Discovery of 442 Hz Pulsations From an X-ray Source In the Globular Cluster NGC 6440

Details Discovery of 442 Hz Pulsations From an X-ray Source In the Globular Cluster NGC 6440 Discovery of 442 Hz Pulsations From an X-ray Source In the Globular Cluster NGC 6440

Sep 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006head....9.0143g&link_type=abstract

American Astronomical Society, HEAD meeting #9, #1.43; Bulletin of the American Astronomical Society, Vol. 38, p.336

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We report on the serendipitous discovery of a 442 Hz pulsation during a Rossi X-ray Timing Explorer (RXTE) observation of the globular cluster NGC 6440. The oscillation is detected following a burst which was decaying at the beginning of the observation. The time scale of the decay suggests we may have seen the tail-end of a long-duration burst. Low-mass X-ray binarie (LMXBs) are known to emit thermonuclear X-ray bursts that are sometimes modulated by the spin frequency of the star, the so called burst oscillations. The pulsations reported here are peculiar if interpreted as canonical burst oscillations. They are more reminiscent of the pulsations observed during the much more energetic "superbursts". For example, the pulse train lasted for ?s, much longer than in standard burst oscillations. The signal was highly coherent and drifted down by ˜2×10-3 Hz, much smaller than the ?Hz drifts typically observed during normal bursts. If the event is a superburst, then the rapidly decaying part preceding the pulse train may be a "precursor" feature like those seen in other superbursts. We note that Kaaret et al. 2006 (ApJ, 598, 581) reported the discovery of a 409.7 Hz burst oscillation from SAX J1748.9-2021, also located in NGC 6440. However, Chandra imaging indicates it contains several point-like X-ray sources, thus the 442 Hz object is likely a different source.

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