Optical Studies of the Ultraluminous X-Ray Source NGC 1313 X-2

Abstract

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 HST ACS 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 <=10⁷ 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_solar about 5×10⁶ yr old, or for an O7 V star at solar metallicity. The counterpart exhibited significant variability of Δm=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.