A MAGNETIC EXPLANATION FOR THE RAPID BURSTER

The main observations of Type II bursts in the low-mass X-ray binary M XB 1730-335 (the Rapid Burster) can be understood if the neutron star' s magnetic field is strong (7 x 10(11) - 2 x 10(12) gauss at the surfa ce) and rotationally symmetric around the axis of a steadily accreting disk. We show that a Keplerian disk of matter develops inside the pre ssure balance radius where it is compressed into a very thin disk by t he external magnetic field. At the inner edge of the disk a ring of ma tter is suspended in the stellar magnetic field and corotates with the star. During quiescence the disk penetrates more and more into the ma gnetosphere, the ring accumulates gas and shifts to lower altitudes. T he stellar field sets a maximum to the amount of supported gas. As soo n as the threshold is passed an ideal mhd instability occurs, the matt er ring falls onto the surface and triggers a violent burst of accreti on. As the stellar field relaxes outward accretion onto the star conti nues to be enhanced until a new magnetic equilibrium is attained, the burst ends and a smaller ring is left at higher altitudes. As the (ste ady) external accretion continues the ring load increases again and mo ves inward until the threshold is reached and the sequence repeats. Su ch a bursting behaviour only occurs if the star is an aligned rotator.