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In order to elucidate the emission properties of ultraluminous X-ray sources(ULXs) during their power-law (PL) state, we examined long-term X-ray spectraldata of IC342 X-1 during its PL state by using our own Suzaku data and thearchival data by XMM-Newton, Chandra, and Swift observations. The PL state ofthis source seems to be classified into two sub-states in terms of the X-rayluminosities in 0.5-10 keV: the low luminosity PL state with 4-6*10^{39} erg/sand the high luminosity one with 1.1-1.4*10^{40} erg/s. During the Suzakuobservations which were made in 2010 August and 2011 March, X-1 stayed in thelow luminosity PL state. The observed X-ray luminosity (4.9-5.6*10^{39} erg/s)and the spectral shape (photon index = 1.67-1.83) slightly changed between thetwo observations. Using the Suzaku PIN detector, we for the first timeconfirmed a PL tail extending up to at least 20 keV with no signatures of ahigh-energy turnover in both of the Suzaku observations. In contrast, aturnover at about 6 keV was observed during the high luminosity PL state in2004 and 2005 with XMM-Newton. Importantly, photon indices are similar betweenthe two PL states and so is the Compton y-parameters of y ~ 1, which indicatesa similar energy balance (between the corona and the accretion disk) holding inthe two PL states despite different electron temperatures. From spectralsimilarities with recent studies about other ULXs and the Galactic black holebinary GRS1915+105, IC342 X-1 is also likely to be in a state with asupercritical accretion rate, although more sensitive higher energyobservations would be necessary to conclude.

Two Power-Law States of the Ultraluminous X-ray Source IC342 X-1