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We have studied the rapid X-ray time variability in 99 pointed observationswith the Rossi X-ray Timing Explorer (RXTE)'s Proportional Counter Array of thelow-mass X-ray binary 1E~1724--3045 which includes, for the first time,observations of this source in its island and banana states, confirming theatoll nature of this source. We report the discovery of kilohertzquasi-periodic oscillations (kHz QPOs). Although we have 5 detections of thelower kHz QPO and one detection of the upper kHz QPO, in none of theobservations we detect both QPOs simultaneously. By comparing the dependence ofthe rms amplitude with energy of kHz QPOs in different atoll sources, weconclude that this information cannot be use to unambiguously identify thekilohertz QPOs as was previously thought. We find that Terzan~2 in itsdifferent states shows timing behavior similar to that seen in otherneutron-star low mass X-ray binaries (LMXBs). We studied the flux transitionsobserved between February 2004 and October 2005 and conclude that they are dueto changes in the accretion rate.

Discovery of kilohertz quasi-periodic oscillations and state transitions in the low-mass X-ray binary 1E~1724-3045 (Terzan 2)