Relativistic Diskoseismology

We will summarize results of calculations of the modes of oscillation trapped within the inner region of accretion disks by the strong-field gravitational properties of a black hole (or a compact, weakly-magnetized neutron star). Their driving and damping will also be addressed. The focus will be on the most observable class: the analogue of internal gravity modes in stars. Their frequencies which corrrespond to the lowest mode numbers depend almost entirely upon only the mass and angular momentum of the black hole. Such a feature may have been detected in the X-ray power spectra of two galactic `microquasars', allowing the angular momentum of the black hole to be determined in one case.Comment: To be published in Physics Reports, proceedings of the conference Astrophysical Fluids: From Atomic Nuclei to Stars and Galaxies; 10 pages, 5 postscript figure

Relativistic Stellar Pulsations With Near-Zone Boundary Conditions

A new method is presented here for evaluating approximately the pulsation modes of relativistic stellar models. This approximation relies on the fact that gravitational radiation influences these modes only on timescales that are much longer than the basic hydrodynamic timescale of the system. This makes it possible to impose the boundary conditions on the gravitational potentials at the surface of the star rather than in the asymptotic wave zone of the gravitational field. This approximation is tested here by predicting the frequencies of the outgoing non-radial hydrodynamic modes of non-rotating stars. The real parts of the frequencies are determined with an accuracy that is better than our knowledge of the exact frequencies (about 0.01%) except in the most relativistic models where it decreases to about 0.1%. The imaginary parts of the frequencies are determined with an accuracy of approximately M/R, where M is the mass and R is the radius of the star in question.Comment: 10 pages (REVTeX 3.1), 5 figs., 1 table, fixed minor typos, published in Phys. Rev. D 56, 2118 (1997

Gravitational Effects in Supersymmetric Domain Wall Backgrounds

A recent study of supersymmetric domain walls in $N=1$ supergravity theories revealed a new class of domain walls interpolating between supersymmetric vacua with different non-positive cosmological constants. We classify three classes of domain wall configurations and study the geodesic structure of the induced space-time. Motion of massive test particles in such space-times shows that these walls are always repulsive from the anti-deSitter (AdS) side, while on the Minkowski side test particles feel no force. Freely falling particles far away from a wall in an AdS vacuum experience a constant proper acceleration, \ie\ they are Rindler particles. A new coordinate system for discussing AdS space-time is presented which eliminates the use of a periodic time-like coordinate.Comment: 13 pages + 4 figures (not included

Puncture of gravitating domain walls

We investigate the semi-classical instability of vacuum domain walls to processes where the domain walls decay by the formation of closed string loop boundaries on their worldvolumes. Intuitively, a wall which is initially spherical may `pop', so that a hole corresponding to a string boundary component on the wall, may form. We find instantons, and calculate the rates, for such processes. We show that after puncture, the hole grows exponentially at the same rate that the wall expands. It follows that the wall is never completely thermalized by a single expanding hole; at arbitrarily late times there is still a large, thin shell of matter which may drive an exponential expansion of the universe. We also study the situation where the wall is subjected to multiple punctures. We find that in order to completely annihilate the wall by this process, at least four string loops must be nucleated. We argue that this process may be relevant in certain brane-world scenarios, where the universe itself is a domain wall.Comment: 13 pages REVTeX, 3 .ps figures, added some references - version to appear in Physics Letters

Individual differences in multisensory integration and timing

The senses have traditionally been studied separately, but it is now recognised that the brain is just as richly multisensory as is our natural environment. This creates fresh challenges for understanding how complex multisensory information is organised and coordinated around the brain. Take timing for example: the sight and sound of a person speaking or a ball bouncing may seem simultaneous, but their neural signals from each modality arrive at different multisensory areas in the brain at different times. How do we nevertheless perceive the synchrony of the original events correctly? It is popularly assumed that this is achieved via some mechanism of multisensory temporal recalibration. But recent work from my lab on normal and pathological individual differences show that sight and sound are nevertheless markedly out of synch by different amounts for each individual and even for different tasks performed by the same individual. Indeed, the more an individual perceive the same multisensory event as having an auditory lead and an auditory lag at the same time. This evidence of apparent temporal disunity sheds new light on the deep problem of understanding how neural timing relates to perceptual timing of multisensory events. It also leads to concrete therapeutic applications: for example, we may now be able to improve an individual’s speech comprehension by simply delaying sound or vision to compensate for their individual perceptual asynchrony

On the Geometry of Planar Domain Walls

The Geometry of planar domain walls is studied. It is argued that the planar walls indeed have plane symmetry. In the Minkowski coordinates the walls are mapped into revolution paraboloids.Comment: 11 paghoj, Late

Corotational Damping of Diskoseismic C-modes in Black Hole Accretion Discs

Diskoseismic c-modes in accretion discs have been invoked to explain low-frequency variabilities observed in black-hole X-ray binaries. These modes are trapped in the inner-most region of the disc and have frequencies much lower than the rotation frequency at the disc inner radius. We show that because the trapped waves can tunnel through the evanescent barrier to the corotational wave zone, the c-modes are damped due to wave absorption at the corotation resonance. We calculate the corotational damping rates of various c-modes using the WKB approximation. The damping rate varies widely depending on the mode frequency, the black hole spin parameter and the disc sound speed, and is generally much less than 10% of the mode frequency. A sufficiently strong excitation mechanism is needed to overcome this corotational damping and make the mode observable.Comment: 10 pages, 5 figures, MNRAS in pres