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A four-hour orbital period of the X-ray burster 4U/MXB1636—53

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Abstract

It has been generally accepted that X-ray burst sources and other non-bursting ‘galactic bulge X-ray sources’ are low-mass close binary systems (for a review see ref. 1). The companion stars of the transient burst sources Aql X-1 ( = Aql MXB = 4U1908 + 00) and Cen X-4 ( = XB1455–31) are of spectral types G7–K3V (ref. 2) and K3–7V (ref. 3), respectively. The companion of the non-bursting transient source A0620–00 is of spectral type K4–5V (refs 4, 5). If these stars fill their Roche lobe, which is likely, the orbital periods are in the range 5–8 h. There is some evidence from X-ray observations alone that the orbital period of Cen X-4 is ∼8 h (ref. 6). From an analysis of their average optical properties, one of us (J.vP. ref. 7) estimated that the typical values of the companion star masses and orbital periods of this class of low-mass X-ray binaries are ∼0.6 M and ∼6 h, respectively. The orbital period recently suggested for the burst source 4U/MXB1735–44 (ref. 8) is in accordance with this general picture. We here report variations in the persistent optical flux of 4U/MXB1636–53 which seem to vary periodically with a period near 4 h.

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Pedersen, H., van Paradijs, J. & Lewin, W. A four-hour orbital period of the X-ray burster 4U/MXB1636—53. Nature 294, 725–727 (1981). https://doi.org/10.1038/294725a0

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