{
  "id": "1602.06792",
  "version": "v1",
  "published": "2016-02-22T14:42:09.000Z",
  "updated": "2016-02-22T14:42:09.000Z",
  "title": "Probing the hard and intermediate states of X-ray binaries using short time-scale variability",
  "authors": [
    "Chris J. Skipper",
    "Ian M. McHardy"
  ],
  "comment": "18 pages, 13 figures; accepted for publication in Monthly Notices of the Royal Astronomical Society, 2016 February 22",
  "categories": [
    "astro-ph.HE"
  ],
  "abstract": "Below an accretion rate of approximately a few per cent of the Eddington accretion rate, X-ray binary systems are not usually found in the soft spectral state. However, at accretion rates a factor of a few lower still, in the hard state, there is another spectral transition which is well observed but not well understood. Below ~0.5-1 per cent of the Eddington accretion rate (m_crit), the spectral index hardens with increasing accretion rate, but above m_crit, although still in the hard state, the spectral index softens with increasing accretion rate. Here we use a combination of X-ray spectral fitting and a study of short time-scale spectral variability to examine the behaviour of three well-known X-ray binaries: Cygnus X-1, GX 339-4 and XTE J1118+480. In Cygnus X-1 we find separate hard and soft continuum components, and show using root-mean-square (rms) spectra that the soft component dominates the variability. The spectral transition at m_crit is clearly present in the hard-state hardness-intensity diagrams of Cygnus X-1. Above m_crit, GX 339-4 shows similar softer-when-brighter behaviour at both long and short time-scales. Similarly, XTE J1118+480, which remains well below m_crit, is harder-when-brighter behaviour on all time-scales. We interpret these results in terms of two continuum components: a hard power-law which dominates the spectra when the accretion rate is low, probably arising from Comptonisation of cyclo-synchrotron photons from the corona, and a soft power-law which dominates at higher accretion rates, arising from Comptonisation of seed photons from the accretion disc.",
  "revisions": [
    {
      "version": "v1",
      "updated": "2016-02-22T14:42:09.000Z"
    }
  ],
  "analyses": {
    "keywords": [
      "x-ray binary",
      "short time-scale variability",
      "intermediate states",
      "eddington accretion rate",
      "increasing accretion rate"
    ],
    "publication": {
      "doi": "10.1093/mnras/stw436"
    },
    "note": {
      "typesetting": "TeX",
      "pages": 18,
      "language": "en",
      "license": "arXiv",
      "status": "editable",
      "inspire": 1422760
    }
  }
}