The X-Ray Properties of the Nearby Star-forming Galaxy IC 342: The XMM-Newton View

We present results on the X-ray properties of IC 342 using a 10 ks XMM-Newton observation. Thirty-five sources are detected coincident with the disk of IC 342 (more than tripling the number known), of which ~31 are likely to be intrinsic to IC 342. This point-source population exhibits a diverse range of spectral properties and has an X-ray luminosity function slope and infrared luminosity comparable to that of starburst galaxies such as M82 and the Antennae, while its relative lack of extended X-ray emission is similar to the properties of quiescent spirals. Although we find no evidence of short-term variability in any of the X-ray sources, we do detect long-term variability between this observation and the 1991 ROSAT and 1993/2000 ASCA observations for five sources. Notably, the second most luminous source in IC 342 (X-2) is found to have an absorption-corrected 0.5-10 keV luminosity of 5.75×10³⁹ ergs s^-1 and a spectrum best fitted by an absorbed multicolor accretion disk model with T_in=2.17^+0.25_-0.12 keV and N_H=15.3^+1.9_-1.8×10²¹ cm^-2. This is the lowest luminosity state observed for X-2 to date, although the slope of the spectrum is intermediate between the previously observed low/hard and high/soft states. IC 342 X-1, on the other hand, is found to be in an identical state to that observed in 2000 with ASCA. Assuming X-1 is in an anomalous very high (VH) state, then either (1) X-1 has remained in this state between 2000 and 2002 and is therefore the longest duration VH-state binary ever observed, or (2) it was simply caught in a VH state by chance in both the 2000 ASCA and 2002 XMM-Newton observations. We have also confirmed the ROSAT HRI result that the nucleus of IC 342 is made up of both pointlike and extended emission, with the extended emission contributing ~55% and ~35% in the 0.3-2.0 keV and 2.0-10.0 keV bands, respectively. The spectrum of the nucleus is best fitted by an absorbed two-component model consisting of a thermal plasma with a temperature of kT~0.3 and a power law with a photon index of Γ~2.53. The relative fluxes of the two spectral components suggest that the nucleus is complex, with a soft extended component contributing approximately half of the total luminosity.