{
  "id": "1912.12145",
  "version": "v1",
  "published": "2019-12-27T15:11:10.000Z",
  "updated": "2019-12-27T15:11:10.000Z",
  "title": "Coronal properties of black hole X-ray binaries in the hard state as seen by NuSTAR and Swift",
  "authors": [
    "Zhen Yan",
    "Fu-guo Xie",
    "Wenda Zhang"
  ],
  "comment": "accepted for publication in ApJL",
  "categories": [
    "astro-ph.HE"
  ],
  "abstract": "In this work, we measure two important phenomenological parameters of corona (and hot accretion flow) in black hole X-ray binaries: the photon index $\\Gamma$ and the electron temperature $\\te$. Thanks to the capability of {\\it NuSTAR} in hard X-rays, we measure these two parameters over six orders of magnitude in the 0.1-100 keV X-ray luminosity $\\lx$, from $\\sim 5\\times10^{38} \\ergs$ down to as low as $\\sim 5\\times10^{32} \\ergs$. We confirm the existence of a \"V\"-shaped correlation between $\\Gamma$ and $\\lx$. Surprisingly, we observe a \"$\\Lambda$\"-shaped correlation between $\\te$ and $\\lx$. The \"cooler when brighter\" branch in the high luminosity regime ($\\lx \\ga 3\\times 10^{36} \\ergs$) agrees with previous results and can be understood under the existing model of Compton scattering in the corona. On the other hand, the apparent \"cooler when fainter\" (positive $\\te$-$\\lx$ correlation) branch in the low-luminosity regime ($\\lx \\la 3\\times 10^{36} \\ergs$) is unexpected, thus it puts a new challenge to existing models of hot accretion flow/corona.",
  "revisions": [
    {
      "version": "v1",
      "updated": "2019-12-27T15:11:10.000Z"
    }
  ],
  "analyses": {
    "keywords": [
      "black hole x-ray binaries",
      "coronal properties",
      "hard state",
      "high luminosity regime",
      "correlation"
    ],
    "note": {
      "typesetting": "TeX",
      "pages": 0,
      "language": "en",
      "license": "arXiv",
      "status": "editable"
    }
  }
}