{
  "id": "1711.00034",
  "version": "v1",
  "published": "2017-10-31T18:16:17.000Z",
  "updated": "2017-10-31T18:16:17.000Z",
  "title": "The Sleeping Monster: NuSTAR observations of SGR 1806-20, 11 years after the Giant Flare",
  "authors": [
    "George Younes",
    "Matthew G. Baring",
    "Chryssa Kouveliotou",
    "Alice Harding",
    "Sophia Donavan",
    "Ersin Gogus",
    "Victoria Kaspi",
    "Jonathan Granot"
  ],
  "comment": "10 pages, 5 figures, accepted for publication in ApJ",
  "categories": [
    "astro-ph.HE"
  ],
  "abstract": "We report the analysis of 5 NuSTAR observations of SGR 1806-20 spread over a year from April 2015 to April 2016, more than 11 years following its Giant Flare (GF) of 2004. The source spin frequency during the NuSTAR observations follows a linear trend with a frequency derivative $\\dot{\\nu}=(-1.25\\pm0.03)\\times10^{-12}$ Hz s$^{-1}$, implying a surface dipole equatorial magnetic field $B\\approx7.7\\times10^{14}$ G. Thus, SGR 1806-20 has finally returned to its historical minimum torque level measured between 1993 and 1998. The source showed strong timing noise for at least 12 years starting in 2000, with $\\dot{\\nu}$ increasing one order of magnitude between 2005 and 2011, following its 2004 major bursting episode and GF. SGR 1806-20 has not shown strong transient activity since 2009 and we do not find short bursts in the NuSTAR data. The pulse profile is complex with a pulsed fraction of $\\sim8\\%$ with no indication of energy dependence. The NuSTAR spectra are well fit with an absorbed blackbody, $kT=0.62\\pm0.06$ keV, plus a power-law, $\\Gamma=1.33\\pm0.03$. We find no evidence for variability among the 5 observations, indicating that SGR 1806-20 has reached a persistent and potentially its quiescent X-ray flux level after its 2004 major bursting episode. Extrapolating the NuSTAR model to lower energies, we find that the 0.5-10 keV flux decay follows an exponential form with a characteristic timescale $\\tau=543\\pm75$ days. Interestingly, the NuSTAR flux in this energy range is a factor of $\\sim2$ weaker than the long-term average measured between 1993 and 2003, a behavior also exhibited in SGR $1900+14$. We discuss our findings in the context of the magnetar model.",
  "revisions": [
    {
      "version": "v1",
      "updated": "2017-10-31T18:16:17.000Z"
    }
  ],
  "analyses": {
    "keywords": [
      "nustar observations",
      "giant flare",
      "sleeping monster",
      "showed strong timing noise",
      "surface dipole equatorial magnetic field"
    ],
    "note": {
      "typesetting": "TeX",
      "pages": 10,
      "language": "en",
      "license": "arXiv",
      "status": "editable"
    }
  }
}