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Wu, Kinwah

Publications and source records attributed to Wu, Kinwah.

28 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS2001

ROSAT/Chandra Observations of a Bright Transient in M81

We present a 10-year X-ray light curve and the spectra of a peculiar X-ray transient in the spiral galaxy M81. The source was below the detection limit of ROSAT PSPC before 1993, but it brightened substantially in 1993, with luminosities exceeding the Eddington limit of a 1.5 solar mass compact accretor. It then faded and was not firmly detected in the ROSAT HRI and PSPC observations after 1994. The Chandra image obtained in 2000 May, however, shows an X-ray source at its position within the instrumental uncertainties. The Chandra source is coincident with a star-like object in the Digitized-Sky-Survey. A Hubble image suggests that the optical object may be extended. While these three observations could be of the same object, which may be an X-ray binary containing a black-hole candidate, the possibility that the ROSAT and Chandra sources are two different objects in a dense stellar environment cannot be ruled out. The Hubble data suggests that the optical object may be a globular cluster yet to be identified.

Ghosh, Kajal K.↗
NASA NTRS1998

Simulating AXAF Grating Spectra of Accreting White Dwarfs

We present simulated AXAF spectra of accreting white dwarfs, using parameters appropriate for magnetic cataclysmic variables. The very high spectral resolution that can be obtained with the High-Energy Transmission Grating of AXAF can resolve the keV X-ray emission lines that characterize the temperature, density and velocity profiles of the shock-heated emission regions of these systems. These simulations demonstrate that actual spectra will allow us to place constraints on the white-dwarf mass and the accretion rate of the systems. The high-resolution spectra also allow the measurement of the velocity of the accretion flow in regions close to the white-dwarf surface.

Tennant, Allyn F.↗
NASA NTRS1994

Structure of steady state accretion shocks with several cooling functions: Closed integral-form solution

We present, for the first time, a closed integral-form solution to the accretion shock structures for the case where the cooling is due to optically thin bremsstrahlung emission and a series of power-law cooling functions of density and temperature. Our results can provide useful checks on numerical calculations and simple accurate estimates for valuable parameters such as the shock height. For the case where the cooling rate j = (2/3)Arho(exp 2)(P/rho)(exp 1/2)(1 + epsilon (sub s)(P/P(sub s)(exp alpha)(rho(sub s)/rho)(exp beta)), we find that a substantial amount of the accretion energy is released at the base of the accretion shock in the form of bremsstrahlung radiation. This implies that for a cyclotron-dominated shock (qualitatively given by alpha = 2.0, beta = 3.85, and epsilon(sub s) is much greater than 1), bremsstrahlung cooling still plays a crucial role in determining the shock structure. Our results are shown to be consistent with detailed numerical calculations.

Wu, Kinwah↗
NASA NTRS1993

The magnetic field configurations of AM Herculis binaries

We have considered equilibrium configurations of a white dwarf and a red dwarf companion in AM Herculis type binary system where the red dwarf is assumed to have an intrinsic centered dipole field, and the white dwarf an offset dipole or a centered dipole plus a quadrupole field, and the interaction is purely magnetic. The restoring torque, due to the system being perturbed from its stable equilibrium configuration, is compared with the accretion torque. The results are used to show that the observed alignment of the main cyclotron emission region relative to the red star can be explained quite simply by these models without having to invoke gravitational torques. We also provide a possible explanation of the observation that the more strongly accreting pole has the weaker field strength in the few systems where fields have been determined at both poles.

Wu, Kinwah↗
NASA NTRS1992

Properties of quasi-periodic oscillations in accreting magnetic white dwarfs

Previous studies of time-dependent accretion onto magnetic white dwarfs, in which the cooling was assumed to be due to bremsstrahlung emission, have shown that the accretion shock undergoes oscillations. However, when cyclotron cooling is also included, the oscillations are damped for sufficiently strong magnetic fields. Here we demonstrate that the oscillations can be sustained by accretion-fluctuation-induced excitations. The frequency of the QPOs are shown to increase quadratically with the magnetic field strength. We interpret the oscillations as a two-phase process in which bremsstrahlung cooling dominates in one half-cycle and cyclotron cooling in the other. Such a process may have very different consequences compared to a single-phase process where the functional form of the cooling is essentially the same throughout the cycle. If in the two-phase process damping occurs mainly in the cyclotron cooling half-cycle, there will be a universal effective damping factor which tends to suppress all oscillation modes indiscriminately. The oscillations of the accretion shock also could be a limit cycle process in which the system vacillates between two branches.

Wu, Kinwah↗
NASA NTRS1992

Three-dimensional structured shocks in AM Herculis-type systems. II - Cyclotron emission from ridge-shaped emission regions

The properties of cyclotron emission from accretion shocks in AM Herculis binaries are studied which result from nonuniform accretion onto linearly extended regions on the surface of the magnetic white dwarf. The resulting ridge-shaped postshock structures yield cyclotron spectra with prominent harmonic features over a wide range of viewing aspects, even for high total accretion rates, in contrast to what has been found previously for axisymmetric accretion shocks.

Wu, Kinwah↗
NASA NTRS1992

The accretion halo in AM Herculis systems

Previous phase-resolved spectropolarimetric observations of the AM Herculis systems V834 Centauri (E1405-451) and EF Eridani have shown broad, Zeeman-shifted absorption features during the bright phases. These features are thought to be nonphotospheric in origin, and to arise from a cool 'halo' of unshocked gas surrounding the accretion shock on the surface of the white dwarf primary. Preliminary models for the accretion halo region are presented and these models are used to perform a more detailed analysis of the relevant data for these two systems than has previously been done. To explain the observed halo Zeeman features, geometries which are consistent with the presence of linearly extended cyclotron emission regions are required. Such regions have previously been deduced from different considerations by other investigators. The estimated masses for the accretion halo are comparable to the mass of the cyclotron emission region.

Achilleos, N.↗
NASA NTRS1991

An interpretation of the field alignment in AM Herculis type systems

The magnetic white dwarfs in AM Herculis-type systems are known to have field structures which differ significantly from that of a centered dipole. The implied quadrupole asymmetry is used to explain the observed orientations of the accretion regions relative to the red star. In particular, an interpretation is presented of the result that in four systems where two accretion shocks are observed, the region with the weaker magnetic pole is the stronger accretor.

Wickramasinghe, D. T.↗
NASA NTRS1991

Magnetic moment distribution of magnetic cataclysmic variables

A simulation study is made of the relative numbers of the AM Herculis binaries and the intermediate polars as a function of the orbital period using random variables subject to suitable constraints to describe the various parameters. It is shown that the observations can be matched by a single distribution in the magnetic moment equals 0.7 +/- 0.3. For such an ensemble, the intermediate polars are distributed in the log(Porb) - log(Ps) diagram about the critical disk line but with a larger scatter than observed.

Wu, Kinwah↗