The spectral-timing properties of upper and lower kHz QPOs

Abstract

Soft lags from the emission of the lower kilohertz quasi-periodic oscillations (kHz QPOs) of neutron star low mass X-ray binaries have been reported from 4U1608-522 and 4U1636-536. Those lags hold prospects for constraining the origin of the QPO emission. In this paper, we investigate the spectral-timing properties of both the lower and upper kHz QPOs from the neutron star binary 4U1728-34, using the entire Rossi X-ray Timing Explorer archive on this source. We show that the lag-energy spectra of the two QPOs are systematically different: while the lower kHz QPO shows soft lags, the upper kHz QPO shows either a flat lag-energy spectrum or hard variations lagging softer variations. This suggests two different QPO-generation mechanisms. We also performed the first spectral deconvolution of the covariance spectra of both kHz QPOs. The QPO spectra are consistent with Comptonized blackbody emission, similar to the one found in the time-averaged spectrum, but with a higher seed-photon temperature, suggesting that a more compact inner region of the Comptonization layer (boundary/spreading layer, corona) is responsible for the QPO emission. Considering our results together with other recent findings, this leads us to the hypothesis that the lower kHz QPO signal is generated by coherent oscillations of the compact boundary layer region itself. The upper kHz QPO signal may then be linked to less-coherent accretion-rate variations produced in the inner accretion disk, being detected when they reach the boundary layer.

20 pages, 7 figures, accepted for publication in ApJ

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• NICER Detects a Soft X-ray Kilohertz Quasi-periodic Oscillation in 4U 0614+09
• 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
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• Are the kHz QPO lags in neutron star 4U 1608-52 due to reverberation?
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• Neutron Star QPOs from Oscillating, Precessing Hot, Thick Flow
• The amplitude of the kilohertz quasi-periodic oscillations in 4U 1636$-$53 in the frequency-energy space
• Mass and spin measurements for the neutron star 4U1608-52 through the relativistic precession model
• Spectral-Timing Analysis of the Lower kHz QPO in the Low-Mass X-ray Binary Aquila X-1
• Study on the energy limits of kHz QPOs in Sco X-1 with $RXTE$ and $Insight$-HXMT observations
• Kilohertz quasi-periodic oscillations in neutron-star X-ray binaries: Flattening of the lag spectrum with increasing luminosity
• Modeling the upper kHz QPOs of 4U 1728-34 with X-ray reverberation
• Spectro-temporal evolution of 4U 1702-429 using AstroSat-NICER
• QPO signatures of disk restoration after type-I X-ray bursts from 4U~1636$-$536
• Lags of the KiloHertz Quasi-Periodic Oscillations in the transient source XTE J1701$-$462