We report on an X-ray observing campaign of the ultraluminous X-ray source IC 342 X-1 with NuSTAR and Swift in 2016 October , in which we captured the very moment when the source showed spectral variation .
The Swift/XRT spectrum obtained in October 9–11 has a power-law shape and is consistent with those observed in the coordinated XMM-Newton and NuSTAR observations in 2012 .
In October 16–17 , when the 3–10 keV flux became \approx 4 times higher , we performed simultaneous NuSTAR and Swift observations .
In this epoch , the source showed a more round-shaped spectrum like that seen with ASCA 23 years ago .
Thanks to the wide energy coverage and high sensitivity of NuSTAR , we obtained hard X-ray data covering up to \sim 30 keV for the first time during the high luminosity state of IC 342 X-1 .
The observed spectrum has a broader profile than the multi-color disk blackbody model .
The X-ray flux decreased again in the last several hours of the NuSTAR observation , when the spectral shape approached those seen in 2012 and 2016 October 9–11 .
The spectra obtained in our observations and in 2012 can be commonly described with disk emission and its Comptonization in cool ( T _ { e } \approx 4 keV ) , optically-thick ( \tau \approx 5 ) plasma .
The spectral turnover seen at around 5–10 keV shifts to higher energies as the X-ray luminosity decreases .
This behavior is consistent with that predicted from recent numerical simulations of super-Eddington accretion flows with Compton-thick outflows .
We suggest that the spectral evolution observed in IC 342 X-1 can be explained by a smooth change in mass accretion rate .