Postburst Quasi-periodic Oscillations from GRO J1744−28 and from the Rapid Burster

The repetitive X-ray bursts from the accretion-powered pulsar GRO J1744-28show similarities to the type II X-ray bursts from the Rapid Burster. Severalauthors (notably Lewin et al.) have suggested that the bursts from GRO J1744-28are type II bursts (which arise from the sudden release of gravitationalpotential energy). In this paper, we present another similarity between thesesources. Rossi X-ray Timing Explorer observations of GRO J1744-28 show that atleast 10 out of 94 bursts are followed by quasi-periodic oscillations (QPO)with frequencies of about 0.4 Hz. The period of the oscillations decreases overtheir 30--80 s lifetime, and they occur during a spectrally hard ``shoulder''(or ``plateau'') which follows the burst. In one case the QPO show a modulationenvelope which resembles simple beating between two narrow-band oscillations at0.325 and 0.375 Hz. Using EXOSAT observations, Lubin et al. found QPO withfrequencies of 0.039 to 0.056 Hz following 10 out of 95 type II bursts from theRapid Burster. As in GRO J1744-28, the period of these oscillations decreasedover their 100 s lifetime, and they occurred only during spectrally hard``humps'' in the persistent emission. Even though the QPO frequencies differ bya factor of 10, we believe that this is further evidence that a similaraccretion disk instability is responsible for the type II bursts from these twosources.