Robert V. Wagoner

SN 1992A : ultraviolet and optical studies based on HST, IUE, and CTIO observations

The Type Ia supernova SN 1992A in the SO galaxy NGC 1380 was observed as a target of opportunity by the International Ultrauiolet Explorer (IUE) and with great alacrity by the Hubble Space Telescope (HST). Here we present the HST and IUE spectra and photometry that we obtained, as well as optical spectra obtained at the Cerro Tololo Inter-American Observatory (CTIO). The HST Faint Object Spectrograph (FOS) spectra, from 5 and 45 days past maximum light, are the best UV spectra of a Type Ia supernova and reveal for the first time with good signal-to-noise ratio the Type Ia spectral region blueward of ∼2650 A

“Stable” Quasi-periodic Oscillations and Black Hole Properties from Diskoseismology

We compare our calculations of the frequencies of the fundamental g-, c-, and p-modes of relativistic thin accretion disks with recent observations of high-frequency quasi-periodic oscillations (QPOs) in X-ray binaries with black hole candidates. These classes of modes encompass all adiabatic perturbations of such disks. The frequencies of these modes depend mainly on the mass and angular momentum of the black hole; their weak dependence on disk luminosity is also explicitly indicated. Identifying the recently discovered, relatively stable QPO pairs with the fundamental g- and c-modes provides a determination of the mass and angular momentum of the black hole. For GRO J1655-40, M = 5.9 ± 1.0 M☉ and J = (0.917 ± 0.024)GM2/c, in agreement with spectroscopic mass determinations. For GRS 1915+105, M = 42.4 ± 7.0 M☉ and J = (0.926 ± 0.020)GM2/c or (less favored) M = 18.2 ± 3.1 M☉ and J = (0.701 ± 0.043)GM2/c. We briefly address the issues of the amplitude, frequency width, and energy dependence of these QPOs.

Diskoseismology: Probing accretion disks. I - Trapped adiabatic oscillations

The normal modes of acoustic oscillations within thin accretion disks which are terminated by an innermost stable orbit around a slowly rotating black hole or weakly magnetized compact neutron star are analyzed. The dominant relativistic effects which allow modes to be trapped within the inner region of the disk are approximated via a modified Newtonian potential. A general formalism is developed for investigating the adiabatic oscillations of arbitrary unperturbed disk models. The generic behavior is explored by way of an expansion of the Lagrangian displacement about the plane of symmetry and by assuming separable solutions with the same radial wavelength for the horizontal and vertical perturbations. The lowest eigenfrequencies and eigenfunctions of a particular set of radial and quadrupole modes which have minimum motion normal for the plane are obtained. These modes correspond to the standard dispersion relation of disk theory.

Relativistic Diskoseismology. I. Analytical Results for “Gravity Modes”

We generalize previous calculations to a fully relativistic treatment of adiabatic oscillations that are trapped in the inner regions of accretion disks by non-Newtonian gravitational effects of a black hole. We employ the Kerr geometry within the scalar potential formalism of Ipser and Lindblom, neglecting the gravitational field of the disk. This approach treats perturbations of arbitrary stationary, axisymmetric, perfect fluid models. It is applied here to thin accretion disks. Approximate analytic eigenfunctions and eigenfrequencies are obtained for the most robust and observable class of modes, which corresponds roughly to the gravity (internal) oscillations of stars. The dependence of the oscillation frequencies on the mass and angular momentum of the black hole is exhibited. These trapped modes do not exist in Newtonian gravity, and thus provide a signature and probe of the strong-field structure of black holes. Our predictions are relevant to observations that could detect modulation of the X-ray luminosity from stellar mass black holes in our Galaxy and the UV and optical luminosity from supermassive black holes in active galactic nuclei.

Gravitational radiation from accreting neutron stars

Neutron stars with relatively weak magnetic fields which are accreting matter from a disk may acquire enough angular momentum to become unstable to the emission of gravitational radiation. When a steady states is reached (viscous damping time equals instability growth time), the metric perturbation h is related to the observed time-averaged X-ray flux l/sub ..gamma../ by happrox.3 x 10/sup -27/ (1 kHz/mf)/sup 1/2/ (l/sub ..gamma..//10/sup -8/ ergs cm/sup -2/ s/sup -1/)/sup 1/2/, where mapprox.4 is the mode number, and f is the frequency of the monochromatic gravitational waves. The small nonaxisymmetric distortion of the neutron star should produce some modulation of the X-ray emission at the gravitational-wave frequency. This frequency should lie in the range 200< or approx. =f< or approx. =800 Hz, well below the rotational frequency of the neutron star.

Direct Analysis of Spectra of the Type Ic Supernova SN 1994I

Synthetic spectra generated with the parameterized supernova synthetic-spectrum code SYNOW are compared to observed photospheric-phase spectra of the Type Ic supernova SN 1994I. The observed optical spectra can be well matched by synthetic spectra that are based on the assumption of spherical symmetry. We consider the identification of the infrared absorption feature observed near 10250 ?, which previously has been attributed to He I ?10830 and regarded as strong evidence that SN 1994I ejected some helium. We have difficulty accounting for the infrared absorption with He I alone. It could be a blend of He I and C I lines. Alternatively, we find that it can be fitted by Si I lines without compromising the fit in the optical region. In synthetic spectra that match the observed spectra, from 4 days before to 26 days after the time of maximum brightness, the adopted velocity at the photosphere decreases from 17,500 to 7000 km s-1. Simple estimates of the kinetic energy carried by the ejected mass give values that are near the canonical supernova energy of 1051 ergs. The velocities and kinetic energies that we find for SN 1994I in this way are much lower than those that we find elsewhere for the peculiar Type Ic SNe 1997ef and 1998bw, which therefore appear to have been hyperenergetic.

Aligned Rotating Magnetospheres. General Analysis

We consider the problem of finding self-consistent steady-state axisymmetric solutions for the electron and ion densities and velocities, as well as the electromagnetic field, outside a uniformly rotating, perfectly conducting star. Inertial effects (assumed small) are included order to mare the problem determinate, but collisional and gravitational forces are neglected. The requirement that all physical quantities be nonsingular leads to boundary conditions which appear capable of determining complete solutions within the light cylinder, which can then be analytically extended beyond the light cylinder. The special case of a toroidal magnetic field which is a linear function of the poloidal magnetic potential A is treated in detail. The behavior of A is then governed by a homogeneous linear separable partial differential equation. Its eigenfunctions and eigenvalues are investigated. Some particular results are presented for the region occupied by the polar field lines. Future papers will consider the detailed properties of various regions of the magnetosphere. (auth)

Diskoseismology: Probing accretion disks. II - G-modes, gravitational radiation reaction, and viscosity

A scalar potential is used to derive a single partial differential equation governing the oscillation of a disk. The eigenfunctions and eigenfrequencies of a variety of disk models are found to fall into two main classes which are analogous to the p-modes and g-modes in the sun. Specifically, the eigenfunctions and eigenfrequencies of isothermal disks are computed, and the way in which these results can be generalized to other disk models is indicated. The (assumed) relatively small rates of growth or damping of the modes due to various mechanisms, in particular gravitational radiation reaction and parameterized models of viscosity are also computed. It is found that for certain parameters the p-modes are unstable to gravitational radiation reaction (CFS instability), while both the p-modes and g-modes are unstable to viscosity unless highly anisotropic viscosity models are considered.

Determining the properties of accretion-gap neutron stars

If neutron stars have radii as small as has been argued by some, observations of accretion-powered X-rays could verify the existence of innermost stable circular orbits (predicted by general relativity) around weakly magnetized neutron stars. This may be done by detecting X-ray emission from clumps of matter before and after they cross the gap (where matter cannot be supported by rotation) between the inner accretion disk and the stellar surface. Assuming the validity of general relativity, it would then be possible to determine the masses of such neutron stars independently of any knowledge of binary orbital parameters. If an accurate mass determination were already available through any of the methods conventionally used, the new mass determination method proposed here could then be used to quantitatively test strong field effects of gravitational theory. 67 refs.

Post-Newtonian gravitational radiation from orbiting point masses

General formulae are derived which describe the gravitational radiation at large distances from a system of bodies whose sizes are small compared with their separations. The calculation is carried out through post-Newtonian order within general relativity. More explicit formulae are derived for two-body systems, and detailed results are presented for circular orbits, gravitational bremsstrahlung, and head-on collisions. (AIP)