The evolution of X-ray bursts in the `Bursting Pulsar' GRO J1744-28

Abstract

GRO J1744-28, commonly known as the `Bursting Pulsar', is a low-mass X-ray binary containing a neutron star and an evolved giant star. This system, together with the Rapid Burster (MXB 1730-33), are the only two systems that display the so-called type II X-ray bursts. These types of bursts, which last for tens of seconds, are thought to be caused by viscous instabilities in the disc; however, the type II bursts seen in GRO J1744-28 are qualitatively very different from those seen in the archetypal type II bursting source, the Rapid Burster. To understand these differences and to create a framework for future study, we perform a study of all X-ray observations of all three known outbursts of the Bursting Pulsar which contained type II bursts, including a population study of all type II X-ray bursts seen by RXTE. We find that the bursts from this source are best described in four distinct phenomena or `classes' and that the characteristics of the bursts evolve in a predictable way. We compare our results with what is known for the Rapid Burster and put out results in the context of models that try to explain this phenomena.