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Power density spectra of modes of orbital motion in strongly curved space-time: obtaining the observable signal

Journal article
Authors P. Bakala
G. Torok
V. Karas
M. Dovciak
M. Wildner
D. Wzientek
E. Sramkova
Marek A Abramowicz
K. Goluchova
G. P. Mazur
F. H. Vincent
Published in Monthly Notices of the Royal Astronomical Society
Volume 439
Issue 2
Pages 1933-1939
ISSN 0035-8711
Publication year 2014
Published at Department of Physics (GU)
Pages 1933-1939
Language en
Links dx.doi.org/10.1093/mnras/stu076
https://gup.ub.gu.se/file/130539
Keywords accretion, accretion discs, black hole physics, gravitation, methods: numerical, X-rays: binaries, QUASI-PERIODIC OSCILLATIONS, X-RAY BINARIES, ROTATING BLACK-HOLE, PROPORTIONAL COUNTER ARRAY, SOURCE 4U 1636-53, HOT-SPOT MODEL, ACCRETION, DISK, NEUTRON-STAR, KHZ-QPO, MILLISECOND PULSAR
Subject categories Astronomy, Astrophysics and Cosmology

Abstract

High-frequency quasi-periodic oscillations (HF QPOs) appear in the X-ray variability of several accreting low-mass binaries. In a series of works, it was suggested that these QPOs may have connection to inhomogeneities orbiting close to an inner edge of the accretion disc. In this paper, we explore the appearance of an observable signal generated by small radiating circular hotspots moving along quasi-elliptic trajectories close to the innermost stable circular orbit in the Schwarzschild space-time. Our consideration takes into account the capabilities of observatories that have been operating in the past two decades represented by the Rossi X-ray Timing Explorer (RXTE) and 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 light curves comparable to those observed in Galactic black hole sources, in particular the microquasar GRS 1915+105. We find that when a weak signal corresponding to the hotspot 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. We conclude that the ability to recognize the harmonic content of the signal can help to distinguish between the different proposed physical models. RAMOWICZ MA, 1991, ASTRONOMY & ASTROPHYSICS, V245, P454 RAMOWICZ MA, 1992, NATURE, V356, P41 RAMOWICZ MA, 2003, PASJ, V55, P466

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