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ATel #505; C. B. Markwardt (U. Maryland & NASA/GSFC), J. Swank (NASA/GSFC), R. Wijnands (U. Amsterdam), J. in 't Zand (SRON)
on 1 Jun 2005; 17:27 UT
Distributed as an Instant Email Notice (Transients)
Password Certification: Craig B. Markwardt (craigm@lheamail.gsfc.nasa.gov)

Subjects: X-ray, Request for Observations, Binaries, Neutron Stars, Pulsars, Transients
Referred to by ATel #: 506, 507, 513, 515, 517, 524

RXTE PCA Bulge scans on May 31, 23h UTC, indicate that a source consistent with the position of SAX J1808.4-3658 has entered outburst. The previous outburst of J1808 was in October 2002, and this outburst would be consistent with the approximate recurrence period of ~2 years. The 2-10 X-ray flux was approximately 3 mCrab.

A small amount of high time resolution data was available (~150 seconds). The expected 401 Hz pulsations were not detected in a straight un-barycentered FFT, with a 95% confidence upper limit of ~25% r.m.s. This does not rule out the identification of SAX J1808.4-3658, since the pulsation amplitude is typically ~5%.

More RXTE observations are planned.