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) in the timing power spectrum, we find that the soft x-ray emission of the neutron-star X-ray binary and atoll source 4U 1636-536 modulated at the QPO frequency lags behind that of the hard x-ray emission. Emission in the 3.8–6.4 keV band is delayed by 25.0±3.3 μs relative to the 9.3–69 keV band. The delay increases in magnitude with increasing energy. Our results are consistent with those of Vaughan et al. (1997), when the sign is corrected (Vaughan et al. 1998), for the atoll source 4U 1608-52. The soft lag could be produced by Comptonization of hard photons injected into a cooler electron cloud or by intrinsic spectral softening of the emission during each oscillation cycle. Subject headings: accretion, accretion disks — stars: individual (4U 1636-536, 4U 160852) — stars: neutron — X-rays: stars 1Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, MA 02138 2Istituto di Fisica Cosmica ed Applicazioni dell’Informatica, CNR, Via Ugo La Malfa 153, 90146 Palermo, Italy 3Istituto di Fisica, Universita’ di Palermo, via Archirafi 36, 90123 Palermo, Italy 4Astronomical Institute, “Anton Pannekoek”, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands