A Possible X-Ray Periodicity at Several Tens of Hours of an Ultraluminous Compact X-Ray Source in IC 342

A long (155 hr) ASCA observation was performed of two ultraluminous compact X-ray sources, source 1 and source 2, in the spiral galaxy IC 342. While source 1, which was in a hard spectral state, varied little, source 2, which was in its soft spectral state, varied significantly on a timescale of about 1 day. The rms variation amplitude amounts to 5% in the 2-10 keV band but is less than 4% in 0.7-2 keV. The variation involves statistically significant changes in the parameters describing multicolor disk blackbody emission from this source. The variation is possibly periodic, with a period of either 31 ± 2 or 41 ± 3 hr. Both are consistent with the orbital period of a semidetached binary formed by a black hole and a main-sequence star of several tens of solar masses. These results reinforce the interpretation of these X-ray objects in terms of accreting massive stellar black holes.