Fractional amplitude of kilohertz quasi-periodic oscillation from 4U 1728-34: evidence of decline at higher energies

Abstract

A kilohertz quasi-periodic oscillation (kHz QPO) is an observationally robust high-frequency timing feature detected from neutron star low-mass X-ray binaries (LMXBs). This feature can be very useful to probe the superdense core matter of neutron stars, and the strong gravity regime. However, although many models exist in the literature, the physical origin of kHz QPO is not known, and hence this feature cannot be used as a tool yet. The energy dependence of kHz QPO fractional rms amplitude is an important piece of the jigsaw puzzle to understand the physical origin of this timing feature. It is known that the fractional rms amplitude increases with energy at lower energies. At higher energies, the amplitude is usually believed to saturate, although this is not established. We combine tens of lower kHz QPOs from a neutron star LMXB 4U 1728-34 in order to improve the signal-to-noise-ratio. Consequently, we, for the first time to the best of our knowledge, find a significant and systematic decrease of the fractional rms amplitude with energy at higher photon energies. Assuming an energy spectrum model, blackbody+powerlaw, we explore if the sinusoidal variation of a single spectral parameter can reproduce the above mentioned fractional rms amplitude behavior. Our analysis suggests that the oscillation of any single blackbody parameter is favored over the oscillation of any single powerlaw parameter, in order to explain the measured amplitude behavior. We also find that the quality factor of a lower kHz QPO does not plausibly depend on photon energy.

arXiv comment15 pages, 8 figures, published in The Astrophysical Journal; a more detailed (and rigorous) version may be available later

References in corpus (9)

• Sonic-Point Model of Kilohertz Quasi-Periodic Brightness Oscillations in Low-Mass X-ray Binaries
• The Amplitude of the Kilohertz Quasi-periodic Oscillations in 4U 1728-34, 4U 1608-52, and Aql X-1, as a Function of X-ray Intensity
• The Amplitude Evolution and Harmonic Content of Millisecond Oscillations in Thermonuclear X-ray Bursts
• Study of the Temporal Behavior of 4U 1728-34 as a Function of its Position in the Color-Color Diagram
• Higher order nonlinearity in accretion disks: QPOs of black hole and neutron star sources and their spin
• Suzaku and BeppoSAX X-ray Spectra of the Persistently Accreting Neutron-Star Binary 4U 1705-44
• Evolution of Spectral States of Aql X-1 during the 2000 Outburst
• Studying Frequency Relations of kHz QPOs for 4U 1636-53 and Sco X-1: Observations Confront Theories
• On the Constancy of the Photon Index of X-ray spectra of 4U~1728-34 through all spectral states

Cited by in corpus (11)

• Energy Dependent Time Delays of kHz Oscillations due to Thermal Comptonization
• AstroSat/LAXPC detection of milli-second phenomena in 4U 1728-34
• A Systematic Spectral-Timing Analysis of Kilohertz Quasi-Periodic Oscillations in the Rossi X-ray Timing Explorer Archive
• Constraining the size of the Comptonizing medium by modeling the energy dependent time-lags of kHz QPOs of Neutron star system
• Study of the X-ray properties of the neutron-star binary 4U 1728$-$34 from the soft to hard state
• High-frequency variability in neutron-star low-mass X-ray binaries
• The amplitude of the kilohertz quasi-periodic oscillations in 4U 1636$-$53 in the frequency-energy space
• Changes in the pulse phase dependence of X-ray emission lines in 4U 1626-67 with a torque reversal
• The kHz QPOs as a probe of the X-ray color-color diagram and accretion-disk structure for the atoll source 4U 1728-34
• Study on the energy limits of kHz QPOs in Sco X-1 with $RXTE$ and $Insight$-HXMT observations
• Timing and Spectral Analysis of 4U 1626-67 Using AstroSat/LAXPC