Tracking Hidden Magnetospheric Fluctuations in Accretion-powered Pulsars With a Kalman Filter

X-ray flux and pulse period fluctuations in an accretion-powered pulsar convey important information about the disk–magnetosphere interaction. It is shown that simultaneous flux and period measurements can be analysed with a Kalman filter based on the standard magnetocentrifugal accretion torque to generate accurate time-dependent estimates of three hidden state variables, which fluctuate stochastically and cannot be measured directly: the mass accretion rate, the Maxwell stress at the disk–magnetosphere boundary, and the radiative efficiency of accretion onto the stellar surface. The inferred fluctuation statistics carry implications for the physics of hydromagnetic instabilities at the disk–magnetosphere boundary and searches for continuous gravitational radiation from low-mass X-ray binaries.