# NuSTAR and Swift observations of the ultraluminous X-ray source IC 342 X-1 in 2016: witnessing spectral evolution

## TL;DR

This study captures spectral evolution of the ultraluminous X-ray source IC 342 X-1 during a 2016 observing campaign, revealing changes in spectral shape and flux linked to accretion rate variations, supported by broad energy coverage data.

## Contribution

First simultaneous NuSTAR and Swift observations of IC 342 X-1 during spectral variation, providing insights into super-Eddington accretion flow behavior.

## Key findings

- Spectral shape varies with luminosity, shifting the spectral turnover to higher energies at lower flux.
- Spectra are consistent with disk emission plus Comptonization in cool, optically-thick plasma.
- Spectral evolution aligns with numerical simulations of super-Eddington accretion flows.

## Abstract

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_{\rm 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.