X-ray and UV observations of omega Centauri with EXOSAT.

Abstract

The globular cluster ω Cen has been observed on two occasions, 6 months apart, with the Channel Multiplier Array on board EXOSAT, in the energy band 0.02 keV - 2.5 keV. The aim of the present investigation is to study the diffuse emission as well as the point sources discovered in the X-ray range with the EINSTEIN satellite (Hartwick et al., 1982). A diffuse UV emission due to the unresolved hot horizontal branch stars of ω Cen is observed. The diffuse X-ray source discovered by Hartwick et al. (1982) is not detected but the authors sensitivity threshold gives an upper limit of the flux which is not inconsistent with the EINSTEIN result. Four UV and three X-ray point sources are detected. One X-ray source shows variability of about a factor 4.5 in flux between the two EXOSAT observations. Flux variations for the X-ray sources between the two EXOSAT observations or with respect to the EINSTEIN ones are discussed.