{
  "id": "2104.03867",
  "version": "v1",
  "published": "2021-04-08T16:22:13.000Z",
  "updated": "2021-04-08T16:22:13.000Z",
  "title": "New X-ray observations of the hot subdwarf binary HD49798 / RXJ0648.0-4418",
  "authors": [
    "S. Mereghetti",
    "F. Pintore",
    "T. Rauch",
    "N. La Palombara",
    "P. Esposito",
    "S. Geier",
    "I. Pelisoli",
    "M. Rigoselli",
    "V. Schaffenroth",
    "A. Tiengo"
  ],
  "comment": "Accepted for publication on MNRAS",
  "categories": [
    "astro-ph.SR",
    "astro-ph.HE"
  ],
  "abstract": "HD49798 / RXJ0648.0-4418 is the only confirmed X-ray binary in which the mass donor is a hot subdwarf star of O spectral type and, most likely, it contains a massive white dwarf (1.28$\\pm$0.05 M$_{\\rm SUN}$) with a very fast spin period of 13.2 s. Here we report the results of new XMM-Newton pointings of this peculiar binary, carried out in 2018 and in 2020, together with a reanalysis of all the previous observations. The new data indicate that the compact object is still spinning-up at a steady rate of $(-2.17\\pm0.01)\\times10^{-15}$ s s$^{-1}$, consistent with its interpretation in terms of a young contracting white dwarf. Comparison of observations obtained at similar orbital phases, far from the ecplise, shows evidence for long term variability of the hard ($>$0.5 keV) spectral component at a level of $\\sim$(70$\\pm$20)\\%, suggesting the presence of time-dependent inhomogeneities in the weak stellar wind of the HD49798 subdwarf. To investigate better the soft spectral component that dominates the X-ray flux from this system, we computed a theoretical model for the thermal emission expected from an atmosphere with element abundances and surface gravity appropriate for this massive white dwarf. This model gives a best fit with effective temperature of T$_{\\rm eff}$=2.25$\\times$10$^5$ K and an emitting area with radius of $\\sim$1600 km, larger than that found with blackbody fits. This model also predicts a contribution of the pulsed emission from the white dwarf in the optical band significantly larger than previously thought and possibly relevant for optical variability studies of this system.",
  "revisions": [
    {
      "version": "v1",
      "updated": "2021-04-08T16:22:13.000Z"
    }
  ],
  "analyses": {
    "keywords": [
      "hot subdwarf binary hd49798",
      "x-ray observations",
      "massive white dwarf",
      "spectral component",
      "fast spin period"
    ],
    "note": {
      "typesetting": "TeX",
      "pages": 0,
      "language": "en",
      "license": "arXiv",
      "status": "editable"
    }
  }
}