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 spectral data of IC 342 X-1 during its PL state by using our own Suzaku data and the archival data by XMM-Newton , Chandra , and Swift observations .
The PL state of this source seems to be classified into two sub-states in terms of the X-ray luminosities in 0.5–10 keV : the low luminosity PL state with ( 4–6 ) \times 10 ^ { 39 } erg s ^ { -1 } and the high luminosity one with ( 1.1–1.4 ) \times 10 ^ { 40 } erg s ^ { -1 } .
During the Suzaku observations which were made in 2010 August and 2011 March , X-1 stayed in the low luminosity PL state .
The observed X-ray luminosity ( 4.9–5.6 \times 10 ^ { 39 } erg s ^ { -1 } ) and the spectral shape ( photon index = 1.67–1.83 ) slightly changed between the two observations .
Using the Suzaku PIN detector , we for the first time confirmed a PL tail extending up to at least 20 keV with no signatures of a high-energy turnover in both of the Suzaku observations .
In contrast , a turnover at about 6 keV was observed during the high luminosity PL state in 2004 and 2005 with XMM-Newton .
Importantly , photon indices are similar between the two PL states and so is the Compton y -parameters of y \sim 1 , which indicates a similar energy balance ( between the corona and the accretion disk ) holding in the two PL states despite different electron temperatures .
From spectral similarities with recent studies about other ULXs and the Galactic black hole binary GRS 1915 + 105 , IC 342 X-1 is also likely to be in a state with a supercritical accretion rate , although more sensitive higher energy observations would be necessary to conclude .