  Observations with RXTE (Rossi X-ray Timing Explorer) revealed the presence of
High Frequency Quasi-Periodic Oscillations (HFQPOs) of the X-ray flux from
several accreting stellar mass Black Holes. HFQPOs (and their counterparts at
lower frequencies) may allow us to study general relativity in the strong
gravity regime. However, the observational evidence today does not yet allow us
to distinguish between different HFQPO models. In this paper we use a general
relativistic ray-tracing code to investigate X-ray timing-spectroscopy and
polarization properties of HFQPOs in the orbiting Hotspot model. We study
observational signatures for the particular case of the 166 Hz quasi-periodic
oscillation (QPO) in the galactic binary GRS 1915+105. We conclude with a
discussion of the observability of spectral signatures with a
timing-spectroscopy experiment like the LOFT (Large Observatory for X-ray
Timing) and polarization signatures with space-borne X-ray polarimeters such as
IXPE (Imaging X-ray Polarimetry Explorer), PolSTAR (Polarization Spectroscopic
Telescope Array), PRAXyS (Polarimetry of Relativistic X-ray Sources), or XIPE
(X-ray Imaging Polarimetry Explorer). A high count-rate mission like LOFT would
make it possible to get a QPO phase for each photon, enabling the study of the
QPO-phase-resolved spectral shape and the correlation between this and the flux
level. Owing to the short periods of the HFQPOs, first-generation X-ray
polarimeters would not be able to assign a QPO phase to each photon. The study
of QPO-phase-resolved polarization energy spectra would thus require
simultaneous observations with a first-generation X-ray polarimeter and a
LOFT-type mission.
