Orbital motion in strongly curved spacetime: obtaining the observable signal

Presentation duration: 15 minutes

Presenter: Pavel Bakala

High frequency quasi-periodic oscillations (HF QPOs) appear in the X-ray variability of several accreting low-mass binaries. In a serie of works it was suggested that these QPOs may have connection to inhomogenities orbiting close to an inner edge of the accretion disc. Here we briefly explore the appearance of the observable signal generated by radiating small circular hot spots moving around quasi-elliptic trajectories close to the innermost stable circular orbit in the Schwarzschild spacetime. Our consideration takes into account the capabilities of the present observatories represented by the Rossi X-ray Timing Explorer (RXTE) as well as the proposed future instruments represented by the Large Observatory for X-ray Timing (LOFT). For these purposes we choose such model parameters that lead to lightcurves roughly comparable to those observed in Galactic black hole sources, in particular the microquasar GRS~1915+105. We find that when the weak signal corresponding to the hot-spot Keplerian frequency is around the limits of the RXTE detectability, the LOFT observations can clearly reveal its first and second harmonics. Moreover, in some specific situations the radial epicyclic frequency of the spot can be detected as well. Finally we also compare the signal produced by the spots to the signal produced by axisymmetric epicyclic disc-oscillation modes and discuss the key differences that could be identified via the proposed future technology.

G. Torok
V. Karas
M. Dovciak
M. Wildner
D. Wzientek
E. Sramkova
M. Abramowicz
K. Goluchova
G. Mazur
F. H. Vincent