Optical studies of the ultraluminous X-ray source NGC 1313 X-2

NGC 1313 X-2 was among the first ultraluminous X-ray sources discovered, and has been a frequent target of X-ray and optical observations. Using the HSTACS multiband observations, this source is identified with a unique counterpart within an error circle of 0.2 ''. The counterpart is a blue star on the edge of a young cluster of <= 10(7) yr amid a dominant old stellar population. Its spectral energy distribution is consistent with that for a Z = 0.004 star with 8.5 M-circle dot about 5 x 10(6) yr old, or for an O7 V star at solar metallicity. The counterpart exhibited significant variability of Delta m = 0.153 +/- 0.033 mag between two F555W observations separated by 3 months, reminiscent of the ellipsoidal variability due to the orbital motion of this ULX binary.