{
  "id": "2306.14946",
  "version": "v1",
  "published": "2023-06-26T18:00:01.000Z",
  "updated": "2023-06-26T18:00:01.000Z",
  "title": "Hourslong Near-UV/Optical Emission from Mildly Relativistic Outflows in Black Hole-Neutron Star Mergers",
  "authors": [
    "Ore Gottlieb",
    "Danat Issa",
    "Jonatan Jacquemin-Ide",
    "Matthew Liska",
    "Alexander Tchekhovskoy",
    "Francois Foucart",
    "Daniel Kasen",
    "Rosalba Perna",
    "Eliot Quataert",
    "Brian D. Metzger"
  ],
  "comment": "For movies of the simulations, see https://oregottlieb.com/bhns.html",
  "categories": [
    "astro-ph.HE"
  ],
  "abstract": "The ongoing LIGO-Virgo-KAGRA observing run O4 provides an opportunity to discover new multi-messenger events, including binary neutron star (BNS) mergers such as GW170817, and the highly anticipated first detection of a multi-messenger black hole-neutron star (BH-NS) merger. While BNS mergers were predicted to exhibit early optical emission from mildly-relativistic outflows, it has remained uncertain whether the BH-NS merger ejecta provides the conditions for similar signals to emerge. We present the first modeling of early near-ultraviolet/optical emission from mildly-relativistic outflows in BH-NS mergers. Adopting optimal binary properties, mass ratio of $q=2$ and rapidly rotating BH, we utilize numerical-relativity and general-relativistic magnetohydrodynamic (GRMHD) simulations to follow the binary's evolution from the pre-merger to homologous expansion. We use an M1 neutrino transport GRMHD simulation to self-consistently estimate the opacity distribution in the outflows, and find a bright near-ultraviolet/optical signal that emerges due to jet-powered cocoon cooling emission, outshining the kilonova emission at early time. The signal peaks at an absolute magnitude of $-14$ to $-15$ a few hours after the merger, longer than previous estimates, which did not consider the first-principles-based jet launching. By late 2024, the Rubin Observatory will have the capability to track the entire signal evolution, or detect its peak up to distances $\\gtrsim1$ Gpc. In 2026, ULTRASAT will conduct all-sky surveys within minutes, detecting some of these events within $ \\sim 200 $ Mpc. BH-NS mergers with higher mass ratios or lower BH spins would produce shorter and fainter signals.",
  "revisions": [
    {
      "version": "v1",
      "updated": "2023-06-26T18:00:01.000Z"
    }
  ],
  "analyses": {
    "keywords": [
      "black hole-neutron star mergers",
      "hourslong near-uv/optical emission",
      "mildly relativistic outflows",
      "neutrino transport grmhd simulation",
      "ligo-virgo-kagra observing run o4"
    ],
    "note": {
      "typesetting": "TeX",
      "pages": 0,
      "language": "en",
      "license": "arXiv",
      "status": "editable"
    }
  }
}