{
  "id": "0907.5415",
  "version": "v1",
  "published": "2009-07-30T20:03:52.000Z",
  "updated": "2009-07-30T20:03:52.000Z",
  "title": "X-Ray Monitoring of Ultraluminous X-Ray Sources",
  "authors": [
    "Philip Kaaret",
    "Hua Feng"
  ],
  "comment": "4 pages, to appear in ApJ",
  "categories": [
    "astro-ph.HE"
  ],
  "abstract": "X-ray monitoring observations were performed with the Swift observatory of the ultraluminous X-ray sources Holmberg IX X-1, NGC 5408 X-1, and NGC 4395 X-2 and also of the nuclear X-ray source in NGC 4395. Holmberg IX X-1 remains in the hard X-ray spectral state as its flux varies by a factor of 7 up to an (isotropic) luminosity of 2.8E40 erg/s. This behavior may suggest an unusually massive compact object. We find excess power at periods near 60 days and 28 days in the X-ray emission from Holmberg IX X-1. Additional monitoring is required to test the significance of these signals. NGC 5408 X-1 and NGC 4395 X-2 appear to remain in the soft spectral state found by Chandra and XMM with little variation in spectral hardness even as the luminosity changes by a factor of 9. We found an outburst from the nuclear source in NGC 4395 reaching an X-ray luminosity of 9E40 erg/s, several times higher than any previously reported.",
  "revisions": [
    {
      "version": "v1",
      "updated": "2009-07-30T20:03:52.000Z"
    }
  ],
  "analyses": {
    "keywords": [
      "x-ray monitoring",
      "ultraluminous x-ray sources holmberg ix",
      "hard x-ray spectral state",
      "nuclear x-ray source",
      "soft spectral state"
    ],
    "tags": [
      "journal article"
    ],
    "publication": {
      "doi": "10.1088/0004-637X/702/2/1679",
      "journal": "The Astrophysical Journal",
      "year": 2009,
      "month": "Sep",
      "volume": 702,
      "number": 2,
      "pages": 1679
    },
    "note": {
      "typesetting": "TeX",
      "pages": 4,
      "language": "en",
      "license": "arXiv",
      "status": "editable",
      "inspire": 827543,
      "adsabs": "2009ApJ...702.1679K"
    }
  }
}