{
  "id": "2009.07279",
  "version": "v1",
  "published": "2020-09-15T18:00:01.000Z",
  "updated": "2020-09-15T18:00:01.000Z",
  "title": "Polarized kilonovae from black hole-neutron star mergers",
  "authors": [
    "M. Bulla",
    "K. Kyutoku",
    "M. Tanaka",
    "S. Covino",
    "J. R. Bruten",
    "T. Matsumoto",
    "J. R. Maund",
    "V. Testa",
    "K. Wiersema"
  ],
  "comment": "8 pages, 4 figures; to be submitted to MNRAS; comments are welcome",
  "categories": [
    "astro-ph.HE"
  ],
  "abstract": "We predict linear polarization for a radioactively-powered kilonova following the merger of a black hole and a neutron star. Specifically, we perform 3-D Monte Carlo radiative transfer simulations for two different models, both featuring a lanthanide-rich dynamical ejecta component from numerical-relativity simulations while only one including an additional lanthanide-free disk wind component. We calculate polarization spectra for nine different orientations at 1.5, 2.5 and 3.5 d after the merger and in the $0.1-2\\,\\mu$m wavelength range. We find that both models are polarized at a detectable level 1.5 d after the merger while show negligible levels thereafter. The polarization spectra of the two models are significantly different. The model lacking a disk wind shows no polarization in the optical, while a signal increasing at longer wavelengths and reaching $\\sim1\\%-6\\%$ at $2\\,\\mu$m depending on the orientation. The model with a disk-wind component, instead, features a characteristic \"double-peak\" polarization spectrum with one peak in the optical and the other in the infrared. Polarimetric observations of future events will shed light on the debated neutron richness of the disk-wind component. The detection of optical polarization would unambiguously reveal the presence of a lanthanide-free disk-wind component, while polarization increasing from zero in the optical to a peak in the infrared would suggest a lanthanide-rich composition for the whole ejecta. Future polarimetric campaigns should prioritize observations in the first $\\sim48$ hours and in the $0.5-2\\,\\mu$m range, where polarization is strongest, but also explore shorter wavelengths/later times where no signal is expected from the kilonova and the interstellar polarization can be safely estimated.",
  "revisions": [
    {
      "version": "v1",
      "updated": "2020-09-15T18:00:01.000Z"
    }
  ],
  "analyses": {
    "keywords": [
      "black hole-neutron star mergers",
      "polarized kilonovae",
      "carlo radiative transfer simulations",
      "polarization spectrum",
      "disk-wind component"
    ],
    "note": {
      "typesetting": "TeX",
      "pages": 8,
      "language": "en",
      "license": "arXiv",
      "status": "editable"
    }
  }
}