Discovery of Microsecond Soft Lags in the X-Ray Emission of the Atoll Source 4U 1636−536

Exploiting the presence of kilohertz quasi-periodic oscillations (QPOs) inthe timing power spectrum, we find that the soft x-ray emission of theneutron-star X-ray binary and atoll source 4U1636-536 modulated at the QPOfrequency lags behind that of the hard x-ray emission. Emission in the 3.8-6.4keV band is delayed by 25.0 +/- 3.3 microseconds relative to the 9.3-69 keVband. The delay increases in magnitude with increasing energy. Our results areconsistent with those of Vaughan et al. (1997), when the sign is corrected(Vaughan et al. 1998), for the atoll source 4U1608-52. The soft lag could beproduced by Comptonization of hard photons injected into a cooler electroncloud or by intrinsic spectral softening of the emission during eachoscillation cycle.Comment: Accepted to the Astrophysical Journal Letters, 4 page