1303.0750 (Didier Barret)
Didier Barret
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 where the clocking mechanism is, it is generally assumed that the kHz Quasi-Periodic Oscillation (QPO) emission, with its hard spectrum, arises from the boundary layer between the neutron star surface and the disk. In this paper, we search for the signature of reverberation of the kHz QPO emission, by measuring the soft lags of the lower kHz QPOs from the transient neutron star low mass X-ray binary 4U1608-522. Soft lags, ranging from ~15 to 40 micro-sec, are detected between 565 and 890 Hz. We show that the soft lags are not constant with frequency. In particular, a clear trend is observed for the soft lags to decrease with frequency between 680 Hz and 890 Hz. We also model the broad band X-ray spectrum as the sum of a disk and a thermal comptonized components, plus a relativistically smeared iron line emission, expected from the reflection of hard X-ray photons onto a ionized disk. Although hints for the signature of reverberation, e.g. an excess around the iron line energy in the lag energy spectrum may be already present in our data, the magnitude of the lag changes in response to a change of the inner disk radius, as inferred from spectral fitting or from QPO frequency variation, and the presence of an Iron line, suggest that the soft lags may indeed involve reverberation of the hard pulsating QPO source irradiating the accretion disk. The distance between the irradiating source and the reflector would vary in a way that can be tracked by the frequency of the lower kHz QPOs, which could then provide an orbital frequency.
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http://arxiv.org/abs/1303.0750
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