The dynamics of the plasma in the inner regions of an accretion disk around accreting millisecond X-ray pulsars is controlled by the magnetic field of the neutron star .
The interaction between an accretion disk and a strong magnetic field is not well-understood , particularly at low accretion rates ( the so-called “ propeller regime ” ) .
This is due in part to the lack of clear observational diagnostics to constrain the physics of the disk-field interaction .
Here we associate the strong \sim 1 Hz modulation seen in the accreting millisecond X-ray pulsar NGC 6440 X-2 with an instability that arises when the inner edge of the accretion disk is close to the corotation radius ( where the stellar rotation rate matches the Keplerian speed in the disk ) .
A similar modulation has previously been observed in another accreting millisecond X-ray pulsar ( SAX J1808.4–3658 ) and we suggest that the two phenomena are related and that this may be a common phenomenon among other magnetized systems .
Detailed comparisons with theoretical models suggest that when the instability is observed , the interaction region between the disk and the field is very narrow – of the order of 1 km .
Modeling further suggests that there is a transition region ( \sim 1 - 10 km ) around the corotation radius where the disk-field torque changes sign from spin up to spin down .
This is the first time that a direct observational constraint has been placed on the width of the disk-magnetosphere interaction region , in the frame of the trapped-disk instability model .