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The dynamics of the plasma in the inner regions of an accretion disk aroundaccreting millisecond X-ray pulsars is controlled by the magnetic field of theneutron star. The interaction between an accretion disk and a strong magneticfield is not well-understood, particularly at low accretion rates (theso-called ``propeller regime'). This is due in part to the lack of clearobservational diagnostics to constrain the physics of the disk-fieldinteraction. Here we associate the strong ~1 Hz modulation seen in theaccreting millisecond X-ray pulsar NGC 6440 X-2 with an instability that ariseswhen 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). Asimilar modulation has previously been observed in another accretingmillisecond X-ray pulsar (SAX J1808.4-3658) and we suggest that the twophenomena are related and that this may be a common phenomenon among othermagnetized systems. Detailed comparisons with theoretical models suggest thatwhen the instability is observed, the interaction region between the disk andthe field is very narrow -- of the order of 1 km. Modelling further suggeststhat there is a transition region (~1-10 km) around the corotation radius wherethe disk-field torque changes sign from spin up to spin down. This is the firsttime that a direct observational constraint has been placed on the width of thedisk-magnetosphere interaction region, in the frame of the trapped-diskinstability model.

1 Hz Flaring in the Accreting Millisecond Pulsar NGC 6440 X-2: Disk Trapping and Accretion Cycles