The amplitude of the kilohertz quasi-periodic oscillations in 4U 1636$-$53 in the frequency-energy space

Abstract

We present for the neutron-star low-mass X-ray binary 4U 1636$-$53, and for the first time for any source of kilohertz quasi-periodic oscillations (kHz QPOs), the two-dimensional behaviour of the fractional rms amplitude of the kHz QPOs in the parameter space defined by QPO frequency and photon energy. We find that the rms amplitude of the lower kHz QPO increases with energy up to $\sim 12$ keV and then decreases at higher energies, while the rms amplitude of the upper kHz QPO either continues increasing or levels off at high energies. The rms amplitude of the lower kHz QPO increases and then decreases with frequency, peaking at $\sim 760$ Hz, while the amplitude of the upper kHz QPO decreases with frequency, with a local maximum at around $\sim 770$ Hz, and is consistent with becoming zero at the same QPO frequency, $\sim 1400$ Hz, in all energy bands, thus constraining the neutron-star mass at $M_{NS}\le 1.6M_{\odot }$, under the assumption that this QPO reflects the Keplerian frequency at the inner edge of the accretion disc. We show that the slope of the rms energy spectrum is connected to the changing properties of the kHz QPOs in different energy bands as its frequencies change. Finally, we discuss a possible mechanism responsible for the radiative properties of the kHz QPOs and, based on a model in which the QPO arises from oscillations in a Comptonising cloud of hot electrons, we show that the properties of the kHz QPOs can constrain the thermodynamic properties of the inner accretion flow.

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Cited by in corpus (8)

• The Comptonising medium of the neutron star in 4U 1636-53 through its lower kilohertz Quasi-Periodic Oscillations
• Evidence for a Compact Object in the Aftermath of the Extra-Galactic Transient AT2018cow
• Wide-band view of High Frequency QPOs of GRS 1915+105 in 'softer' variability classes observed with AstroSat
• High-frequency variability in neutron-star low-mass X-ray binaries
• Kilohertz quasi-periodic oscillations in neutron-star X-ray binaries: Flattening of the lag spectrum with increasing luminosity
• Study on the energy limits of kHz QPOs in Sco X-1 with $RXTE$ and $Insight$-HXMT observations
• Modeling the upper kHz QPOs of 4U 1728-34 with X-ray reverberation
• Lags of the KiloHertz Quasi-Periodic Oscillations in the transient source XTE J1701$-$462