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Discovery of a ~205 Hz X-ray pulsar in the globular cluster NGC 6440

ATEL # 2182; D. Altamirano (UvA), T. E. Strohmayer (NASA/GSFC), Craig O. Heinke(Univ. of Alberta), C. B. Markwardt (U. Maryland & NASA/GSFC), J. H. Swank (NASA/GSFC), D. Pereira (GSFC), E. Smith (GSFC), R. Wijnands, M. Linares, A. Patruno, P. Casella and M. van der Klis (UvA)
on 31 Aug 2009; 19:36 UT
Distributed as an Instant Email Notice (Request for Observations)
Password Certification: Diego Altamirano (

Subjects: X-ray, Request for Observations, Binaries, Transients

Discovery of a 205 Hz X-ray pulsar in the globular cluster NGC 6440

The globular cluster NGC 6440 was observed by the PCA instrument aboard RXTE on August 30, 2009 at 01:42 (UTC). The observation lasted for approximately 3000 seconds and the source was detected with an intensity of ~7 mCrab (2-10 keV).

Pulsations were detected at a barycentric frequency of 205.893 Hz, with a pulsed amplitude of approximately 7%. The data suggest an orbital period between 40 and 70 minutes, but more data is needed to better constrain this, as well as, the other orbital parameters.

The ~205 Hz spin frequency is inconsistent with the ~442 Hz spin frequency of the other accreting millisecond X-ray pulsar (AMXP) which is in this globular cluster (SAX J1748.9-2021, Altamirano et al. 2008). Although a new transient has recently been identified in the globular cluster NGC 6440 (see Atels #2139, #2143 and #2180), images with higher spatial resolution are necessary to identify this source. In any case, it is interesting to note that NGC 6440 has two AMXPs, while no AMXPs have been detected in any other globular cluster.

We request immediate observations of this new transient in all wavelengths. Further RXTE, Swift and Gemini observations are planned.

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R. E. Rutledge , Editor
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