Discovery of Nearly Coherent Oscillations with a Frequency of ∼567 H[CLC]z[/CLC] 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 of567 Hz during type-I X-ray bursts from the X-ray transient and eclipsing binaryX 1658-298. If these oscillations are directly related to the neutron starrotation then the spin period of the neutron star in X 1658-298 is 1.8 ms. Theoscillations can be present during the rise or decay phase of the bursts.Oscillations during the decay phase of the bursts show an increase in thefrequency of 0.5 to 1 Hz. However, in one particular burst the oscillationsreappear at the end of the decay phase at about 571.5 Hz. This represents anincrease in oscillation frequency of about 5 Hz which is the largest frequencychange seen so far in a burst oscillation. It is unclear if such a large changecan be accommodated by present models used to explain the frequency evolutionof the oscillations. The oscillations at 571.5 Hz are unusually soft comparedto the oscillations found at 567 Hz. We also observed several bursts duringwhich the oscillations are 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 theoscillations (although several complications exist with this simple picture).We discuss our discovery in the framework of the neutron star spininterpretation.Comment: Accepted for publication in ApJ Letters (21 December 2000). Addition of a table containing the burst propertie