Abstract
WE suggest faint (B ∼ 18), blue stars as the optical counterparts of two X-ray bursters, 4U1636–53 (MXB1636–53)1–4 and 4U1735–44 (MXB1735–44; KGX345–6?)1,4–6, and for the 7-s X-ray pulsar, 4U1626–671,7,8. The candidate stars have large ultraviolet excesses and were discovered well within the 20″ and 30″ X-ray error radii determined using the rotating modulation collimator experiment aboard the SAS-3 X-ray Observatory9,10. Photographic observations of several X-ray error boxes were performed (by C.R.C. and W.A.H.) in April 1976 (before final SAS-3 X-ray positions were known) with the 4-m telescope at the Cerro Tololo Interamerican Observatory (CTIO). R, B and U plates were obtained for the regions including 4U1636–53 and 4U1735–44. The R plates (RG610 filter with 127-04 emulsion) are shown as finding charts in ref. 9. Preliminary iris photometry of some of the plates was performed to identify the bluest stars in each error box for further study.
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McCLINTOCK, J., CANIZARES, C., BRADT, H. et al. Optical candidates for two X-ray bursters and an X-ray pulsar. Nature 270, 320–321 (1977). https://doi.org/10.1038/270320a0
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DOI: https://doi.org/10.1038/270320a0
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