Quasi-periodic X-ray brightness oscillations of GRO J1744-28

The newly discovered source GRO J1744-28 is remarkable in many respects. It is a low-mass X-ray binary system that shows 2.1 Hz coherent pulsations. It also produces type II X-ray bursts similar to those seen in the Rapid Burster. In this Letter we report results from a series of observations with the Rossi X-Ray Timing Explorer (RXTE) during which GRO J1744-28 showed quasi-periodic X-ray brightness oscillations (QPOs) at 20, 40, and 60 Hz. Their fractional rms amplitudes are 0.19%, 5.9%, and 0.42%, respectively. The QPO centroid frequency and its rms amplitude as a function of source brightness in the band of 2-60 keV indicate that these QPOs are of a different kind from the beat-frequency QPOs that have been observed among Z sources as well as X-ray pulsars, We discuss possible interpretations of these QPOs in the context of neutron star g-mode oscillations and photon bubble oscillations.