5,614 research outputs found

    Approaches to Teaching Tolkien\u27s The Lord of the Rings and Other Works (2015) edited by Leslie A. Donovan

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    Book review of Approaches to Teaching Tolkien\u27s The Lord of the Rings and Other Works (2015), edited by Leslie A. Donova

    Quasi-Periodic Oscillations from Low-Mass X-Ray Binaries with Neutron Stars

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    Before the launch of the Rossi X-ray Timing Explorer (RXTE) it was recognized that neutron star accretion disks could extend inward to very near the neutron star surface, and thus be governed by millisecond timescales. Previous missions lacked the sensitivity to detect them. The kilohertz quasi-periodic oscillations (QPO) that RXTE discovered are often, but not always, evident in the X-ray flux. In 8 years RXTE has found kilohertz signals in about a fourth of 100 low-mass X-ray binaries (LMXB) containing neutron stars. The observed power spectra have simple dominant features, the two kilohertz oscillations, a low frequency oscillation, and band-limited white noise. They vary systematically with changes in other source properties and offer the possibility of comparison with model predictions. New information from the millisecond pulsars resolves some questions about the relations of the QPO and the spin. Coherence, energy spectrum and time lag measurements have indicated systematic behaviors, which should constrain mechanisms.Comment: 8 pages, 8 figures, to appear in the proceedings of "X-Ray Timing 2003: Rossi and Beyond", eds. P. Kaaret, F.K. Lamb, & J.H. Swank (Melville, NY: AIP

    X-Ray Observations of Low-Mass X-Ray Binaries: Accretion Instabilities on Long and Short Time-Scales

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    X-rays trace accretion onto compact objects in binaries with low mass companions at rates ranging up to near Eddington. Accretion at high rates onto neutron stars goes through cycles with time-scales of days to months. At lower average rates the sources are recurrent transients; after months to years of quiescence, during a few weeks some part of a disk dumps onto the neutron star. Quasiperiodic oscillations near 1 kHz in the persistent X-ray flux attest to circular motion close to the surface of the neutron star. The neutron stars are probably inside their innermost stable circular orbits and the x-ray oscillations reflect the structure of that region. The long term variations show us the phenomena for a range of accretion rates. For black hole compact objects in the binary, the disk flow tends to be in the transient regime. Again, at high rates of flow from the disk to the black hole there are quasiperiodic oscillations in the frequency range expected for the innermost part of an accretion disk. There are differences between the neutron star and black hole systems, such as two oscillation frequencies versus one. For both types of compact object there are strong oscillations below 100 Hz. Interpretations differ on the role of the nature of the compact object.Comment: 12 pages, 5 Postscript figures, in "Astrophysical Sources of Gravitational Radiation for Ground-Based Detectors", American Institute of Physics, 200

    Observations of Type I Bursts from Neutron Stars

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    Observations of Type I X-ray bursts have long been taken as evidence that the sources are neutron stars. Black body models approximate the spectral data and imply a suddenly heated neutron star cooling over characteristic times of seconds to minutes. The phenomena are convincingly explained in terms of nuclear burning of accreted gas on neutron stars with low mass companion stars. Prospects are promising that detailed theory and data from RXTE and future missions will lead to better determinations of important physical parameters (neutron star mass and radius, composition of the accreting gas, distance of the source). Among the variety of bursts observed, there are probably representatives of different kinds of explosive burning. RXTE's discovery of a 2.5 ms persistent coherent period from one Type I burster has now linked bursters indisputably to the epitome of a neutron star, a fast spinning magnetic compact object. Oscillations in some bursts had already been thought to arise from the neutron stars' rotations. Detailed observations of these oscillations are touchstones of how the explosive bursts originate and progress, as well as independent measures of the neutron star parameters.Comment: 10 pages, 4 figures, to appear in "Cosmic Explosions, Proceedings of the 10th October Astrophysics in Maryland, AIP Conf. Proceedings 522, ed. S. S. Holt & W. W. Zhang, (AIP: Woodbury, N. Y.), 200

    Creating a Legacy: Building a Planned Giving Program From the Ground Up

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    This book explores if, when, and how to use planned giving as part of a fundraising strategy. Includes tips and practical examples, as well as the dos and don'ts associated with building a well-integrated planned giving program
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