The ultraluminous X-ray source X-37 is a background quasar in the Antennae galaxies

In this Letter we report that a bright, X-ray source in the Antennae galaxies (NGC 4038/9), previously identified as an ultraluminous X-ray source (ULX), is in fact a background quasar. We identify an isolated infrared and optical counterpart within 0.'' 3 +/- 0.'' 5 of the X-ray source X-37. After acquiring an optical spectrum of its counterpart, we use the narrow [O III] and broad H alpha emission lines to identify X-37 as a quasar at a redshift of z = 0.26. Through a U, V, and photometric analysis, we demonstrate that most of the observable light along this line of sight is from the quasar. We discuss the implications of this discovery and the importance of acquiring spectra for optical and IR counterparts to ULXs.