RXTE Observation of the X-ray burster 1E1724-3045. I : Timing study of the persistent X-ray emission with the PCA

We report on the RXTE/PCA observation of the X-ray burster 1E1724-3045 located in the globular cluster Terzan 2. The persistent X-ray emission shows a high level of noise variability, the so-called High Frequency Noise (HFN) with a fractional Root Mean Squared (RMS) of about 25% in the 2E-3 - 40 Hz range. The strong HFN together with the hardness of its X-ray spectrum suggest that 1E1724-3045 is an ``Atoll'' source which was in its ``Island'' state during the observation. The Fourier Power Density Spectra (PDS) can be modeled in terms of the sum of two ``shot noise'' components for which the shots have a single-side exponential shape. The characteristic shot decay timescales inferred from the best fitting of the PDS are ~680 and 16 msec respectively. The two components contribute similarly to the total RMS (about 15%). The PDS contains also a third component: a broad and asymmetric peaked noise feature centered at 0.8 Hz. This Quasi Periodic Oscillation-like (QPO) feature contributes at the level of ~ 10% to the total RMS. The integrated RMS of all three components shows a positive correlation with energy up to at least 40 keV. We also show that 1E1724-3045 has striking timing similarities with the black hole candidate GRO J0422+32 (Nova Persei 1992). Our observation demonstrates that a low frequency QPO simultaneously with a high level of RMS is not a timing signature unique to black holes. This extends the growing list of similarities between Atoll sources and black hole systems.