On-the-fly computation of frontal orbitals in density matrix expansions

Linear scaling density matrix methods typically do not provide individual eigenvectors and eigenvalues of the Fock/Kohn-Sham matrix, so additional work has to be performed if they are needed. Spectral transformation techniques facilitate computation of frontal (homo and lumo) molecular orbitals. In the purify-shift-and-square method the convergence of iterative eigenvalue solvers is improved by combining recursive density matrix expansion with the folded spectrum method [J. Chem. Phys. 128, 176101 (2008)]. However, the location of the shift in the folded spectrum method and the iteration of the recursive expansion selected for eigenpair computation may have a significant influence on the iterative eigenvalue solver performance and eigenvector accuracy. In this work, we make use of recent homo and lumo eigenvalue estimates [SIAM J. Sci. Comput. 36, B147 (2014)] for selecting shift and iteration such that homo and lumo orbitals can be computed in a small fraction of the total recursive expansion time and with sufficient accuracy. We illustrate our method by performing self-consistent field calculations for large scale systems

No periodicity revealed for an "eclipsing" ultraluminous supersoft X-ray source in M81

Luminous supersoft X-ray sources found in the Milky Way and Magellanic Clouds are likely white dwarfs that steadily or cyclically burn accreted matter on their surface, which are promising type Ia supernova progenitors. Observations of distant galaxies with Chandra and XMM-Newton have revealed supersoft sources that are generally hotter and more luminous, including some ultraluminous supersoft sources (ULSs) that are possibly intermediate mass black holes of a few thousand solar masses. In this paper we report our X-ray spectral and timing analysis for M81-ULS1, an ultraluminous supersoft source in the nearby spiral galaxy M81. M81-ULS1 has been persistently supersoft in 17 Chandra ACIS observations spanning six years, and its spectrum can be described by either a $kT_{bb}\approx70$ eV blackbody for a $\sim1.2M_\odot$ white dwarf, or a $kT_{in} \approx 80$ eV multicolor accretion disk for a $\gtrsim10^3M_\odot$ intermediate mass black hole. In two observations, the light curves exhibited dramatic flux drop/rise on time scales of $10^3$ seconds, reminiscent of eclipse ingress/egress in eclipsing X-ray binaries. However, the exhaustive search for periodicity in the reasonable range of 50 ksec to 50 days failed to reveal an orbital period. The failure to reveal any periodicity is consistent with the long period ($\ge30$ yrs) predicted for this system given the optical identification of the secondary with an asymptotic giant star. Also, the eclipse-like dramatic flux changes in hours are hard to explain under the white dwarf model, but can in principle be explained by disk temperature changes induced by accretion rate variations under the intermediate mass black hole model.Comment: 19 pages, 7 figures, 1 table, to appear in ApJ

Reconstructing supernova-neutrino spectra using low-energy beta-beams

Only weakly interacting, neutrinos are the principal messengers reaching us from the center of a supernova. Terrestrial neutrino telescopes, such as SNO and SuperKamiokande, can provide precious information about the processes in the core of a collapsing and exploding star. But the information about the supernova that a neutrino detector can supply, is restricted by the fact that little experimental data on the neutrino-nucleus cross sections exists and by the uncertainties in theoretical calculations. In this letter, we propose a novel procedure that determines the response of a target nucleus in a supernova-neutrino detector directly, by using low-energy beta-beams. We show that fitting 'synthetic' spectra, constructed by taking linear combinations of beta-beam spectra, to the original supernova-neutrino spectra reproduces the folded differential cross sections very accurately. Comparing the response in a terrestrial detector to these synthetic responses provides a direct way to determine the main parameters of the supernova-neutrino energy-distribution.Comment: 4 page

Compact silicon photonics circuit to extract multiple parameters for process control monitoring

We present a compact interferometer circuit to extract multiple model parameters of on-chip waveguides and directional couplers from optical measurements. The compact design greatly improves the accuracy of extraction with fewer measurements, making it useful for process monitoring and detailed wafer-level variability analysis. We discuss the design requirements and illustrate the extraction using the Restart-CMA-ES global optimization algorithm. (C) 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreemen

Co-populated Conformational Ensembles of β(2)-Microglobulin Uncovered Quantitatively by Electrospray Ionization Mass Spectrometry

Ordered assembly of monomeric human β(2)-microglobulin (β(2)m) into amyloid fibrils is associated with the disorder hemodialysis-related amyloidosis. Previously, we have shown that under acidic conditions (pH <5.0 at 37 °C), wild-type β(2)m assembles spontaneously into fibrils with different morphologies. Under these conditions, β(2)m populates a number of different conformational states in vitro. However, this equilibrium mixture of conformationally different species is difficult to resolve using ensemble techniques such as nuclear magnetic resonance or circular dichroism. Here we use electrospray ionization mass spectrometry to resolve different species of β(2)m populated between pH 6.0 and 2.0. We show that by linear deconvolution of the charge state distributions, the extent to which each conformational ensemble is populated throughout the pH range can be determined and quantified. Thus, at pH 3.6, conditions under which short fibrils are produced, the conformational ensemble is dominated by a charge state distribution centered on the 9+ ions. By contrast, under more acidic conditions (pH 2.6), where long straight fibrils are formed, the charge state distribution is dominated by the 10+ and 11+ ions. The data are reinforced by investigations on two variants of β(2)m (V9A and F30A) that have reduced stability to pH denaturation and show changes in the pH dependence of the charge state distribution that correlate with the decrease in stability measured by tryptophan fluorescence. The data highlight the potential of electrospray ionization mass spectrometry to resolve and quantify complex mixtures of different conformational species, one or more of which may be important in the formation of amyloid

The Electronic Spectrum of Fullerenes from the Dirac Equation

The electronic spectrum of sheets of graphite (plane honeycomb lattice) folded into regular polihedra is studied. A continuum limit valid for sufficiently large molecules and based on a tight binding approximation is derived. It is found that a Dirac equation describes the flat graphite lattice. Curving the lattice by insertion of odd numbered rings can be mimicked by coupling effective gauge fields. In particular the $C_{60}$ and related molecules are well described by the Dirac equation on the surface of a sphere coupled to a color monopole sitting at its center.Comment: 29 pages, 7 figures. IASSNS-HEP-92/5

Follow-up Studies of the Pulsating Magnetic White Dwarf SDSS J142625.71+575218.3

We present a follow-up analysis of the unique magnetic luminosity-variable carbon-atmosphere white dwarf SDSS J142625.71+575218.3. This includes the results of some 106.4 h of integrated light photometry which have revealed, among other things, the presence of a new periodicity at 319.720 s which is not harmonically related to the dominant oscillation (417.707 s) previously known in that star. Using our photometry and available spectroscopy, we consider the suggestion made by Montgomery et al. (2008) that the luminosity variations in SDSS J142625.71+575218.3 may not be caused by pulsational instabilities, but rather by photometric activity in a carbon-transferring analog of AM CVn. This includes a detailed search for possible radial velocity variations due to rapid orbital motion on the basis of MMT spectroscopy. At the end of the exercise, we unequivocally rule out the interacting binary hypothesis and conclude instead that, indeed, the luminosity variations are caused by g-mode pulsations as in other pulsating white dwarfs. This is in line with the preferred possibility put forward by Montgomery et al. (2008).Comment: 11 pages in emulateApJ, 12 figures, accepted for publication in Ap

A Detailed Study of 2S 0114+650 with the RXTE

We present the results of a detailed study of the high mass X-ray binary 2S 0114+650 made with the pointed instruments onboard the Rossi X-ray Timing Explorer. The spectral and temporal behaviour of this source was examined over the pulse, orbital, and super-orbital timescales, covering $\sim$2 cycles of the 30.7 d super-orbital modulation. Marginal evidence for variability of the power law photon index over the pulse period was identified, similar to that observed from other X-ray pulsars. If this variability is real it can be attributed to a varying viewing geometry of the accretion region with the spin of the neutron star. Variability of the neutral hydrogen column density over the orbital period was observed, which we attribute to the line of sight motion of the neutron star through the dense circumstellar environment. A reduction in the power law photon index was observed during the orbital maximum, which we speculate is due to absorption effects as the neutron star passes behind a heavily absorbing region near the base of the supergiant companion's wind. No significant variability of the column density was observed over the super-orbital period, indicating that variable obscuration by a precessing warp in an accretion disc is not the mechanism behind the super-orbital modulation. In contrast, a significant increase in the power law photon index was observed during the super-orbital minimum. We conclude that the observed super-orbital modulation is tied to variability in the mass accretion rate due to some as yet unidentified mechanism.Comment: 22 pages, 27 figures, accepted for publication in MNRAS after moderate revisio