Detection of high-frequency X-ray variability in a black hole transient with USA

Studies of high-frequency variability (above ~100 Hz) in X-ray binaries provide a unique opportunity to explore the fundamental physics of spacetime and matter, since the orbital timescale on the order of several milliseconds is a timescale of the motion of matter through the region located in close proximity to a compact stellar object. The detection of weak high-frequency signals in X-ray binaries depends on how well we can understand and model the detector deadtime and other instrumental systematic effects. We describe the preflight timing calibration work performed on the Unconventional Stellar Aspect (USA) X-ray detector to study deadtime and timing issues. We developed a Monte Carlo deadtime model and deadtime correction methods for the USA experiment. The instrumental noise power spectrum can be estimated within ~0.1% accuracy in the case when no energy-dependent instrumental effect is present. We also developed correction techniques to account for an energy-dependent instrumental effect. The developed methods were successfully tested on USA Cas A and Cygnus X-1 data. We report the 20 detection of high-frequency variability (~40-300 Hz) with 7.7% fractional rms amplitude in the BHC X-ray transient XTE J1859+226 with USA. This detection is the first confirmation of high-frequency variability in BHC binaries with a different instrument. The Fourier spectral method was applied to the XTE J1859+226 data with corrections for the deadtime and instrumental effects. The USA measurements can be interpreted as indicating a QPO signal that changes its frequency between three dominant ``states'' at 67 Hz, 134 Hz and 193 Hz. We discuss the high-frequency QPO detections in black hole candidate binaries, in the context of Keplerian, diskoseismic and relativistic precession theoretical models. Currently, the interpretation of these signals within existing models is ambiguous. We performed an energy-dependent timing analysis of a low-frequency 4.9 Hz QPO, its harmonics, and broad-band noise in XTE J1859+226. The increase of fractional rms amplitude of all noise components with energy is seen. The result, if interpreted within the Compton microscope theoretical model, may indicate that the variability in this frequency range could be confined to the outer parts of the accretion disk.