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Astron. Astrophys. 333, 942-951 (1998)

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RXTE observation of the X-ray burster 1E 1724-3045

I. Timing study of the persistent X-ray emission with the PCA

J.F. Olive 1, D. Barret 1, L. Boirin 1, J.E. Grindlay 2, J.H. Swank 3 and A.P. Smale 3

1 Centre d'Etude Spatiale des Rayonnements, CNRS/UPS, 9 Avenue du Colonel Roche, F-31028 Toulouse Cedex 04, France
(email: barret@cesr.fr)
2 Harvard Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
3 Laboratory for High Energy Astrophysics, NASA Goddard Space Flight Center Greenbelt, MD 20771, USA

Received 4 December 1997 / Accepted 2 January 1998

Abstract

We report on the Rossi X-ray Timing Explorer Proportional Counter Array (PCA) observation of the X-ray burster 1E1724-3045 located in the globular cluster Terzan 2. The observation lasted for about 100 kiloseconds and spanned from November 4th to 8th, 1996. The PCA source count rate in the 2-20 keV range was about 470 cts/s. No large spectral variations were observed within our observation as inferred from color-color and hardness-intensity diagram analysis. 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 [FORMULA] 25% in the [FORMULA] 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 (i.e. instantaneous rise and exponential decay). The characteristic shot decay timescales inferred from the best fitting of the PDS are [FORMULA] and 16 msec respectively. The two components contribute similarly to the total RMS ([FORMULA] 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 [FORMULA] 10% to the total RMS. Neither the shot timescales nor the QPO frequency vary with energy. On the other hand, the integrated RMS of all three components shows a positive correlation with energy up to at least 40 keV. In addition, 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 Lorentzian fit of this last component implies a shot decay timescale of [FORMULA] ms.

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.

Key words: stars: individual: 1E 1724-3045 – globular clusters: individual: Terzan 2 – X-rays: bursts – X-rays: stars

Send offprint requests to: D. Barret

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© European Southern Observatory (ESO) 1998

Online publication: April 28, 1998

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