{
  "id": "2012.04024",
  "version": "v1",
  "published": "2020-12-07T20:03:11.000Z",
  "updated": "2020-12-07T20:03:11.000Z",
  "title": "Observations of the Disk/Jet Coupling of MAXI J1820+070 During its Descent to Quiescence",
  "authors": [
    "A. W. Shaw",
    "R. M. Plotkin",
    "J. C. A. Miller-Jones",
    "J. Homan",
    "E. Gallo",
    "D. M. Russell",
    "J. A. Tomsick",
    "P. Kaaret",
    "S. Corbel",
    "M. Espinasse",
    "J. Bright"
  ],
  "comment": "19 pages, 5 figure, 1 appendix. Accepted for publication in ApJ",
  "categories": [
    "astro-ph.HE"
  ],
  "abstract": "Black hole X-ray binaries in the quiescent state (Eddington ratios typically $\\lesssim$10$^{-5}$) display softer X-ray spectra (photon indices $\\Gamma\\sim2$) compared to higher-luminosity black hole X-ray binaries in the hard state ($\\Gamma\\sim1.7$). However, the cause of this softening, and its implications for the underlying accretion flow, are still uncertain. Here, we present quasi-simultaneous X-ray and radio spectral monitoring of the black hole X-ray binary MAXI J1820$+$070 during the decay of its 2018 outburst and of a subsequent re-flare in 2019, providing an opportunity to monitor a black hole X-ray binary as it actively transitions into quiescence. We probe 1-10 keV X-ray luminosities as low as $L_{\\rm X}\\sim4\\times10^{32}$ erg s$^{-1}$, equivalent to Eddington fractions of $\\sim4\\times10^{-7}$. During its decay towards quiescence, the X-ray spectrum of MAXI J1820$+$070 softens from $\\Gamma\\sim1.7$ to $\\Gamma\\sim2$, with the softening taking $\\sim30$d, and completing at $L_{\\rm X}\\approx10^{34}$ erg s$^{-1}$ ($\\approx10^{-5} L_{\\rm Edd}$). While the X-ray spectrum softens, the radio spectrum generally remains flat/inverted throughout the decay. We also find that MAXI J1820$+$070 follows a radio ($L_{\\rm R}$) -- X-ray luminosity correlation of the form $L_{\\rm R} \\propto L_{\\rm X}^{0.52\\pm0.07}$, making it the fourth black hole system to follow the so-called `standard track' unbroken over several (in this case, four) decades in $L_{\\rm X}$. Comparing the radio/X-ray spectral evolution(s) with the $L_{\\rm R}$ -- $L_{\\rm X}$ plane, we find that the X-ray softening is consistent with X-rays produced by Comptonization processes in a radiatively inefficient accretion flow. We generally disfavor X-ray emission originating solely from within the jet, with the possible exception of X-rays produced via synchrotron self-Compton processes.",
  "revisions": [
    {
      "version": "v1",
      "updated": "2020-12-07T20:03:11.000Z"
    }
  ],
  "analyses": {
    "keywords": [
      "x-ray spectrum",
      "quiescence",
      "disk/jet coupling",
      "black hole x-ray binary maxi",
      "hole x-ray binary maxi j1820"
    ],
    "note": {
      "typesetting": "TeX",
      "pages": 19,
      "language": "en",
      "license": "arXiv",
      "status": "editable"
    }
  }
}