On the geometric nature of low-frequency quasi-periodic oscillations in neutron-star low-mass X-ray binaries

Abstract

We report on a detailed analysis of the so-called ~1 Hz quasi-periodic oscillation (QPO) in the eclipsing and dipping neutron-star low-mass X-ray binary EXO 0748-676. This type of QPO has previously been shown to have a geometric origin. Our study focuses on the evolution of the QPO as the source moves through the color-color diagram, in which it traces out an atoll-source-like track. The QPO frequency increases from ~0.4 Hz in the hard state to ~25 Hz as the source approaches the soft state. Combining power spectra based on QPO frequency reveals additional features that strongly resemble those seen in non-dipping/eclipsing atoll sources. We show that the low-frequency QPOs in atoll sources and the ~1 Hz QPO in EXO 0748-676 follow similar relations with respect to the noise components in their power spectra. We conclude that the frequencies of both types of QPOs are likely set by (the same) precession of a misaligned inner accretion disk. For high-inclination systems, like EXO 0748-676, this results in modulations of the neutron-star emission due to obscuration or scattering, while for lower-inclination systems the modulations likely arise from relativistic Doppler boosting and light-bending effects.

Updated to published version (ApJ, 812, 80)

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Cited by in corpus (8)

• Inclination dependence of QPO phase lags in black hole X-ray binaries
• High-Frequency and Type-C QPOs from Oscillating, Precessing Hot, Thick Flow
• The mass of the black hole in RE J1034+396
• A detailed view of low-frequency quasi-periodic oscillation in the broadband 0.2-200 keV with Insight-HXMT and NICER
• Neutron Star QPOs from Oscillating, Precessing Hot, Thick Flow
• Ocean g-modes on transient neutron stars
• Detection of the thermonuclear X-ray bursts and dips from the X-ray binary 4U 1323-62 with AstroSat/LAXPC
• X-ray and optical orbital modulation of EXO 0748-676: A co-variability study using XMM-Newton