EXO 1745-248 is an 11 Hz Eclipsing Pulsar

ATel #2929; T. E. Strohmayer (NASA/GSFC), C. B. Markwardt (NASA/GSFC)
on 13 Oct 2010; 05:29 UT
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Credential Certification: Craig B. Markwardt (craigm@lheamail.gsfc.nasa.gov)

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RXTE pointed observations of the ongoing outburst of EXO 1745-284 (ATEL #2919, #2920, #2922, #2924) began at approximately 2010-10-13 at 00:13 UTC, for an exposure of 3.2 ksec. The flux of the source is approximately 95 mCrab (2-10 keV). The observation reveals strong pulsations at a barycentric frequency of 11.0452(2) Hz.

In addition, we observe an eclipse ingress/egress during the observation, beginning at 00:50:24.5 UTC and ending at 00:58:56.2 UTC, implying an eclipse duration of 511.7 ± 1 sec. The eclipse is not associated with earth occultation of the RXTE satellite, and X-ray pulsations disappear during the eclipse, firmly establishing the X-ray pulsator and eclipser as the same object. The intensity level during the eclipse is consistent with contributions solely from instrumental and X-ray background. Both the ingress and egress are observed and are quite sharp, with the transition occurring in less than 1 second.

A thermonuclear X-ray burst was also observed beginning near 00:37. The burst lasts for 150 seconds, a duration suggestive of significant hydrogen in the accreted matter. Further RXTE observations are being planned. Observations at other wavelengths are strongly encouraged.