Discovery of nearly coherent oscillations with a frequency of similar to 567 Hz during type I X-ray bursts of the X-ray transient and eclipsing binary X1658-298

We report the discovery of nearly coherent oscillations with a frequency of similar to 567 Hz during type I X-ray bursts from the X-ray transient and eclipsing binary X1658-298. If these oscillations are directly related to t he neutron star rotation, then the spin period of the neutron star in X1658 -298 is similar to1.8 ms. The oscillations can be present during the rise o r decay phase of the bursts. Oscillations during the decay phase of the bur sts show an increase in frequency of similar to0.5-1 Hz. However, in one pa rticular burst the oscillations reappear at the end of the decay phase at a bout 571.5 Hz. This represents an increase in oscillation frequency of abou t 5 Hz, which is the largest frequency change seen so far in a burst oscill ation. It is unclear if such a large change can be accommodated by present models used to explain the frequency evolution of the oscillations. The osc illations at 571.5 Hz are unusually soft compared to the oscillations found at 567 Hz. We also observed several bursts during which the oscillations a re detected at much lower significance or not at all. Most of these bursts happen during periods of X-ray dipping behavior, suggesting that the X-ray dipping might decrease the amplitude of the oscillations (although several complications exist with this simple picture). We discuss our discovery in the framework of the neutron star spin interpretation.