VLT Observations of the Ultraluminous X-Ray Source NGC 1313 X-2

We present archive ESO VLT photometric and spectroscopic data of the ultraluminous X-ray source NGC 1313 X-2. The superb quality of the VLT images reveals that two distinct objects, with R magnitudes 23.7 and 23.6, are visible inside the Chandra error box. The two objects, separated by 0…75, were unresolved in our previous ESO 3.6 m+EFOSC image. We show that both are stars in NGC 1313, the first a B0-O9 main-sequence star of ~20 M(circle dot operator), the second a G supergiant of ~10 M(circle dot operator) . Irrespective of which of the two objects the actual counterpart is, this implies that NGC 1313 X-2 is a high-mass X-ray binary with a very massive donor.