Measuring the Magnetic Dipole Moment and Magnetospheric Fluctuations of Accretion-powered Pulsars in the Small Magellanic Cloud with an Unscented Kalman Filter

Many accretion-powered pulsars rotate in magnetocentrifugal disequilibrium, spinning up or down secularly over multiyear intervals. The magnetic dipole moment μ of such systems cannot be inferred uniquely from the time-averaged aperiodic X-ray flux 〈 L ( t )〉 and pulse period 〈 P ( t )〉, because the...

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Bibliographic Details
Published in:The Astrophysical journal 2024-08, Vol.971 (2), p.126
Main Authors: O’Leary, Joseph, Melatos, Andrew, Kimpson, Tom, O’Neill, Nicholas J., Meyers, Patrick M., Christodoulou, Dimitris M., Bhattacharya, Sayantan, Laycock, Silas G. T.
Format: Article
Language:English
Subjects:
Accretion disks
Bayesian statistics
Charged particles
Complex variables
Cross correlation
Dipole moments
Fluctuations
High energy astrophysics
High mass x-ray binary stars
Kalman filters
Magellanic clouds
Magnetic dipoles
Magnetic flux
Pulsar magnetospheres
Pulsars
X-ray fluxes
X-rays
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Summary:Many accretion-powered pulsars rotate in magnetocentrifugal disequilibrium, spinning up or down secularly over multiyear intervals. The magnetic dipole moment μ 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 μ 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 ∼16 yr. As well as independent estimates of μ , 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.
ISSN:0004-637X
1538-4357
DOI:10.3847/1538-4357/ad53c2