  An unusual double-lobed extended X-ray source (RX J105343+5735) is detected
in the ROSAT ultra-deep HRI image of the Lockman Hole. The angular size of the
source is 1.7 X 0.7 arcmin^2 and its X-ray flux is 2 X 10^-14 erg cm^-2 s^-1.
R-band imaging from the Keck telescope revealed a marginal excess of galaxies
brighter than R=24.5, but Keck LRIS spectroscopy of 24 objects around the X-ray
centroid did not yield a significant number of concordant redshifts. The
brightest galaxy close to the centre of the eastern emission peak appears to be
a gravitationally lensed arc at z=2.570, suggesting that the X-ray object is
associated with the lens, most likely a cluster of galaxies. Based on a
comparison of lensing surface mass density, X-ray luminosity, morphology and
galaxy magnitudes with clusters of known distance, we argue that RX
J105343+5735 is a cluster at a redshift around 1. Future X-ray, ground-based
optical/NIR and high resolution HST observations of the system will be able to
clarify the nature of the object.
