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The optical counterpart of SAX J1808.4-3658, the transient bursting millisecond X-ray pulsar

A. B. Giles·K. M. Hill·J. G. Greenhill
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Abstract

A set of CCD images have been obtained during the decline of the X-ray transient SAX J1808.4-3658 during April-June 1998. The optical counterpart has been confirmed by several pieces of evidence. The optical flux shows a modulation on several nights which is consistent with the established X-ray binary orbit period of 2 hours. This optical variability is roughly in antiphase with the weak X-ray modulation. The source mean magnitude of V=16.7 on April 18 declined rapidly after April 22. From May 2 onwards the magnitude was more constant at around V=18.45 but by June 27 was below our sensitivity limit. The optical decline precedes the rapid second phase of the X-ray decrease by 3 +/- 1 days. The source has been identified on a 1974 UK Schmidt plate at an estimated magnitude of ~20. The nature of the optical companion is discussed.

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A. B. Giles, K. M. Hill, J. G. Greenhill. 1999-10-07. The optical counterpart of SAX J1808.4-3658, the transient bursting millisecond X-ray pulsar. https://doi.org/10.1046/j.1365-8711.1999.02361.x

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