Optical Counterpart of the Ultraluminous X-ray Source Ic 342 X-1

We present Chandra and HST observations of the ultraluminous X-ray source (ULX) IC 342 X-1. The Chandra and HST images are aligned using two X-ray emitting foreground stars. The astrometry corrected position for X-1 is R.A. = 03 h 45 m 55. s 61, Decl. = +68 • 04 ′ 55. ′′ 3 (J2000) with an error circle of 0.2 ′′. One extended optical source is found in the error circle, which could be the optical counterpart of X-1. The source shows an extended feature in HST images at long wavelengths, which is likely to be a superposition of two point sources, although it is possible that the dimmer one could be a jet. Both sources are much redder than typical for ULX optical counterparts. The brighter one has an absolute magnitude M V = −5.2 ± 0.2 and (B − V) 0 = 0.66 ± 0.13 and the dimmer star is not detected in B and has (B − V) 0 > 2.1. Their colors are consistent with an F8 to G0 Ib supergiant or a carbon star, respectively. However, it is likely that part or most of the optical emission may be due to X-rays reprocessed by the companion star or the accretion disk. The stellar neighborhood of IC 342 X-1 lacks O stars and has a minimum age of ∼ 10 Myr. This excludes the possibility that the surrounding nebula is powered by an energetic explosion of a single massive star that formed a black hole. We suggest that the nebula is most likely powered by an outflow from the X-ray source. 1. INTRODUCTION Ultraluminous X-ray sources, the brightest nonnu-clear X-ray point sources in external galaxies, still hide their nature from us. Their luminosity of 3 × 10 39 to 10 41 ergs s −1 , inferred assuming isotropic emission , is too high to be powered by accretion onto stellar mass black holes that are formed in the collapse of normal stars with normal metallicity, and thus may suggest the presence of intermediate mass black holes (IMBHs; Colbert & Mushotzky 1999; Kaaret et al. 2001). However, if the emission is beamed (King et al. 2001; Körding et al. 2002) or truly super-Eddington (Watarai et al. 2001; Begelman 2002), IMBHs are not required. Multiwavelength observations have provided powerful means to determine the nature of ULXs. Pakull & Mirioni (2003) carried out an optical survey of bright ULXs …