Postburst Quasi-periodic Oscillations from Gro J1744228 and from the Rapid Burster

The repetitive X-ray bursts from the accretion-powered pulsar GRO J1744228 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 J1744228 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 J1744228 show that at least 10 out of 94 bursts are followed by quasi-periodic oscillations (QPOs) with frequencies of 10.4 Hz. The period of the oscillations decreases over their 130–80 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 10.325 and 10.375 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 J1744228, the period of these oscillations decreased over their 1100 s lifetime, and they occurred only during spectrally hard “humps” in the persistent emission. Even though the QPO frequencies differ by a factor of 110, we believe that this is further evidence that a similar accretion disk instability is responsible for the type II bursts from these two sources. Subject headings: X-rays: bursts — stars: neutron—X-rays: general