Measuring the magnetic dipole moment and magnetospheric fluctuations of accretion-powered pulsars in the Small Magellanic Cloud with an unscented Kalman filter

Abstract

Many accretion-powered pulsars rotate in magnetocentrifugal disequilibrium, spinning up or down secularly over multi-year intervals. The magnetic dipole moment $\mu$ of such systems cannot be inferred uniquely from the time-averaged aperiodic X-ray flux $⟨L(t)⟩$ and pulse period $⟨P(t)⟩$, because the radiative efficiency of the accretion is unknown and degenerate with the mass accretion rate. Here we circumvent the degeneracy by tracking the fluctuations in the unaveraged time series $L(t)$ and $P(t)$ using an unscented Kalman filter, whereupon $\mu$ can be estimated uniquely, up to the uncertainties in the mass, radius and distance of the star. The analysis is performed on Rossi X-ray Timing Explorer observations for $24$ X-ray transients in the Small Magellanic Cloud, which have been monitored regularly for $\sim 16$ years. As well as independent estimates of $\mu$, the analysis yields time-resolved histories of the mass accretion rate and the Maxwell stress at the disk-magnetosphere boundary for each star, and hence auto- and cross-correlations involving the latter two state variables. The inferred fluctuation statistics convey important information about the complex accretion physics at the disk-magnetosphere boundary.

42 pages, 10 figures, accepted for publication in The Astrophysical Journal

References in corpus (17)

• X-ray Properties of Black-Hole Binaries
• Accretion to Magnetized Stars through the Rayleigh-Taylor Instability: Global Three-Dimensional Simulations
• The Magnetic Rayleigh-Taylor Instability in Three Dimensions
• A Long Look at the Be/X-Ray Binaries of the Small Magellanic Cloud
• Accretion-Powered Stellar Winds III: Spin Equilibrium Solutions
• The Radiative Efficiency of a Radiatively Inefficient Accretion Flow
• Detection of Cyclotron Resonance Scattering Feature in High Mass X-ray Binary Pulsar SMC X-2
• Spin up and phase fluctuations in the timing of the accreting millisecond pulsar XTE J1807-294
• Magnetic Fields of Neutron Stars
• SXP 5.05 = IGR J00569-7226 : using X-rays to explore the structure of a Be stars circumstellar disk
• Indications of non-conservative mass-transfer in AMXPs
• Rapid parameter estimation of a two-component neutron star model with spin wandering using a Kalman filter
• Three new high-mass X-ray binaries in the Large Magellanic Cloud
• A Bayesian approach for torque modelling of BeXRB pulsars with application to super-Eddington accretors
• Tracking hidden magnetospheric fluctuations in accretion-powered pulsars with a Kalman filter
• Pulse Frequency Fluctuations of Persistent Accretion Powered Pulsars
• Measuring the magnetic dipole moment and magnetospheric fluctuations of SXP 18.3 with a Kalman filter