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      Ginga observations of XI820 — 303 in the globular cluster NGC 6624

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      Author(s)
      Ercan, E. N.
      Cruise, A. M.
      Kellett, B. J.
      Saygili, K.
      Date
      1993
      Source Title
      Royal Astronomical Society
      Print ISSN
      0035-8711
      Publisher
      Oxford University Press
      Volume
      262
      Issue
      2
      Pages
      511 - 520
      Language
      English
      Type
      Article
      Item Usage Stats
      111
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      36
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      Abstract
      We present the first spectral observations obtained by Ginga of the low-mass X-ray binary source X1820-303 located in the globular cluster NGC6624. Two twocomponent spectral models are found to provide good fits to the data: a bremsstrahlung component plus a blackbody, or a power law with exponential cut-off plus a blackbody. In both cases, inclusion of an iron line at 6.7 keV improves the fits. After considering the importance of Comptonization for the spectral and temporal behaviour, we select the power law plus blackbody model. This model then provides simple explanations for many of the observed features present in our data. The model requires a scattering cloud surrounding the neutron star, with a Thomson optical depth of~ 7. By studying the variation in the amphtude of the 685-s binary period with source luminosity, it is possible to gain some insight into the source geometry. This reveals that our line of sight to X1820 - 303 is close to some ‘critical’ angle. We suggest that gravitationally induced shocks in the rim of the accretion disc cause the height of the rim to vary with azimuthal angle, resulting in the quasi-sinusoidal X-ray orbital modulations. Superposed on this are random ‘dips’ which result from chaotic, timevarying structures in the disc which, principally, only cover the blackbody component of the spectrum.
      Keywords
      Accretion
      Accretion discs
      Binaries: close
      Stars
      Individual
      XI820 - 303 - globular clusters
      NGC 6624
      X-rays: stars
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      http://hdl.handle.net/11693/49379
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