{
  "id": "0906.1984",
  "version": "v2",
  "published": "2009-06-10T18:28:18.000Z",
  "updated": "2010-02-09T06:31:24.000Z",
  "title": "Discovery of A 693.5 s Period in the X-ray Binary 4U 1820-30: A Superhump Interpretation",
  "authors": [
    "Zhongxiang Wang",
    "Deepto Chakrabarty"
  ],
  "comment": "5 pages, 5 figures, Accepted for publication in ApJ",
  "categories": [
    "astro-ph.HE"
  ],
  "abstract": "The X-ray source 4U1820-30 in the globular cluster NGC 6624 is known as the most compact binary among the identified X-ray binaries. Having an orbital period of 685.0 s, the source consists of a neutron star primary and likely 0.06--0.08 Msun white dwarf secondary. Here we report on far-ultraviolet (FUV) observations of this X-ray binary, made with the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope. From our Fourier spectral analysis of the FUV timing data, we obtain a period of 693.5+/-1.3 s, which is significantly different from the orbital period. The light curve folded at this period can be described by a sinusoid, with a fractional semiamplitude of 6.3% and the phase zero (maximum of the sinusoid) at MJD 50886.015384+/-0.000043 (TDB). While the discovered FUV period may be consistent with a hierarchical triple system model that was previously considered for 4U 1820-30, we suggest that it could instead be the indication of superhump modulation, which arises from an eccentric accretion disk in the binary. The X-ray and FUV periods would be the orbital and superhump periods, respectively, indicating a 1% superhump excess and a white-dwarf/neutron-star mass ratio around 0.06. Considering 4U 1820-30 as a superhump source, we discuss the implications.",
  "revisions": [
    {
      "version": "v2",
      "updated": "2010-02-09T06:31:24.000Z"
    }
  ],
  "analyses": {
    "keywords": [
      "x-ray binary 4u",
      "superhump interpretation",
      "orbital period",
      "space telescope",
      "fuv period"
    ],
    "tags": [
      "journal article"
    ],
    "publication": {
      "doi": "10.1088/0004-637X/712/1/653",
      "journal": "The Astrophysical Journal",
      "year": 2010,
      "month": "Mar",
      "volume": 712,
      "number": 1,
      "pages": 653
    },
    "note": {
      "typesetting": "TeX",
      "pages": 5,
      "language": "en",
      "license": "arXiv",
      "status": "editable",
      "inspire": 822691,
      "adsabs": "2010ApJ...712..653W"
    }
  }
}