The supergiant optical counterpart of ULX P13 in NGC 7793

We have identified the optical counterpart of the ULX source P13 in the nearby spiral galaxy NGC 7793. The object is a V similar to 20.5 mag star, ten times brighter than any other established counterpart of a ULX in nearby galaxies. Medium resolution optical spectroscopy carried out in 2008 and 2009 with the ESO-VLT reveals the presence of narrow high order Balmer, He I and Mg II absorption lines indicating a late B-type supergiant companion star with mass between 10 and 20M(circle dot). Stellar H beta and He II lambda 4686 angstrom emission lines are also seen superposed on the photospheric spectrum. We detect different patterns of radial velocity variations from the emission and absorption lines over a time interval of one month. The velocity of the high order Balmer absorption lines changes by similar to 100 km s(-1) while the H beta and He II lambda 4686 angstrom emission components vary by about the same amount but with a different phasing. Assuming that the observed velocity changes trace the motion of the mass-donor star and of the X-ray source implies a mass of the accreting black hole in the range of 3 to 100M(circle dot) with a most probable value of similar to 10 to 20M(circle dot). We expect an orbital period in the range of 20 to 40 days based on the low density of the supergiant star. P13 is likely in a short-lived, and thus rare high X-ray luminosity evolutionary state associated with the ascension of the donor star onto the supergiant stage. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim