Soft lags in neutron star kHz Quasi Periodic Oscillations: evidence for reverberation?

Abstract

High frequency soft reverberation lags have now been detected from stellar mass and super massive black holes. Their interpretation involves reflection of a hard source of photons onto an accretion disk, producing a delayed reflected emission, with a time lag consistent with the light travel time between the irradiating source and the disk. Independently of the location of the clock, the kHz Quasi-Periodic Oscillation (QPO) emission is thought to arise from the neutron star boundary layer. Here, we search for the signature of reverberation of the kHz QPO emission, by measuring the soft lags and the lag energy spectrum of the lower kHz QPOs from 4U1608-52. Soft lags, ranging from ~15 to ~40 microseconds, between the 3-8 keV and 8-30 keV modulated emissions are detected between 565 and 890 Hz. The soft lags are not constant with frequency and show a smooth decrease between 680 Hz and 890 Hz. The broad band X-ray spectrum is modeled as the sum of a disk and a thermal comptonized component, plus a broad iron line, expected from reflection. The spectral parameters follow a smooth relationship with the QPO frequency, in particular the fitted inner disk radius decreases steadily with frequency. Both the bump around the iron line in the lag energy spectrum, and the consistency between the lag changes and the inferred changes of the inner disk radius, from either spectral fitting or the QPO frequency, suggest that the soft lags may indeed involve reverberation of the hard pulsating QPO source on the disk.

28 pages, 2 tables, 5 figures, Accepted for publication in the Astrophysical Journal

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• Shapiro delay measurement of a two solar mass neutron star
• Relativistic X-ray Lines from the Inner Accretion Disks Around Black Holes
• Evaluating Spectral Models and the X-ray States of Neutron-Star X-ray Transients
• The coherence of kHz quasi-periodic oscillations in the X-rays from accreting neutron stars
• Discovery of Fe K-alpha X-ray reverberation around the black holes in MCG-5-23-16 and NGC 7314
• INTEGRAL and BeppoSAX observations of the transient atoll source 4U 1608-522: from quiescent to hard spectral state

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