What stops satellites from dropping out of space?
Professor Fred Lamb received his bachelor's degree in physics from the California Institute of Technology in 1967, and his D.Phil. in theoretical physics from Oxford University in 1970. He was a fellow at Magdalen College, Oxford, from 1970 to 1972, when he joined the physics faculty at the University of Illinois as an assistant professor.
Profressor Lamb has made profound and lasting contributions to theoretical high-energy and relativistic astrophysics. He is recognized as an international leader in the field of X-ray astrophysics and neutron stars, where his work has provided both a unified model for understanding the basic phenomena of X-ray emission from neutron stars and the tools for exploring other areas of physics, such as general relativity and the theory of dense matter. His particular strength has been to bring the analytical and quantitative tools of physics to bear on a wide variety of astrophysical problems, and he is widely recognized as providing fundamental theoretical understanding of many important astrophysical phenomena--from the quasi-periodic oscillations seen in the emission from bright galactic X-ray sources to the radiation processes in accretion systems.
One of the original proposers (nearly twenty years ago) of the spectacularly successful NASA Rossi X-ray Timing Explorer (RXTE) satellite project, Professor Lamb tenaciously supported it, guided it through its development, and served as the chair of the NASA RXTE Science Committee. Further, he has devoted his considerable physics skills to the public good by playing a central role (in collaboration with Professor Jeremiah Sullivan) in calculations underpinning arms control, including methods of estimating yields from nuclear detonations--a key to compliance verification for international nuclear test ban treaties.
Professor Lamb is also a gifted teacher. He regularly involves undergraduate students in his research group, and he is known for his skilled guidance and training of his graduate students and exceptional mentoring for his post-doctoral associates and younger colleagues.
Theoretical Studies of X-Ray and Gamma-Ray Emission by Neutron Stars, White Dwarfs, and Black Holes
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 Properties of Accreting and Isolated Neutron Stars
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, RXTE, and AXAF.
Study of Kilohertz QPOs in Z Sources
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.
A Comprehensive Survey of Atoll Sources
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.
Rapid X-Ray Variability of Z Sources
We are using NASA's Rossi X-ray Timing Explorer (RXTE) 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) discovered by the team in four 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.
High-Time-Resolution X-Ray Spectroscopy of Z Sources
NASA's Rossi 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.
Luminosity Dependence of the X-Ray Spectra and Variability of Atoll Sources
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 Rossi 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.
Analysis of X-Ray Emission from the Bursting Pulsar
The Bursting Pulsar is a rotating neutron star that was discovered on December 2, 1996, as it began a giant outburst that lasted six months. This pulsar is unique among all known pulsars in producing both periodic x-ray oscillations with a frequency of 2 Hz and powerful x-ray bursts at intervals ranging from three minutes to ten hours. The cause of the outburst and the mechanism that generates the x-ray bursts are both unknown. This grant is supporting a year-long monitoring campaign in which 10 ksec of data on the Bursting Pulsar are collected approximately every two weeks using the Rossi X-Ray Timing Explorer. Analysis of these data will be used to develop and test models of the accretion torque on the neutron star and models of the outburst and x-ray bursts.
Analysis of Unusual X-Ray Behavior of the Bursting Pulsar
This grant will support collection, analysis, and interpretation of x-ray and gamma-ray data on the Bursting Pulsar taken with the Rossi X-ray Timing Explorer when the pulsar displays unusual behavior, such as a pronounced increase in its brightness between x-ray bursts, a substantial increase in the rate or brightness of the bursts, or unusual changes in its spin rate. These data will be used to develop and test models of the accretion torque on the neutron star and models of the outburst and x-ray bursts.
Further Studies of Rapid X-Ray Variability in Z Sources
The immediate goal of this project is to explore further the relatively coherent pairs of quasi-periodic brightness oscillations (QPOs) with frequencies in the kilohertz range that we discovered in the x-ray stars Sco X-1 and GX 5-1, and the anomalous frequency behavior of the horizontal-branch QPO that we discovered in the x-ray star GX 172 using NASA's Rossi X-Ray Timing Explorer satellite. The kilohertz QPOs we have discovered are the highest frequency coherent behavior ever detected in neutron stars and have important implications for their structure and the equation of state of dense matter. We are modeling these phenomena using analytical methods as well as radiation- and magnetohydrodynamic computer simulations.
237B Loomis Laboratory
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