1994A&A...286..491K

The main observations of Type II bursts in the low-mass X-ray binary MXB 1730-335 (the Rapid Burster) can be understood if the neutron star's magnetic field is strong (7 x 10¹¹ - 2 x 10¹² gauss at the surface) and rotationally symmetric around the axis of a steadily accreting disk. We show that a Keplerian disk of matter develops inside the pressure balance radius where it is compressed into a very thin disk by the external magnetic field. At the inner edge of the disk a ring of matter is suspended in the stellar magnetic field and corotates with the star. During quiescence the disk penetrates more and more into the magnetosphere, the ring accumulates gas and shifts to lower altitudes. The stellar field sets a maximum to the amount of supported gas. As soon as the threshold is passed an ideal mhd instability occurs, the matter ring falls onto the surface and triggers a violent burst of accretion. As the stellar field relaxes outward accretion onto the star continues to be enhanced until a new magnetic equilibrium is attained, the burst ends and a smaller ring is left at higher altitudes. As the (steady) external accretion continues the ring load increases again and moves inward until the threshold is reached and the sequence repeats. Such a bursting behaviour only occurs if the star is an aligned rotator.