In collaboration with Thomas Steiman-Cameron of NASA/Ames Research Center, John Middleditch of Los Alamos National Laboratory, and Jeff Scargle of NASA/Ames Research Center, Imamura obtains, analyzes, and interprets optical and x-ray high-speed photometry of black hole candidates, neutron stars, and white dwarfs. The data were obtained at the Cerro Tololo Inter-American Observatory and using the EXOSAT and RXTE satellite observatories. The observations are used to motivate and test Imamura's theoretical modeling efforts. As such, the studies focus primarily on accreting white dwarfs and neutron stars although observations of GX339-4 are also routinely made. Time series analysis tools (e.g., wavelet transforms, multi-tapering techniques, ... ) are investigated for their usefulness to astrophysical problems as part of the work.

A typical source of optical quasi-periodic oscillations is the the AM Herculis (white dwarf) x-ray binary VV Puppis. The AM Herculis objects are low mass, short orbital period (P ~ hours), mass transfer binary systems composed of a strongly magnetic (B ~7 - 230 MegaGauss) white dwarf and a late-type companion star.

• optical light curve -- obtained at CTIO using the 1.5 m telescope on Jan 25, 1991 using the ASCAP with an unfiltered Hamamatsu GaAs phototube and TSPHOT
• orbital phase resolved power spectral densities for the data shown above. Note the excess power between ~ 0.5 Hz and 1 Hz and the strong dependence on orbital phase.
• time-averaged scalegram (amplitudes of the wavelet coefficients) for the data shown above. The other curves are based on the dripping handrail nonlinear dynamical model.

The last few years Imamura, Steiman-Cameron, Middleditch, and Scargle obtained optical photometry (from CTIO) and x-ray photometry (PCA data from XTE) for the AM Herculis QPO sources, VV Pup, V834 Cen, and EF Eri.

We have also studied the LMXB Sco X-1.