"Disk Accretion Onto Magnetic Neutron Stars: Some Surprises and a New Toy Model"

The features of accretion torque models for disk accreting magnetic neutron stars are summarized. Observations of alternating spin-up and spin-down episodes in several accreting X-ray pulsars contradict some of the basic predictions of standard models. We consider a toy model for the disk-magnetosphere interaction in which matter accretion, ejection and recycling can take place at the magnetospheric boundary. We show that a limit cycle behavior ensues over a wide range of parameters, causing the neutron star to flip between spin up and spin down intervals.