{
  "id": "2406.01827",
  "version": "v1",
  "published": "2024-06-03T22:54:11.000Z",
  "updated": "2024-06-03T22:54:11.000Z",
  "title": "Measuring the magnetic dipole moment and magnetospheric fluctuations of accretion-powered pulsars in the Small Magellanic Cloud with an unscented Kalman filter",
  "authors": [
    "Joseph O'Leary",
    "Andrew Melatos",
    "Tom Kimpson",
    "Nicholas J. O'Neill",
    "Patrick M. Meyers",
    "Dimitris M. Christodoulou",
    "Sayantan Bhattacharya",
    "Silas G. T. Laycock"
  ],
  "comment": "42 pages, 10 figures, accepted for publication in The Astrophysical Journal",
  "categories": [
    "astro-ph.HE"
  ],
  "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 $\\langle L(t) \\rangle$ and pulse period $\\langle P(t) \\rangle$, 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.",
  "revisions": [
    {
      "version": "v1",
      "updated": "2024-06-03T22:54:11.000Z"
    }
  ],
  "analyses": {
    "keywords": [
      "magnetic dipole moment",
      "small magellanic cloud",
      "unscented kalman filter",
      "accretion-powered pulsars",
      "fluctuation statistics convey important"
    ],
    "note": {
      "typesetting": "TeX",
      "pages": 42,
      "language": "en",
      "license": "arXiv",
      "status": "editable"
    }
  }
}