Astrometry of the globular cluster 47 Tuc and possible optical identification of X-ray sources

Abstract

Positions of stars of 47 Tuc have been derived by the use of photographic plates and CCD frames (ESO La Silla) and data from the Hubble Space Telescope (HST). The positions have been determined with respect to the PPM, which is based on the FK5 system. We have compared the positions of variable and blue stars in the core of 47 Tuc with those of the X-ray sources found by Hasinger et al. (1994). Taking into account a possible constant shift of the X-ray positions of up to 10", there are three different solutions which would give identifications of four of the central X-ray sources with blue stragglers and variable stars. We discuss the nature of the X-ray sources and the different possible identifications. Since it is not possible to give an unique identification based on pure astrometric arguments, our positions may be taken for future identifications based on additional astrophysical arguments or coincident observations in different wavelengths. An identification of the X-ray source No. 3 of Hasinger et al. (1994) in the outer field of 47 Tuc with a galaxy was found.