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