Pointed RXTE observations of SAX J1808.4-3658

ATel #507; R. Wijnands, M. Klein Wolt, M. Linares, M. van der Klis (U. of Amsterdam), D. Chakrabarty, E. H. Morgan (MIT), C. B. Markwardt (U. Maryland & NASA/GSFC)
on 2 Jun 2005; 17:25 UT
Credential Certification: Rudy Wijnands (rudy@space.mit.edu)

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Follow-up RXTE PCA observations of the transient source consistent with the position of SAX J1808.4-3658 (ATEL#505) showed clear coherent pulsations at approximately 401 Hz. This frequency is identical to the spin frequency of the neutron star in SAX J1808.4-3658 demonstrating that it is indeed SAX J1808.4-3658 which is in outburst again.

During the three pointed observations the 2-10 keV X-ray fluxes were approximately 19 mCrab on June 1, 17:35 - 18:42 UTC, 31 mCrab on June 2 00:13 - 00:51 UTC, and 34 mCrab on June 2 10:56 - 11:46 UTC indicating that the X-ray flux is still rising. One type-I X-ray burst was detected during the second observation. Millisecond oscillations at the spin frequency were detected during the burst.

During all observations, the aperiodic variability was dominated by strong broad band-limited noise (roughly 25% r.m.s. amplitude in the frequency range 0.1-100 Hz) similar to what has been observed during previous outbursts of SAX J1808.4-3658. During the individual observations no significant features above 100 Hz were detected. However, when combining the data of all three observations and fitting the power density spectrum with a combination of Lorentzians, a broad peaked noise component can be detected (3.7 sigma) at a frequency of around 300 Hz. This high frequency feature might be related to the kilohertz quasi-periodic oscillations (kHz QPOs) previously seen for this source during its October 2002 outburst. If correct, then this broad noise feature should evolve into a true kHz QPO if the X-ray flux continues to rise, similar to what was seen during the October 2002 outburst of SAX J1808.4-3658.

Additional RXTE observations are planned.