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 ~500 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, 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.