Five fake ultra-luminous X-ray sources

Abstract

Context. The nature of ultra-luminous X-ray (ULX) sources is still a matter of debate. Studies of their counterparts in other spectral ranges will help to clarify the nature of those sources.
Aims: Our goal was to identify and characterize the optical counterparts of five sources catalogued as ULXs.
Methods: We obtained optical imaging and intermediate resolution spectroscopy of these optical counterparts.
Results: We demonstrate that five sources previously catalogued as ULXs, two are foreground stars in our galaxy and three are background active galactic nuclei.