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RXTE and Swift: IGR J17511-3057 fading, XTE J1751-305 detected

ATel #2237; C. B. Markwardt (U. Maryland & NASA/GSFC), D. Altamirano (U. Amsterdam), T. E. Strohmayer (NASA/GSFC), J. H. Swank (NASA/GSFC)
on 9 Oct 2009; 20:25 UT
Credential Certification: Craig B. Markwardt (craigm@lheamail.gsfc.nasa.gov)

Subjects: X-ray, Binary, Neutron Star, Transient, Pulsar

In continuing follow-up observations of the millisecond pulsar IGR J17511-3057 (ATEL #2197), the pulsations from this source have become difficult to detect. On date 2009-10-08 UT, RXTE PCA serendipitously detected 435 Hz X-ray pulsations, identified as the nearby pulsar XTE J1751-305, which is in the same field of view. This is consistent with the recent INTEGRAL report (ATEL #2235). In a pointed RXTE PCA observation on 2009-10-09 at 04:00 UT, pulsations at 435 Hz were only weakly detected.

A Swift XRT pointed observation was performed on 2009-10-08. The source XTE J1751-305 was detected at a 2-10 keV flux of about 2 mCrab, whereas IGR J17511-3057 has faded to about 0.2 mCrab. Thus, both high resolution imaging and high precision timing both show that XTE J1751-305 had become detectable while IGR J17511-3057 has faded.

XTE J1751-305 was most recently detected in April 2007 (ATEL #1051). Follow-up observations are encouraged. We thank the RXTE and Swift schedulers for rapid follow-up response.