{
  "id": "1709.01559",
  "version": "v1",
  "published": "2017-09-05T19:21:17.000Z",
  "updated": "2017-09-05T19:21:17.000Z",
  "title": "Truncation of the Accretion Disk at One Third of the Eddington Limit in the Neutron Star Low-Mass X-ray Binary Aquila X-1",
  "authors": [
    "Renee M. Ludlam",
    "Jon M. Miller",
    "Nathalie Degenaar",
    "Andrea Sanna",
    "Edward M. Cackett",
    "Diego Altamirano",
    "Ashley L. King"
  ],
  "comment": "Accepted for publication in ApJ, 7 pages, 2 Tables, 5 Figures",
  "categories": [
    "astro-ph.HE"
  ],
  "abstract": "We perform a reflection study on a new observation of the neutron star low-mass X-ray binary Aquila X-1 taken with NuSTAR during the August 2016 outburst and compare with the July 2014 outburst. The source was captured at $\\sim32\\%\\ L_{\\mathrm{Edd}}$, which is over four times more luminous than the previous observation during the 2014 outburst. Both observations exhibit a broadened Fe line profile. Through reflection modeling, we determine that the inner disk is truncated $R_{in,\\ 2016}=11_{-1}^{+2}\\ R_{g}$ (where $R_{g}=GM/c^{2}$) and $R_{in,\\ 2014}=14\\pm2\\ R_{g}$ (errors quoted at the 90% confidence level). Fiducial neutron star parameters (M$_{NS}=1.4$ M$_{\\odot}$, $R_{NS}=10$ km) give a stellar radius of $R_{NS}=4.85\\ R_{g}$; our measurements rule out a disk extending to that radius at more than the $6\\sigma$ level of confidence. We are able to place an upper limit on the magnetic field strength of $B\\leq3.0-4.5\\times10^{9}$ G at the magnetic poles, assuming that the disk is truncated at the magnetospheric radius in each case. This is consistent with previous estimates of the magnetic field strength for Aquila X-1. However, if the magnetosphere is not responsible for truncating the disk prior to the neutron star surface, we estimate a boundary layer with a maximum extent of $R_{BL,\\ 2016}\\sim10\\ R_{g}$ and $R_{BL,\\ 2014}\\sim6\\ R_{g}$. Additionally, we compare the magnetic field strength inferred from the Fe line profile of Aquila X-1 and other neutron star low-mass X-ray binaries to known accreting millisecond X-ray pulsars.",
  "revisions": [
    {
      "version": "v1",
      "updated": "2017-09-05T19:21:17.000Z"
    }
  ],
  "analyses": {
    "keywords": [
      "neutron star low-mass x-ray binary",
      "star low-mass x-ray binary aquila",
      "magnetic field strength",
      "accretion disk",
      "eddington limit"
    ],
    "note": {
      "typesetting": "TeX",
      "pages": 7,
      "language": "en",
      "license": "arXiv",
      "status": "editable"
    }
  }
}