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

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