The ultraluminous X-ray source NGC 1313 X-2 (MS 0317.7-6647) and its environment

We present new optical and Chandra observations of the field containing the ultraluminous X-ray source NGC 1313 X-2. On an ESO 3.6 m image, the Chandra error box embraces an R=21.6 pointlike object and excludes a previously proposed optical counterpart. The resulting X-ray/optical flux ratio of NGC 1313 X-2 is ~500. The value of fX/fopt, the X-ray variability history, and the spectral distribution derived from a reanalysis of the ROSAT, ASCA, and XMM-Newton data indicate a luminous X-ray binary in NGC 1313 as a likely explanation for NGC 1313 X-2. If the X-ray soft component observed in the XMM-Newton EPIC spectrum originates from an accretion disk, the inferred mass of the compact remnant is ~100 Msolar, making it an intermediate-mass black hole. The derived optical luminosity (L~105Lsolar) is consistent with that of a ~15-20 Msolar companion. The properties of the environment of NGC 1313 X-2 are briefly discussed.