2008AJ....136..212R

We present radio through X-ray observations of a bright (>10⁴⁰ erg/s in the 0.5-8.0 keV band) ultraluminous X-ray source (ULX), CXOU J153434.9+151149, in the starburst, interacting pair of galaxies NGC 5953/5954. A Chandra image of this ULX shows that it is elongated. From HST/WFPC2/F606W data we have detected a counterpart of the ULX system with M_F606W∼ -7.1±0.7 mag. This optical counterpart may be either an O-type supergiant star or a young star cluster. From our Fabry-Perot interferometric observations, we have detected Hα and [N II] (6584 Å) diffuse emission, with velocity gradients up to 60 km/s at the astrometric corrected Chandra position of the ULX. Different scenarios have been invoked as to explain the possible nature of CXOU J153434.9+151149. Based on the observed X-ray morphology of the ULX, we estimate that the inclination between the line of sight to the observer and the direction of a possibly beamed outflow is ∼53°. Beaming with this geometry from a stellar-mass black hole system will be inadequate to explain the observed X-ray luminosity of this ULX. Finally, we suggest that mild-beaming from a binary black hole with mass more than 50 M_☉ associated with a young star cluster, is the most favorable scenario that describes the multiwavelength properties of this ULX. Future observations are definitely needed to determine the nature of this rare object.