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

The repetitive X-ray bursts from the accretion-powered pulsar GRO J1744-28 show similarities to the type II X-ray bursts from the Rapid Burster. Several authors (notably, Lewin et al.) have suggested that the bursts from GRO J1744-28 are type II bursts (which arise from the sudden release of gravitational potential energy). In this paper, we present another similarity between these sources. Rossi X-ray Timing Explorer observations of GRO J1744-28 show that at least 10 out of 94 bursts are followed by quasi-periodic oscillations (QPOs) with frequencies of ~0.4 Hz. The period of the oscillations decreases over their ~30-80 s lifetime, and they occur during a spectrally hard “shoulder” (or “plateau”) that follows the burst. In one case, the QPOs show a modulation envelope that resembles simple beating between two narrow-band oscillations at ~0.325 and ~0.375 Hz. Using EXOSAT observations, Lubin et al. found QPOs with frequencies of 0.039-0.056 Hz following 10 out of 95 type II bursts from the Rapid Burster. As in GRO J1744-28, the period of these oscillations decreased over their ~100 s lifetime, and they occurred only during spectrally hard “humps” in the persistent emission. Even though the QPO frequencies differ by a factor of ~10, we believe that this is further evidence that a similar accretion disk instability is responsible for the type II bursts from these two sources.