Lag of Low-Energy Photons in an X-ray Burst Oscillation: Doppler Delays

Numerous X-ray bursts show strong oscillations in their flux at several hundred Hz as revealed by RXTE. Analyzing one such oscillation from the X-ray binary Aql X-1, I find that low energy photons (3.5-5.7 keV) lag high energy photons (>5.7 keV) by approximately 1 radian. The oscillations are thought to be produced by hot spots on the spinning neutron star. The lags can then be explained by a Doppler shifting of emission from the hot spots; higher energy photons being emitted earlier in the spin phase as the spot approaches the observer. A quantitative test of this simple model shows a remarkable agreement with the data. Similar low energy lags have been measured in kilohertz quasi-periodic oscillations and in the accreting millisecond pulsar SAX J1808.4-3658. A Doppler delay mechanism may be at work there as well.Comment: accepted ApJ Letter

Discovery of Microsecond Soft Lags in the X-Ray Emission of the Atoll Source 4U1636-536

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 4U1636-536 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 microseconds 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 4U1608-52. 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.Comment: Accepted to the Astrophysical Journal Letters, 4 page

The Amplitude of the Kilohertz Quasi-periodic Oscillations in 4U 1728-34, 4U 1608-52, and Aql X-1, as a Function of X-ray Intensity

We study the kilohertz quasi-periodic oscillations (kHz QPOs) in the low-mass X-ray binaries 4U 1728-34, 4U 1608-52, and Aql X-1. Each source traces out a set of nearly parallel lines in a frequency vs. X-ray count rate diagram. We find that between two of these tracks, for similar QPO frequency, the source count rate can differ by up to a factor of ~ 4, whereas at the same time the rms amplitude of the kHz QPOs is only a factor of ~ 1.1 different. We also find that, for 4U 1608-52 and Aql X-1, the rms spectrum of the kHz QPOs does not depend upon which track the source occupies in the frequency vs. X-ray count rate diagram. Our results for 4U 1728-34, 4U 1608-52, and Aql X-1 are inconsistent with scenarios in which the properties of the kHz QPOs are only determined by the mass accretion rate through the disk, whereas X-ray count rate also depends upon other sources of energy that do not affect the QPOs.Comment: Submitted to ApJ.(7 pages; 9 figures