Discovery of Microsecond Soft Lags in the X-Ray Emission of the Atoll Source 4U 1636-536
Abstract
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. 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.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- March 1999
- DOI:
- 10.1086/311941
- arXiv:
- arXiv:astro-ph/9901349
- Bibcode:
- 1999ApJ...514L..31K
- Keywords:
-
- ACCRETION;
- ACCRETION DISKS;
- STARS: INDIVIDUAL (4U 1608-52;
- 4U 1636-536);
- STARS: NEUTRON;
- X-RAYS: STARS;
- Accretion;
- Accretion Disks;
- stars: individual (4U 1608-52;
- Stars: Neutron;
- X-Rays: Stars;
- Astrophysics
- E-Print:
- Accepted to the Astrophysical Journal Letters, 4 pages