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C.B. Markwardt (NASA/GSFC), H.C. Lee (Northwestern Univ.), J.H. Swank (NASA/GSFC)
The measurement of energy dependent time lags in the X-ray emission from galactic low mass X-ray binaries holds the promise of constraining the emission geometry and mechanisms. This hope is especially strong for sources with kilohertz quasiperiodic oscillations (QPOs), whose mechanism is largely still a mystery. From the energy spectrum of these sources, one might infer the presence of a hot corona which should in turn inverse Compton scatter softer seed photons, leading to hard lags. Such lags are present at low frequencies (< 30 Hz). However, we find soft lags in the ``lower'' kilohertz QPO peak for the Atoll sources 4U~1608-52 (see Vaughan et al 1998 [erratum]), 4U~1636-536 (also Kaaret et al 1999), 4U~1728-34, and 4U~1702-429, with magnitudes reaching ~50 \mus. Further, we show that the lag spectrum of upper kilohertz peak in at least one of the sources is entirely inconsistent with soft lags, and indeed suggests a hard lag. This fact alone strongly implies that either the emission mechanisms for the oscillations are distinct, or the emission regions are distinct.
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