James N. Imamura

The quasi-periodic oscillations and very low frequency noise of Scorpius X-1 as transient chaos - A dripping handrail?

We present evidence that the quasi-periodic oscillations (QPO) and low frequency noise (LFN) characteristic of many accretion sources are different aspects of the same physical process. We analyzed a long, high time resolution EXOSAT observation of Sco X-1. The x-ray luminosity varies stochastically on time scales from milliseconds to hours. The nature of this variability -- as quantified with both power spectrum analysis and new wavelet technique, the {\it scalegram} - agrees well with the {\it dripping handrail} accretion model, a simple dynamical system which exhibits {\it transient chaos}. In this model both the QPO and LFN are produced by radiation from blobs with a wide size distribution, resulting from accretion and subsequent diffusion of hot gas, the density of which is limited by an unspecified instability to lie below a threshold.

A numerical study of the stability of radiative shocks

Attention is given to the oscillatory instability of optically thin radiative shocks in time-dependent numerical calculations of accretion flows onto degenerate dwarfs. The present nonlinear calculations yield good quantitative agreement with the linear results obtained for oscillation frequencies, damping rates, and critical alpha-values. The fundamental mode and the first overtone in the shock radius and luminosity variations can be clearly identified, and evidence is sometimes seen for the second overtone. Time-dependent calculations are also performed which include additional physics relevant to degenerate dwarf accretion, such as electron thermal conduction, unequal electron and ion temperatures, Compton cooling, and relativistic corrections to the bremsstrahlung cooling law. All oscillatory modes are found to be damped, and hence stable, in the case of a 1-solar mass white dwarf accreting in spherical symmetry.

Nonaxisymmetric Dynamic Instabilities of Rotating Polytropes. II. Torques, Bars, and Mode Saturation with Applications to Protostars and Fizzlers

Dynamic nonaxisymmetric instabilities in rapidly rotating stars and protostars have a range of potential applications in astrophysics, including implications for binary formation during protostellar cloud collapse and for the possibility of aborted collapse to neutron star densities at late stages of stellar evolution (fizzlers). We have recently presented detailed linear analyses for polytropes of the most dynamically unstable global modes, the barlike modes. These produce bar distortions in the regions near the rotation axis but have trailing spiral arms toward the equator. In this paper, we use our linear eigenfunctions to predict the early nonlinear behavior of the dynamic instability and compare these quasi-linear predictions with several fully nonlinear hydrodynamics simulations. The comparisons demonstrate that the nonlinear saturation of the barlike instability is due to the self-interaction gravitational torques between the growing central bar and the spiral arms, where angular momentum is transferred outward from bar to arms. We also find a previously unsuspected resonance condition that accurately predicts the mass of the bar regions in our own simulations and in those published by other researchers. The quasi-linear theory makes other accurate predictions about consequences of instability, including properties of possible end-state bars and increases in central density, which can be large under some conditions. We discuss in some detail the application of our results to binary formation during protostellar collapse and to the formation of massive rotating black holes.

Nonaxisymmetric Dynamic Instabilities of Rotating Polytropes. I. The Kelvin Modes

We study the dynamic instabilities of rotating polytropes in the linear regime using an approximate Lagrangian technique and a more precise Eulerian scheme. We consider nonaxisymmetric modes with azimuthal dependence proportional to exp (im), where m is an integer and is the azimuthal angle, for polytropes with a wide range of compressibilities and angular momentum distributions. We determine stability limits for the m = 2-4 modes and find the eigenvalue and eigenfunction of the most unstable m-mode for given equilibrium models. To the extent that we have explored parameter space, we find that the onset of instability is not very sensitive to the compressibility or angular momentum distribution of the polytrope when the models are parameterized by T/| W |. Here T is the rotational kinetic energy, and W is the gravitational energy of the polytrope. The m = 2, 3, and 4 modes become unstable at T/| W | ≈ 0.26-0.28, 0.29-0.32, and 0.32-0.35, respectively, limits consistent with those of the Maclaurin spheroids to within ±0.015 in T/| W |. The only exception to this occurs for the most compressible polytrope we test and then only for m = 4, where instability sets in at T/| W | ≈ 0.37-0.39. The eigenfunctions for the fastest growing low m-modes are similar to those of the Maclaurin spheroid eigenfunctions in that they do not show large vertical motions, are only weakly dependent on z, and increase strongly in amplitude as the equatorial radius of the spheroid is approached. The polytrope eigenfunctions are, however, qualitatively different from the Maclaurin eigenfunctions in one respect: they develop strong spiral arms. The spiral arms are stronger for more compressible polytropes and for polytropes whose angular momentum distributions deviate significantly from those of the Maclaurin spheroids. Nevertheless, our approximate Lagrangian method, which explicitly assumes nonspiral Maclaurin-like trial functions, yields reasonable estimates for the pattern periods and e-folding times of unstable m = 2 modes even for highly compressible and strongly differentially rotating polytropes. Comparisons for m = 2 between the linear analyses in this paper and nonlinear hydrodynamic simulations give excellent quantitative agreement in eigenfunctions, pattern speeds, and e-folding times for the dynamically unstable modes.

X-ray and ultraviolet radiation from accreting white dwarfs. IV - Two-temperature treatment with electron thermal conduction

Results are reported from two-temperature calculations of the structures and X-ray spectra of radiation shocks generated by accretion onto nonmagnetic white dwarfs. The approach was necessitated by the domination of bremsstrahlung in the emission region by Compton cooling. Features of the shock model, which includes steady, spherical infall of fully ionized plasma and dominance of the stand-off shock by collisional processes, are summarized. A maximum hard X-ray temperature of about 50 keV and a maximum hard X-ray luminosity of 2 x 10 to the 36th ergs/sec were obtained. The results prove that the bulk of accretion energy cannot be transported to the star by electron thermal conduction, provided that bremsstrahlung cooling is dominant over cyclotron cooling. 52 references.

Secular stability limits for rotating polytropic stars

On utilise le principe variationnel Lagrangien pour localiser numeriquement les limites de stabilite seculaires de polytropes en rotation par rapport a des perturbations sans symetrie axiale

X-Ray and Optical Observations of BL Hydri

We acquired hard X-ray and unfiltered optical photometric data of the AM Herculis system BL Hydri. The X-ray data were obtained using the Proportional Counter Array detector of the Rossi X-Ray Timing Explorer satellite on 1997 October 17. The optical data were obtained using the 1.5 m telescope of the Cerro Tololo Inter-American Observatory on 1997 September 24. At the time of our observations, BL Hyi was in an unusual high-luminosity state with average visual magnitude mV ≈ 14.4 and orbital phase-averaged 2-10 keV X-ray flux ≈1.5 × 10-11 ergs cm-2 s-1, roughly 50% larger than had been seen previously. The X-ray light curves suggested that two accretion poles were active. The primary X-ray emission region was extended, covering ~45° in longitude on the white dwarf, while the secondary X-ray emission region was consistent with a point source with flux ~33% that of the primary region. The X-ray spectra were well fitted by absorbed bremsstrahlung with Gaussian emission-line models; and absorbed power law with Gaussian emission-line models. For the thermal models, the X-ray spectra were consistent with kTX ≈ 11 keV and an emission line at E ≈ 6.7-6.8 keV with equivalent width EW = 0.86-1.2 keV. The absorbed power-law models had slope α ≈ 2.1 and an emission line at E = 6.7-6.8 keV with EW = 1.1-1.5 keV. The optical data showed, 2.7% ± 0.2% rms quasi-periodic oscillations (QPOs) over the frequency range 0.2-0.8 Hz modulated strongly on the orbital period suggesting that the QPOs were connected with the dominant accretion hot spot. We did not find corresponding QPOs in the X-ray emission for an upper limit of 20% rms. There were no detections of other short-period QPOs or coherent features in either the optical or the X-ray data. The X-ray and optical data were consistent with a radiative shock model. Based primarily on the X-ray continuum and line spectrum, we infer that the mass of the white dwarf in the BL Hyi system is 0.3-0.7 M☉, depending on the relative cyclotron and hard X-ray luminosities.

16 Second Optical Quasi-Periodic Oscillations in GX 339?4

We report the results of high-speed optical white-light photometry of the black hole candidate GX 339-4, obtained at the Cerro Tololo Inter-American Observatory during 1996 April. We searched the data for strictly periodic features, quasi-periodic oscillations (QPOs), and aperiodic variability (noise) using Fourier and wavelet techniques. (1) We found a QPO with amplitude 4.5%-6%, frequency f ~ 0.064 Hz, and width Δf/f ~ 0.3-0.5. The QPO was long-lived in that it was present in all segments of the data, even in sets acquired one day apart. (2) In addition to the 0.064 Hz QPO there are indications of low-amplitude QPOs at f = 0.02, 0.03, 0.08, 0.3, and 3 Hz. None of the features appeared in more than one day of data (except for the 0.08 Hz feature) and none of the features were as strong as the 0.064 Hz QPO, and so they should be taken with some caution. (3) We found power in excess of counting statistics out to frequencies of several hertz on both nights. The origin of this excess power is not clear.

Coordinated Optical and X-Ray Observations of the AM Herculis Objects VV Puppis, V834 Centauri, and EF Eridani

We obtained coordinated broadband optical photometry and hard X-ray data on the AM Her objects VV Pup, V834 Cen, and EF Eri. The X-ray data were obtained using the Proportional Counter Array detectors of the Rossi X-Ray Timing Explorer, and the optical data were obtained using the 1 m telescope of the Cerro Tololo Inter-American Observatory. During our observations, VV Pup and V834 Cen were in their high optical luminosity states with average mV ~ 17 and 15.1, respectively. EF Eri was in a very low state at the time of our observation: mV > 17.7. VV Pup and V834 Cen had average 2-10 keV X-ray fluxes of 1.7 × 10-12 and 2.13 × 10-11 ergs cm–2 s–1, respectively. EF Eri was not detected in X-rays. The X-ray spectrum of V834 Cen was adequately modeled using an absorbed bremsstrahlung continuum with a Gaussian emission line. The best-fit model had continuum temperature kTX = 11.8 ± 1.0 keV and absorption column nH = (1.03 ± 0.39) × 1022 cm–2. The Gaussian emission line had line center energy E = 6.7 ± 0.1 keV and equivalent width EW = 0.64 keV. VV Pup was too faint to produce reliable spectral fits. The 2-10 keV flux for VV Pup quoted above was based on the best-fit spectral model. Holding kTX fixed in the fitting process yielded roughly the same flux for kTX = 0.5-30 keV.We detected ~1 Hz optical quasi-periodic oscillations (QPOs) with amplitude ~2.2% from VV Pup and ~1% from V834 Cen. We did not detect optical QPOs over the range 0.2-1 Hz from EF Eri at a 90% confidence upper limit of 4.5%. The QPO properties are consistent with those found in the literature. None of the targets showed X-ray QPO activity. However, only V834 Cen yielded a meaningful upper limit: ~14% at the 90% confidence level over 0.2-1.2 Hz.

V2301 Ophiuchi: An X-Ray-bright Eclipsing AM Herculis Object

We present hard X-ray and optical observations of the eclipsing AM Her system V2301 Oph. The X-ray data were obtained using the PCA detector of the Rossi X-Ray Timing Explorer satellite during 1997 May, and the optical data were obtained using the 1 and 1.5 m telescopes of the Cerro Tololo Inter-American Observatory during 1996 May and 1997 June. V2301 Oph was bright in both the optical and hard X-rays during our observations. This, when coupled with its eclipsing nature, makes V2301 Oph an ideal testbed for theories of the large-scale topology of AM Her flows and the radiative shocks in AM Her systems. The X-ray emission from V2301 Oph was modulated strongly on the orbital period. During the bright orbital phases, the X-ray flux was Fx≈3.6 × 10−11 ergs cm-2 s-1 over the energy range E=2-10 keV. The X-ray emission did not go to zero during the faint orbital phases; it was ~10% of the bright phase level. The X-ray spectrum could be fitted by (1) optically thin thermal bremsstrahlung (temperature kTx≈9-19 keV) models with an absorption line at 5.1-5.2 keV or an emission line at ~7 keV, and (2) power-law continuum (index ≈2) models with an absorption line at 5.1-5.2 keV or an emission line at ~7 keV. The absorption columns were large for all fits, nH~(3-10) × 1022 cm-2. The nH are model dependent, but their large sizes are secure because they are set by the rollover in the X-ray spectrum at 3-4 keV. The hardness of the X-ray spectrum was roughly constant during the bright orbital phases. During the faint orbital phases, the X-ray spectral properties were not well determined, but it did appear that the spectrum hardened. There were total eclipses in both the X-ray and optical light curves. The X-ray light curves and eclipses were consistent with a dominant hot spot and a secondary hot spot. The dominant hot spot was not a point source; it had to cover about 50° in longitude on the surface of the white dwarf. We argue that the X-ray light curve and eclipse shape also suggest that the accretion occurs in a sheetlike geometry rather than in a columnar geometry. The optical light curves and eclipses were consistent with emission from the white dwarf photosphere, an extended emission region that sat above the surface of the white dwarf, and the X-ray-heated face of the companion star.