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

NGC1313 X-2 was among the first ultraluminous X-ray sources discovered, andhas been a frequent target of X-ray and optical observations. Using the HST/ACSmulti-band observations, this source is identified with a unique counterpartwithin an error circle of $0\farcs2$. The counterpart is a blue star on theedge of a young cluster of $\le10^7$ years amid a dominant old stellarpopulation. Its spectral energy distribution is consistent with that for aZ=0.004 star with 8.5 $M_\odot$ about $5\times10^6$ years old, or for an O7 Vstar at solar metallicity. The counterpart exhibited significant variability of$\Delta m = 0.153\pm0.033$ mag between two F555W observations separated bythree months, reminiscent of the ellipsoidal variability due to the orbitalmotion of this ULX binary.Comment: 21 pages, 7 figures, scheduled for the ApJ June 10, 2007, v662n 1 issu