Two ASCA observations were made of two ultra-luminous compact X-ray sources ( ULXs ) , Source 1 and Source 2 , in the spiral galaxy IC 342 .
In the 1993 observation , Source 2 showed a 0.5–10 keV luminosity of 6 \times 10 ^ { 39 } ergs s ^ { -1 } ( assuming a distance of 4.0 Mpc ) , and a hard power-law spectrum of photon index \sim 1.4 .
As already reported , Source 1 was \sim 3 times brighter on that occasion , and exhibited a soft spectrum represented by a multi-color disk model of inner-disk temperature \sim 1.8 keV .
The second observation made in February 2000 revealed that Source 1 had made a transition into a hard spectral state , while Source 2 into a soft spectral state .
The ULXs are therefore inferred to exhibit two distinct spectral states , and sometimes make transitions between them .
These results significantly reinforce the scenario which describes ULXs as mass-accreting black holes .