Discovery of Kilohertz Quasi-Periodic Oscillations and State Transitions in the Low-Mass X-Ray Binary 1E 1724–3045 (Terzan 2)

We have studied the rapid X-ray time variability in 99 pointed observations with the Rossi X-Ray Timing Explorer (RXTE) Proportional Counter Array of the low-mass X-ray binary 1E 1724–3045, including, for the first time, observations of this source in its island and banana states, confirming the atoll nature of this source. We report the discovery of kilohertz quasi-periodic oscillations (kHz QPOs). Although we have five detections of the lower kHz QPO and one detection of the upper kHz QPO, in none of the observations we detect both QPOs simultaneously. By comparing the dependence of the rms amplitude with energy of kHz QPOs in different atoll sources, we conclude that this information cannot be use to unambiguously identify the kilohertz QPOs as was previously thought. We find that Terzan 2 in its different states shows timing behavior similar to that seen in other neutron-star low-mass X-ray binaries (LMXBs). We studied the flux transitions observed between 2004 February and 2005 October and conclude that they are due to changes in the accretion rate.