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Discovery of kilohertz QPOs in RXTE observations of SAX J1748.9-2021.

ATEL # 2426; D. Altamirano, A. Patruno, A. Watts , M. Armas Padilla, Y. Cavecchi, N. Degenaar, M. Kalamkar, R. Kaur, P. Soleri, Y.J. Yang, M. van der Klis, R. Wijnands (University of Amsterdam); P. Casella (Southampton), M. Linares (MIT) and N. Rea (IEEC-CSIC)
on 8 Feb 2010; 13:30 UT
Distributed as an Instant Email Notice (Transients)
Password Certification: Diego Altamirano (

Subjects: X-ray, Binaries, Globular Clusters, Neutron Stars, Pulsars, Transients

We report on RXTE observations of the ongoing outburst of the accreting millisecond X-ray pulsar SAX J1748.9-2021 (Altamirano et al. 2008, 674, 45A, see ATEL #2360 and #2407).

Since the first detection of intermittent pulsations on January 27th, 2010, observations have been carried out on a daily basis. The source remained between 120 and 160 mCrab (2-16 keV - see ATEL #2407) until February 3rd, when the intensity started to decrease. A preliminary study of the energy spectra shows that since then, the source has slowly been undergoing a soft-to-hard state transition. As expected from this type of system, the state transition is being accompanied by changes in the power spectra: on February 6th and 7th (at ~60 mCrab) we detected for the first time in this source a single kilohertz quasi-periodic oscillation peak (~760-790Hz, ~9% rms amplitude in the 2-60 keV range, >5 sigma single trial).

There is no evidence for intermittent pulsations near 442 Hz at amplitudes similar to those in the first detection (ATEL #2407)

For light curves of the current outburst in different energy bands, please refer to the MAXI/GSC web-page:

More RXTE observations are planned.

We thank the RXTE team for the scheduling of our observations.

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R. E. Rutledge , Editor
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