{
  "id": "2311.05681",
  "version": "v1",
  "published": "2023-11-09T19:00:01.000Z",
  "updated": "2023-11-09T19:00:01.000Z",
  "title": "Dynamics of baryon ejection in magnetar giant flares: implications for radio afterglows, r-process nucleosynthesis, and fast radio bursts",
  "authors": [
    "Jakub Cehula",
    "Todd A. Thompson",
    "Brian D. Metzger"
  ],
  "comment": "24 pages, 12 figures. Submitted to MNRAS. Videos produced from simulations are publicly available at https://www.youtube.com/playlist?list=PLtfS1xXqUmtC2A48QhRvGXxbeMK1vkNnC. Comments are welcome",
  "categories": [
    "astro-ph.HE",
    "astro-ph.SR"
  ],
  "abstract": "We explore the impact of a magnetar giant flare (GF) on the neutron star (NS) crust, and the associated potential baryon mass ejection. We consider that sudden magnetic energy dissipation creates a thin high-pressure shell above a portion of the NS surface, which drives a relativistic shockwave into the crust, heating a fraction of these layers to sufficiently high energies to become unbound along directions unconfined by the magnetic field. We explore this process by means of spherically-symmetric relativistic hydrodynamical simulations. For an initial shell pressure $P_{\\rm GF}$ we find that the total unbound ejecta mass roughly obeys the relation $M_{\\rm ej}\\sim4-9\\times 10^{24}$ g $(P_{\\rm GF}/10^{30}$ ergs cm$^{-3})^{1.43}$. For $P_{\\rm GF}\\sim10^{30}-10^{31}$ ergs cm$^{-3}$ corresponding to the dissipation of a magnetic field of strength $\\sim10^{15.5}-10^{16}$ G, we find $M_{\\rm ej}\\sim10^{25}-10^{26}$ g with asymptotic velocities $v_{\\rm ej}/c\\sim 0.3-0.6$ compatible with the ejecta properties inferred from the radio afterglow of the GF from SGR 1806-20. Because the flare excavates crustal material to a depth characterized by an electron fraction $Y_e \\approx 0.40-0.46$, and is ejected with high entropy and rapid expansion timescale, the conditions are met for heavy element $r$-process nucleosynthesis via the alpha-rich freeze-out mechanism. Given an energetic GF rate of roughly once per century in the Milky Way, we find that GFs could contribute an appreciable heavy $r$-process source that tracks star formation. We predict that GFs are accompanied by short minutes long, luminous $\\sim 10^{39}$ ergs s$^{-1}$ optical transients powered by $r$-process decay (\"nova brevis\"), akin to scaled-down kilonovae. Our findings also have implications for FRBs from repeating magnetar flares, particularly the high rotation measures of the synchrotron nebulae surrounding these sources.",
  "revisions": [
    {
      "version": "v1",
      "updated": "2023-11-09T19:00:01.000Z"
    }
  ],
  "analyses": {
    "keywords": [
      "magnetar giant flare",
      "fast radio bursts",
      "radio afterglow",
      "magnetic energy dissipation creates",
      "r-process nucleosynthesis"
    ],
    "note": {
      "typesetting": "TeX",
      "pages": 24,
      "language": "en",
      "license": "arXiv",
      "status": "editable"
    }
  }
}