Astron. Astrophys. 338, 556-562 (1998)

Progressive covering in dipping and Comptonization in the spectrum of XB 1916-053 from the BeppoSAX observation

M.J. Church ¹, A.N. Parmar ², M. Bauciska-Church ¹, T. Oosterbroek ², D. Dal Fiume ³ and M. Orlandini <sup>3

1</sup> School of Physics and Astronomy, University of Birmingham, Birmingham, B15 2TT, UK
(mjc@star.sr.bham.ac.uk; mbc@star.sr.bham.ac.uk)
² Astrophysics Division, Space Science Department of ESA, ESTEC, Postbus 299, 2200 AG Noordwijk, The Netherlands
(aparmar@astro.estec.esa.nl; toosterb@astro.estec.esa.nl)
³ Istituto TESRE, CNR, via Gobetti 101, I-40129 Bologna, Italy
(daniele@tesre.bo.cnr.it; orlandini@tesre.bo.cnr.it)

Received 27 February 1998 / Accepted 26 May 1998

Abstract

We report results of a BeppoSAX observation of the low-mass X-ray binary (LMXB) dipping source XB 1916-053. The source joins the small group of LMXB detected at energies 100  keV. The non-dip spectrum is well fitted by an absorbed blackbody with a temperature of 1.62 keV and an absorbed cut-off power law with a photon index of 1.61 and a cut-off energy of 80 keV. Below 10 keV, where photoelectric absorption is dominant, the dramatic spectral changes observed during dips can be simply modelled by progressive covering of the blackbody and cut-off power law components. The blackbody component is very rapidly absorbed during dips, consistent with it being point-like, while the cutoff power law is more gradually absorbed, consistent with it being extended. The most likely locations for the blackbody component are the surface of the neutron star or the boundary layer between the neutron star and the accretion disk. The extended emission most probably originates in an accretion disk corona. Above 10 keV, dipping is detected up to 40 keV, and there is some evidence for an energy-independent reduction in intensity of up to 15%. This reduction could be caused by electron scattering or obscuration. In the first case, the change is consistent with an electron column density of  , several times smaller than the average hydrogen column measured simultaneously.

Key words: X-rays: stars stars: individual: XB 1916-053 stars: neutron binaries: close accretion: accretion discs

Send offprint requests to: M.J. Church

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Contents

• 1. Introduction
• 2. Observations
• 3. Results
  • 3.1. Light curves and hardness ratio
  • 3.2. The broad-band spectrum
  • 3.3. Spectral Evolution during dipping
• 4. Discussion
• Acknowledgements
• References

© European Southern Observatory (ESO) 1998

Online publication: September 14, 1998
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