The Nature of the Fluctuations Preceding the Giant Bursts in the Bursting Pulsar GRO J1744−28

Abstract

We examine recent time-dependentaccretion disk calculations by Cannizzo that attempt to reproduce thegiant bursts in GRO J1744-28 and find that the fluctuations thatprecede the main burst resemble the fluctuations that were observedin GRO J1744-28 toward the end of the time during which the giantbursts were seen. The calculations by Cannizzo were tuned to be ata rate of accretion slightly greater than the critical rate M dot_crit that divides steady and bursting behavior. This tuning leadsto a near balance between the heating and cooling rates within theaccretion disk such that oscillations slowly develop and grow, eventuallybecoming unbounded and producing a global instability. The fact that anincrease in the amplitude of the interburst oscillations was seenin GRO J1744-28 most strongly during the late epoch of the time ofseveral months covering the giant bursts is consistent with the massaccretion rate onto the pulsar decreasing and approaching M dot _crit.