We present two epochs of near-infrared spectroscopy of the candidate red supergiant counterpart to RX J004722.4-252051 , a ULX in NGC 253 .
We measure radial velocities of the object and its approximate spectral type by cross-correlating our spectra with those of known red supergiants .
Our VLT/X-shooter spectrum is best matched by that of early M-type supergiants , confirming the red supergiant nature of the candidate counterpart .
The radial velocity of the spectrum , taken on 2014 , August 23 , is 417 \pm 4 km s ^ { -1 } .
This is consistent with the radial velocity measured in our spectrum taken with Magellan/MMIRS on 2013 , June 28 , of 410 \pm 70 km s ^ { -1 } , although the large error on the latter implies that a radial velocity shift expected for a black hole of tens of M _ { \odot } can easily be hidden .
Using nebular emission lines we find that the radial velocity due to the rotation of NGC 253 is 351 \pm 4 km s ^ { -1 } at the position of the ULX .
Thus the radial velocity of the counterpart confirms that the source is located in NGC 253 , but also shows an offset with respect to the local bulk motion of the galaxy of 66 \pm 6 km s ^ { -1 } .
We argue that the most likely origin for this displacement lies either in a SN kick , requiring a system containing a \gtrsim 50 M _ { \odot } black hole , and/or in orbital radial velocity variations in the ULX binary system , requiring a \gtrsim 100 M _ { \odot } black hole .
We therefore conclude that RX J004722.4-252051 is a strong candidate for a ULX containing a massive stellar black hole .