Compact Binaries as Sources of Gravitational Radiation

With current terrestrial gravitational wave detectors working at initial design sensitivities, and upgrades and space missions planned, it is likely that in the next five to ten years gravitational radiation will be detected directly from a variety of classes of objects. The most confidently expected of these classes is compact binaries, involving neutron stars or black holes. Detection of their coalescence, or their long-term orbits, has the potential to inform us about the evolutionary history of compact binaries and possibly even star formation over the past several billion years. We review what is currently known about compact binaries as sources of gravitational radiation, as well as the current uncertainties and what we expect to learn from future detections of gravitational waves from these systems.Comment: To appear in the proceedings of the meeting "The Multicoloured Landscape of Compact Objects and their Explosive Origins", Cefalu, Italy, June 2006, to be published by AIP, Eds. L. Burderi et a

Beat-Frequency Models of Kilohertz QPOs

Kilohertz QPO sources are reasonably well-characterized observationally, but many questions remain about the theoretical framework for these sources and the consequent implications of the observations for disk physics, strong gravity, and dense matter. We contrast the predictions and implications of the most extensively studied class of kilohertz QPO models, the beat-frequency models, with those of alternative classes of models. We also discuss the expected impact of new observations of these sources with satellites such as Chandra, XMM, and Constellation-X.Comment: 10 pages, invited paper at Bologna X-ray Astronomy 1999. To appear in Astrophysical Letters and Communication

Spectral Effects of the Vacuum Resonance in Soft Gamma-Ray Repeaters

The association of all three soft gamma-ray repeaters (SGRs) with supernova remnants has established that SGRs are young neutron stars, and has given us a starting point for detailed modeling. One of the most popular classes of models involves strongly magnetised neutron stars, with surface dipole fields B~ 10^{14}-10^{15} Gauss. In such strong magnetic fields, many otherwise negligible processes can play an important role. Here we consider the effects of vacuum polarisation on Compton scattering. Vacuum polarisation introduces a characteristic density-dependent photon frequency at which the normal modes of polarisation become nonorthogonal and the mean free path of photons decreases sharply. Our analytic results and Monte Carlo simulations of photon propagation through a magnetised plasma show that this effect leads, under a wide range of physical conditions, to a broad absorption-like feature in the energy range ~5 keV---40 keV. We discuss this effect in light of the spectra from SGR 1806-20.Comment: 13 pages Latex , uses mn.sty. Submitted to MNRA