NuSTAR and Swift Observations of the Ultraluminous X-Ray Source IC 342 X-1 in 2016: Witnessing Spectral Evolution

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 ≈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 ASCA23 years ago. Thanks to the wide energy coverage and high sensitivity of NuSTAR, we obtained hard X-ray data covering up to ∼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 ≈ 4 keV), optically thick (τ ≈ 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.