Discovery of spectral transitions from two ultraluminous compact X-ray sources in IC 342

Two ASCA observations were made of two ultraluminous compact X-ray sources (ULXs) in the spiral galaxy IC 342. In the 1993 observation, source 2 showed a 0.5-10 keV luminosity of 6 x 10(39) ergs s(-1) (assuming a distance of 4.0 Mpc) and a hard power-law spectrum of photon index similar to1.4. As already reported, source 1 was similar to3 times brighter on that occasion and exhibited a soft spectrum represented by a multicolor disk model with an inner- disk temperature of similar to 1.8 keV. The second observation, made in 2000 February, revealed that source 1 had made a transition into a hard spectral state, while source 2 made a transition into a soft spectral state. The ULXs are therefore inferred to exhibit two distinct spectral states, and they sometimes make transitions between them. These results significantly reinforce the scenario that describes ULXs as mass-accreting black holes.