Two power-law states of the ultraluminous X-ray source IC 342 X-1

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 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 the 0.5-10 keV band: the low-luminosity PL state with (4-6) × 10³⁹ erg s^-1 and the high-luminosity one with (1.1-1.4) × 10⁴⁰ erg s^-1. During 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 × 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, the photon indices are similar between the two PL states, and so are the Compton y-parameters of y ∼ 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 make any conclusion.