  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.
