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

• 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

Subject headings

NATURVETENSKAP  -- Fysik -- Astronomi, astrofysik och kosmologi (hsv//swe)

NATURAL SCIENCES  -- Physical Sciences -- Astronomy, Astrophysics and Cosmology (hsv//eng)

Keyword

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

Publication and Content Type

ref (subject category)

art (subject category)