The Ultraluminous X-ray Source IC342 X-1 and its Environment

Abstract

We present optical observations of a ULX in the nearby spiral galaxy IC 342. This variable source has an average X-ray luminosity of some 10^{40} erg/s. At the position of the source there is an ionized nebula (the "Tooth") having huge dimensions (280 x 130 pc), much larger than normal supernova remnants. Our optical spectra reveal highly supersonic expansion velocities and emission line ratios typical of SNRs. It has been claimed that two [OIII] 5007 bright regions in the nebula might be indicative of excitation by non-isotropic emission from the ULX. However, our continuum subtracted [OIII] 5007 image reveals that O^{++} ions are rather smoothly distributed in the nebula, fully consistent with shock excitation. Within the nebula we find two candidate stars (V=24.0 & 24.6) in the Chandra X-ray error circle. Both are substantially reddened being consistent with the patchy interstellar absorption. We will discuss the nature of this source in the framework of what is currently known about optical counterparts of ULXs.