the band edge of the pure system. Inside the band of the pure system the density of states and conducting properties can be computed perturbatively. This expansion breaks down near $E_c(λ)$ because of small denominators. We show how it can be resummed by choosing the appropriate scaling of the energy. For energies greater than $E_c(λ)$ we show that non-perturbative effects contribute to the density of states but have been unable tell whether they also contribute to the conducting properties.

cond-mat↗