Discovery of Spectral Transitions from Two Ultra-Luminous Compact X-Ray Sources in Ic342

Abstract

Two {\it ASCA} observations were made of two ultra-luminous compact X-ray sources (ULXs), Source 1 and Source 2, in the spiral galaxy IC 342. In the 1993 observation, Source 2 showed a 0.5--10 keV luminosity of $6 \times 10^{39}$ ergs s$^{-1}$ (assuming a distance of 4.0 Mpc), and a hard power-law spectrum of photon index $\sim 1.4$. As already reported, Source 1 was $\sim 3$ times brighter on that occasion, and exhibited a soft spectrum represented by a multi-color disk model of inner-disk temperature $ \sim 1.8$ keV. The second observation made in February 2000 revealed that Source 1 had made a transition into a hard spectral state, while Source 2 into a soft spectral state. The ULXs are therefore inferred to exhibit two distinct spectral states, and sometimes make transitions between them. These results significantly reinforce the scenario which describes ULXs as mass-accreting black holes.