Discovery of 442 Hz pulsations from an X-ray source in the globular cluster NGC 6440

We report on the serendipitous discovery of a 442 Hz pulsar during a Rossi X-Ray Timing Explorer (RXTE) observation of the globular cluster NGC 6440. The oscillation is detected following a burstlike event which was decaying at the beginning of the observation. The timescale of the decay suggests we may have seen the tail end of a long-duration burst. Low-mass X-ray binaries (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. In particular, the pulse train lasted for similar to 500 s, much longer than in standard burst oscillations. The signal was highly coherent and drifted down by similar to 2 x 10(-3) Hz, much smaller than the similar to Hz drifts typically observed during normal bursts, but consistent with orbital motion of the neutron star. The pulsations are reminiscent of those observed during the much more energetic superbursts; however, the temporal profile and the energetics of the burst suggest that it was not the tail end nor the precursor feature of a superburst. Rather, it is likely that we caught a portion of an outburst from a new intermittent accreting millisecond pulsar, a phenomenon which until now had only been seen in HETE J1900.1-2455.