This work focuses on problems in astrophysics where a knowledge of microscopic and many-body physics will lead to rapid progress. Among the problems being addressed are: the equation of state and transport properties of matter at high densities; the influence of superfluid neutrons on the structure, dynamics, and evolution of neutron stars; and the physics of accretion by magnetic neutron stars and white dwarfs.

Observations of cosmic x-ray sources by a sequence of Earth-orbiting satellites has led to rapid advances in understanding accreting neutron stars, black holes, and white dwarfs. This research supports these orbital missions by investigating accretion by magnetic neutron stars; quasi-periodic and periodic oscillations in the brightness of compact cosmic x-ray sources; the physics of x-ray emission by magnetic and nonmagnetic neutron stars and white dwarfs; and methods of determining the internal and external properties of neutron stars from observations of their pulsed x-ray emission.

This research project uses the physical concepts and mathematical techniques of condensed matter, nuclear, high-energy, gravitational, and plasma physics, and of radiation hydrodynamics to investigate the properties of matter under the extreme conditions found in and near neutron stars, black holes, and white dwarfs, and in the early universe. The results of these investigations are incorporated into models of these objects and of accretion by them. Goals include the development of specific predictions that can be compared directly with observations and the identification of promising new observations that would test current theories.

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.

Much of what we know about neutron stars has come from x-ray observations. We are investigating disk accretion by magnetic and nonmagnetic neutron stars; subcritical and supercritical radial flows onto nonmagnetic and magnetic neutron stars; the x-ray spectra of disk and radially accreting neutron stars; and neutron star structure, cooling processes, and thermal evolution. The results are used to improve understanding of spin-up and spin-down of accretion-powered pulsars, the nature of the Z and atoll sources, quasi-periodic brightness oscillations in neutron stars and black holes, and thermal x-ray emission by isolated neutron stars. These studies directly support NASA's high-energy astrophysics missions, including ROSAT, the Compton Observatory, EVVE, XTE, and AXAF.

We are using NASA's X-ray Timing Explorer (XTE) satellite to study, for the first time, the submillisecond variability of the so-called Z sources, which are among the brightest known x-ray stars. The goals of this project include detailed analysis of the quasi-periodic brightness oscillations (QPOs) of these sources, comparison of the results with the detailed predictions of QPO properties made previously by the theoretical group at Illinois, and searches for predicted new, fast aperiodic variability and millisecond periodic oscillations. We are also using advanced time-series analysis techniques to study variations in the power spectral density of brightness variations on time scales as short as seconds.

NASA's X-ray Timing Explorer satellite is being used to study, for the first time, variations in the x-ray spectra of two Z sources on timescales of milliseconds, which are the timescales of the quasi-periodic oscillations and aperiodic flickering observed in these x-ray stars. We are reconstructing x-ray spectra to determine how the x-ray spectra of these sources vary on very short timescales. We are also developing quantitative models of the x-ray spectrum and the millisecond x-ray variability expected when matter falls onto the compact object in these systems, which is thought to be a neutron star. Successful comparison of the predictions of these models with our analysis results will test the models and allow us to derive the physical properties of the neutron stars and accretion disks in these systems.

At low luminosities, the x-ray properties of the x-ray stars called atoll sources are very similar to those of black hole candidates in their low states. We are using NASA's X-ray Timing Explorer satellite to observe a selected sample of atoll sources, which are thought to be accreting neutron stars, in order to study the luminosity dependence of their x-ray spectra and x-ray variability. This research is aimed at understanding the respective roles of the mass accretion rate and the magnetic field in determining the x-ray spectrum and its variability and the reasons for the similarities between the properties of atoll sources and accreting black holes. Detailed theoretical modeling of x-ray spectral formation in LMXBs is an integral part of the study.

We develop and use state-of-the-art numerical techniques on supercomputers to investigate the role of ambipolar diffusion in the formation of protostars in massive molecular clouds. The effect of interstellar dust particles (grains) in (collisionally) coupling the magnetic field to the dominant neutral species (hydrogen molecules), and thereby affecting the contraction time and the masses and sizes of protostars, is at the heart of the project. Rotation and magnetic braking are also included. The nonlinear interactions of gravitational, magnetic, thermal-pressure, frictional, and centrifugal forces, and the role they play in the formation of protostars, are being elucidated.

Theoretical research is conducted to elucidate physical processes in the diffuse astrophysical environment. Currently, the primary effort is in understanding the transport of maser radiation in rotating disks that occurs in the formation of stars from the interstellar gas.

Several aspects of the microphysics that influences the diffuse galactic and extragalactic medium are being investigated. Effort is devoted to understanding the physics necessary to relate the observed polarization of radiation (at radio and infrared wavelengths) from star-forming and other regions to the magnetic fields in these environments. Effort is also devoted to understanding the transport of maser radiation in rotating, gaseous disks associated with the nuclei of other galaxies.