RXTE OBSERVATION OF THE X-RAY BURSTER 1E 1724-3045 - I - TIMING STUDYOF THE PERSISTENT X-RAY-EMISSION WITH THE PCA

We report on the Rossi X-ray Timing Explorer Proportional Counter Arra y (PCA) observation of the X-ray burster 1E1724-3045 located in the gl obular cluster Terzan 2. The observation lasted for about 100 kiloseco nds and spanned from November 4th to 8th, 1996. The PCA source count r ate in the 2-20 keV range was about 470 cts/s. No large spectral varia tions were observed within our observation as inferred from color-colo r and hardness-intensity diagram analysis. The persistent tent X-ray e mission shows a high level of noise variability, the so-called High Fr equency Noise (HFN) with a fractional Root Mean Squared (RMS) of simil ar to 25% in the 2 x 10(-3) - 40 Hz range. The strong HFN together wit h the hardness of its X-ray spectrum suggest that 1E1724-3045 is an '' Atoll'' source which was in its ''Island'' slate during the observatio n. 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 si ngle-side exponential shape (i.e. instantaneous rise and exponential d ecay). The characteristic shot decay timescales inferred from the best fitting of the PDS are similar to 680 and 16 msec respectively. The t wo components contribute similarly to the total RMS (similar to 15%). The PDS contains also a third component: a broad and asymmetric peaked noise feature centered at 0.8 Hz. This Quasi Periodic Oscillation-lik e (QPO) feature contributes at the level of similar to 10% to the tota l RMS. Neither the shot timescales nor the QPO frequency vary with ene rgy. On the other hand, the integrated RMS of all three components sho ws a positive correlation with energy up to at least 40 keV. Ln additi on, in the 2-20 keV energy band where the signal to noise ratio is the highest, we found evidence for a high frequency component which shows up in the PDS above 100 Hz. In terms of the shot noise model, a Loren tzian fit of this last component implies a shot decay timescale of sim ilar to 1.4 ms. We also show that 1E1724-3045 has striking timing simi larities with the black hole candidate GROJ0422+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 hol es. This extends the growing list of similarities between Atoll source s and black hole systems.