A Comprehensive Survey of Atoll Sources
F. K. Lamb,* L. Zampieri
National Aeronautics and Space Administration, RXTE-30701

With the co-discovery by the UIUC team of quasi-periodic x-ray brightness oscillations (QPOs) at kilohertz frequencies using the Rossi X-Ray Timing Explorer, we appear to be on the threshold of a breakthrough in measuring general relativistic effects in the strong-field regime and in determining the masses and radii of neutron stars and the equation of state of neutron star matter. The major survey of the atoll sources and further theoretical research that this grant is supporting are expected to provide much of the additional results needed to achieve this breakthrough.

Active Galactic Nuclei, Dense Stellar Systems, and Galactic Environment
S. Lamb,* in collaboration with J. Perry, Cambridge, England, and twelve others in the U.S. and Europe
University of Illinois

We are investigating a self-consistent model on a large range of scales to understand the processes leading to nuclear activity in galaxies. Current observations support the view that interactions between galaxies may be crucial in triggering episodes of activity in some active galactic nuclei. Interactions also trigger some starbursts, and we are investigating the relationship between these two phenomena. We employ numerical simulations of colliding galaxies and analytical studies of the physics of the central regions of galaxies to obtain a detailed model that can be compared to observations of these systems.

Applications of Field Theory in the Early Universe
J. D. Cohn,* D. Kaiser, E. Stewart
National Science Foundation, PHY98-96019

Methods from field theory and string theory were applied to processes in the early universe, in particular to inflationary models which produce open universes. The spectrum of scalar fields and gravity waves for a set of models proposed by Hawking and Turok were calculated. Supercurvature modes for a massless field have been expressed in terms of more familiar flat-space modes. Current work is on inflationary model building.

Coalescence of Binary Black Holes and Neutron Stars-Computational Contributions to LIGO
S. L. Shapiro,* T. W. Baumgarte
National Science Foundation, PHY 99-02833

Gravitational wave forms from the final plunge and coalescence of black-hole and neutron-star binaries are primary targets for LIGO and other gravitational wave interferometers. These wave forms can only be predicted by large-scale numerical computations. If such simulated wave forms are to be made available to the LIGO community in a timely fashion, a significant effort to compute them must be continued now. A code for evolving Einstein's equations in three spatial dimensions and with no special symmetries assumed is needed to produce binary coalescence simulations. The researchers will construct such a code to solve the problem of coalescing compact binaries.

Cosmic Microwave Background Anisotropies and Structure Formation
M. White*
University of Illinois

The tens of micro-Kelvin variations in the temperature of the cosmic microwave background (CMB) radiation across the sky encode a wealth of information about the universe. The full-sky, high-resolution maps of the CMB that will be made in the next decade should determine cosmological parameters to unprecedented precision and sharply test inflation and other theories of the early universe. We are working on the theoretical foundations of CMB anisotropy formation and the interface between theory and experiment in this highly active field.

Global Star Formation in Impact-induced Starburst Galaxies
S. Lamb*
University of Illinois

The first impact of two colliding galaxies takes place on a time scale of approximately 108 years, the dynamical time scale. Within this period it is anticipated that much star formation will be triggered as a result of density increases and shocks in the gas which are produced by inflow to the nuclear regions. We are currently comparing our array of simulations of galaxy collisions to observations of collisionally produced starburst galaxies (both our own observations and those of others) and investigating the resulting implications for both the stellar and gaseous components.

Observational Tests of QPO Models
F. K. Lamb*
National Aeronautics and Space Administration, RXTE 4-0018

The special properties of the so-called horizontal branch oscillation (HBO) observed in the x-ray stars GX 17+2 and GX 5-1 are being exploited to test observationally the Lense-Thirring precession and beat-frequency models of the kilohertz quasi-periodic oscillations and the HBO. Extensive observations of these two x-ray stars are being carried out using the Rossi X-Ray Timing Explorer satellite, focusing on the relationship between these oscillations. The data will be analyzed using sophisticated time series analysis and modeled by numerical calculations of gas dynamics and radiation transport in full general relativity.

Rotating and Binary Neutron Stars in General Relativity
S. L. Shapiro*
National Aeronautics and Space Administration, NAG 5-7152

The two-body problem remains the most important unsolved problem in classical general relativity. The solution has important consequences for the detection of gravitational waves from laser interferometers now under construction. Binary and rotating neutron stars require the same computational machinery for analysis: a fully relativistic code in three equations for the gravitational field and the relativistic hydrodynamic equations for the matter.

Simulation of Turbulent Mixing and Propagation of Nuclear Burning in the Surface Layers of Accreting Neutron Stars
F. K. Lamb,* V. A. Simonenko*
National Aeronautics and Space Administration, NRA 98-OSS-08

This is a joint research project involving scientists at the University of Illinois at Urbana-Champaign and the Russian Federal Nuclear Center/Institute of Technical Physics. The goal is to improve understanding of the spread of nuclear burning in the surface layers of accreting neutron stars by comparing the results of large scale two- and three-dimensional radiation-hydrodynamic calculations with data on the spread of thermonuclear burning in several neutron stars obtained using NASA's Rossi X-Ray Timing Explorer (RXTE) satellite. The effects of inhomogeneous fuel loadings, hydrodynamic instabilities, turbulent mixing, and stellar rotation are being investigated.

Studies in Theoretical Physics and Astrophysics
F. K. Lamb,* C. J. Pethick, S. L. Shapiro, O. Markovic
National Science Foundation, AST 96-18524

The project addresses problems in condensed matter physics, nuclear physics, and general relativity, as well as neutrino astrophysics, stellar dynamics, hydrodynamics, supernovae, neutron stars and black holes, and active galactic nuclei. A common theme uniting the diverse theoretical studies is understanding the physics of compact objects. As a consequence, much of the effort is devoted to investigating the properties of matter under extreme conditions and strong gravitational fields. The approach involves large-scale computations as well as analytic modeling.

Study of Kilohertz QPOs in Z Sources
F. K. Lamb,* L. Zampieri
National Aeronautics and Space Administration, RXTE-30040

This research project focuses on additional observations of the Z sources using the Rossi X-Ray Timing Explorer and further development of theoretical models. The project is measuring the photon energy dependence of the kilohertz quasi-periodic x-ray brightness oscillations (QPOs) discovered earlier by the team using the Rossi Explorer, the dependence of QPO frequencies on accretion rate, and microsecond time variability, and is comparing the results with gas dynamical and radiation transport calculations. New data analysis algorithms and advanced time series analysis techniques developed by the team are being used.

Supernova Fallback and the Emergence of Black Holes and Neutron Stars
S. L. Shapiro*
National Aeronautics and Space Administration, NAG 5-8418

We propose to investigate the fallback of gaseous material in a supernova onto the central compact object formed in the aftermath of the explosion. We will construct a radiation-hydrodynamics code to calculate the light curve and the spectrum from the supernova remnant, taking into account the luminosity produced by accretion onto the compact object. Our preliminary calculations for SN 1997D suggest that a black hole may emerge above the emission of the envelope in just a few years. The detection of its accretion luminosity by HST may provide unmistakable evidence for the presence of a black hole.

Theoretical Studies of X-Ray and Gamma-Ray Emission by Neutron Stars, White Dwarfs, and Black Holes
F. K. Lamb,* Y. Chen
National Aeronautics and Space Administration, NAG 5-2925

This project involves theoretical research that directly supports analysis and interpretation of data from recent and forthcoming NASA-supported high-energy astrophysics missions. This research focuses on six main topics: neutron star structure, dynamics and evolution; accretion by magnetic and nonmagnetic neutron stars and black holes; quasi-periodic x-ray brightness oscillations (QPOs), pulse frequency changes in x-ray radio pulsars, and x-ray bursts; x-ray spectroscopy of accretion-powered pulsars, accreting neutron stars in low-mass binary systems, and solitary neutron stars; gamma-ray emission by accreting neutron stars; and feeding of black holes in active galactic nuclei.

X-Ray Spectra and Variability of Accreting Neutron Stars
F. K. Lamb,* D. Psaltis, L. Zampieri
National Aeronautics and Space Administration, NAG 4-2925

The goal of this project is to improve understanding of the x-ray spectra and variability of accreting neutron stars in low-mass binary systems. Specific problems being addressed include formation of the x-ray spectra of the Z and atoll sources and their characteristic variations; interaction of accretion disks with the strong magnetic fields of the neutron stars in these systems and their resulting spin evolution; fully general relativistic radiation hydrodynamics of the inner disk and central corona and possible signatures of strongly curved spacetime; use of burst oscillations and kilohertz QPOs to determine the properties of neutron stars and dense matter.