The nature of the fluctuations preceding the giant bursts in the bursting pulsar GRO J1744-28

We examine recent time-dependent accretion disk calculations by Cannizzo that attempt to reproduce the giant bursts in GRO J1744-28 and find that the fluctuations that precede the main burst resemble the fluctuations that were observed in GRO J1744-28 toward the end of the time during which the giant bursts were seen. The calculations by Cannizzo were tuned to be at a rate of accretion slightly greater than the critical rate M-crit that divides steady and bursting behavior. This tuning leads to a near balance between the heating and cooling rates within the accretion disk such that oscillations slowly develop and grow, eventually becoming unbounded and producing a global instability. The fact that an increase in the amplitude of the interburst oscillations was seen in GRO J1744-28 most strongly during the late epoch of the time of several months covering the giant bursts is consistent with the mass accretion rate onto the pulsar decreasing and approaching M-crit.