Coalescence of black hole–neutron star binaries

We review the current status of general relativistic studies for coalescences of black hole–neutron star binaries. First, high-precision computations of black hole–neutron star binaries in quasiequilibrium circular orbits are summarized, focusing on the quasiequilibrium sequences and the mass-shedding limit. Next, the current status of numerical-relativity simulations for the merger of black hole–neutron star binaries is described. We summarize our understanding for the merger process, tidal disruption and its criterion, properties of the merger remnant and ejected material, gravitational waveforms, and gravitational-wave spectra. We also discuss expected electromagnetic counterparts to black hole–neutron star coalescences

Electronic structure of the candidate 2D Dirac semimetal SrMnSb2: a combined experimental and theoretical study

SrMnSb$_2$ is suggested to be a magnetic topological semimetal. It contains square, 2D Sb planes with non-symmorphic crystal symmetries that could protect band crossings, offering the possibility of a quasi-2D, robust Dirac semi-metal in the form of a stable, bulk (3D) crystal. Here, we report a combined and comprehensive experimental and theoretical investigation of the electronic structure of SrMnSb$_2$, including the first ARPES data on this compound. SrMnSb$_2$ possesses a small Fermi surface originating from highly 2D, sharp and linearly dispersing bands (the Y-states) around the (0,$\pi$/a)-point in $k$-space. The ARPES Fermi surface agrees perfectly with that from bulk-sensitive Shubnikov de Haas data from the same crystals, proving the Y$-$states to be responsible for electrical conductivity in SrMnSb$_2$. DFT and tight binding (TB) methods are used to model the electronic states, and both show good agreement with the ARPES data. Despite the great promise of the latter, both theory approaches show the Y-states to be gapped above E$_F$, suggesting trivial topology. Subsequent analysis within both theory approaches shows the Berry phase to be zero, indicating the non-topological character of the transport in SrMnSb$_2$, a conclusion backed up by the analysis of the quantum oscillation data from our crystals.Comment: 26 pages, 10 figures, revised submission to SciPost after including changes requested by referees. All referee reports are open and can be viewed here: https://scipost.org/submissions/1711.07165v2

Space-based Gravitational Wave Observatories

In this article, which will appear as a chapter in the Handbook of Gravitational Wave Astronomy, we will describe the detection of gravitational waves with space-based interferometric gravitational wave observatories. We will provide an overview of the key technologies underlying their operation, illustrated using the specific example of the Laser Interferometer Space Antenna (LISA). We will then give an overview of data analysis strategies for space-based detectors, including a description of time-delay interferometry, which is required to suppress laser frequency noise to the necessary level. We will describe the main sources of gravitational waves in the millihertz frequency range targeted by space-based detectors and then discuss some of the key science investigations that these observations will facilitate. Once again, quantitative statements given here will make reference to the capabilities of LISA, as that is the best studied mission concept. Finally, we will describe some of the proposals for even more sensitive space-based detectors that could be launched further in the future

A Generalized Multigroup Method

The standard multigroup (MG) method for energy discretization of the transport equation can be sensitive to approximation in the weighting spectrum chosen for cross-section averaging. As a result, MG often inaccurately treats important phenomena such as self-shielding variations across a fuel pin in nuclear reactor simulations. From a finite-element viewpoint, MG uses a single fixed basis function (the pre-selected spectrum) within each group, with no mechanism to adapt to local spatial and angular solution realities. To address these issues, we introduce a Petrov-Galerkin finite-element multigroup (PG-FEMG) method, a generalization of the MG method that is related to the family of multiband (MB) methods. PG-FEMG uses integrals over several discontinuous energy domains within a group as its degrees of freedom, which allows PG-FEMG to be used in standard MG-based computer codes with changes to pre- and post-processing of the data only. We define a problem-wide effective total cross section as the basis of these discontinuous energy domains. We implement the PG-FEMG method for several realistic pin-cell problems and find it to be significantly more accurate per degree of freedom than MG for several quantities of interest, including criticality eigenvalue and power profile shape. We find that PG-FEMG is much less sensitive to errors in weighting spectra compared to standard MG. We discuss straightforward generalizations to multi-dimensional problems of practical interest, including reactor depletion calculations

Precision VLBI astrometry: Instrumentation, algorithms and pulsar parallax determination

(Abridged) This thesis describes the development of DiFX, the first general-purpose software correlator for radio interferometry, and its use with the Australian Long Baseline Array (LBA) to complete the largest VLBI pulsar astrometry program undertaken to date in the Southern Hemisphere. This two year astrometry program has resulted in the measurement of seven new pulsar parallaxes, more than trebling the number of measured VLBI pulsar parallaxes in the Southern Hemisphere. The measurements included a determination of the distance and transverse velocity of PSR J0437-4715 with better than 1% accuracy, enabling improved tests of General Relativity, and the first significant measurement of parallax for the famous double pulsar system PSR J0737-3039A/B, which will allow tests of General Relativity in this system to proceed to the 0.01% level. The DiFX software correlator developed to enable this science has been extensively tested and is now an integral part of the upgraded LBA Major National Research Facility; furthermore, it has been selected to facilitate a substantial sensitivity upgrade for the US Very Long Baseline Array.Comment: PhD Thesis, Swinburne University, accepted January 2009. 202 pages, 51 figures. For a version with high resolution images, see http://www.aoc.nrao.edu/~adeller/AdamDellerPhDThesis.pd

Journey to the Center of GW170817: Bayesian parameter estimation of outflows from binary neutron star mergers

The discovery of GW170817 provided the first empirical evidence that merging binary neutron star systems are both progenitors of short gamma-ray bursts, as well as the primary sites of the nucleosynthetic rapid-neutron capture process. Initially detected as gravitational wave (GW) and gamma-ray burst (GRB) triggers, GW170817 was well-localized and follow-up observations detected a kilonova along with the GRB afterglow. Gamma-ray burst afterglows encode information about jet geometry and the environments in which the relativistic jets propagated. However, the information we can garner is limited to regions that are transparent to radiation—meaning it cannot directly give us information about the central engine, the properties of the jet at injection, or tell us how that jet acquires its structure. By using Bayesian parameter estimation analysis to connect astrophysical data with numerical models, we can begin to constrain the properties of these unobservable regions and improve our understandings of short GRBs and neutron star matter. In this work, we performed two separate analyses using emcee, a Python implementation of the Markov Chain Monte Carlo sampling method. We first constrained jet, environment, and observer parameters of a GRB afterglow resulting from an off-axis structured jet. We then used a subset of those results as data in our second analysis to constrain properties of the central engine, neutron star ejecta, and the jet at injection using an outflow model that simulates baryon-loaded wind and jet interactions. In total we were able to constrain 17 parameters, the largest parameter space so far explored using structured jet outflow and afterglow models. We anticipate that with future joint GW-GRB observations of binary neutron star merger events, similar techniques can be used to further probe the nature of neutron star matter, and by extension a neutron star’s equation of state

Optics and Quantum Electronics

Contains table of contents on Section 3 and reports on nineteen research projects.Defense Advanced Research Projects Agency Grant F49620-96-0126Joint Services Electronics Program Grant DAAH04-95-1-0038National Science Foundation Grant ECS 94-23737U.S. Air Force - Office of Scientific Research Contract F49620-95-1-0221U.S. Navy - Office of Naval Research Grant N00014-95-1-0715Defense Advanced Research Projects Agency/National Center for Integrated Photonics TechnologyMultidisciplinary Research InitiativeU.S. Air Force - Office of Scientific ResearchNational Science Foundation/MRSECU.S. Navy - Office of Naval Research (MFEL) Contract N00014-91-J-1956National Institutes of Health Grant R01-EY11289U.S. Navy - Office of Naval Research (MFEL) Contract N00014-94-0717Defense Advanced Research Projects Agency Contract N66001-96-C-863

UAH/NASA Workshop on Space Science Platform

The scientific user requirements for a space science platform were defined. The potential user benefits, technological implications and cost of space platforms were examined. Cost effectiveness of the platforms' capabilities were also examined