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We report the discovery of millisecond oscillations in an X-ray burst fromthe X-ray transient SAX J1748.9-2021 in the globular cluster NGC 6440.Oscillations at a frequency of 409.7 +/- 0.3 Hz were present in one of nineX-ray bursts observed with the Proportional Counter Array on the Rossi X-rayTiming Explorer during the outburst which occurred in 2001. The burst was oneof the two dimmest and had the longest duration and decay time. The averagepeak luminosity of two bursts showing radius expansion is (3.6 +/- 0.4) x 10^38erg/s, consistent with the Eddington luminosity for a 1.4 solar mass and 10 kmradius neutron star burning hydrogen-poor matter. We speculate that thedichotomy observed between sources with burst oscillations at once versus twicethe frequency difference of kHz quasiperiodic oscillations in the persistentemission may be related to the magnetic field geometry of the neutron stars.Comment: 8 pages, to appear in Ap

Discovery of X‐Ray Burst Oscillations from a Neutron Star X‐Ray Transient in the Globular Cluster NGC 6440