The X-ray properties of the nearby star-forming galaxy IC 342: The XMM-Newton view

摘要

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 approximate to31 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 5.75x10(39) ergs s(-1) and a spectrum best fitted by an absorbed multicolor accretion disk model with T-in=2.17(-0.12)(+0.25) keV and N-H=15.3(-1.8)(+1.9)x10(21) 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 approximate to55 and approximate to35 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 kTapproximate to0.3 and a power law with a photon index of Gammaapproximate to2.53. 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. References: 34