XMM-Newton view of the ultra-luminous X-ray sources in M51
  (Abridged) We present results based on XMM-Newton observation of the nearby spiral galaxy M51 (NGC5194 and NGC5195). Two ULXs in NGC5194 show evidence for short-term variability, and all but two ULXs vary on long time scales (over a baseline of 2.5 years), providing strong evidence that these are accreting sources. One ULX in NGC5194, source 69, shows possible periodic behavior in its X-ray flux. We derive a period of 5925\pm200s at a confidence level of 95%, based on three cycles. This period is lower than the period of 7620\pm500s derived from a Chandra observation in 2000. The higher effective area of XMM-Newton enables us to identify multiple components in the spectra of ULXs. Most ULXs require at least two components -- a power law and a soft X-ray excess component which is modeled by an optically thin plasma or multicolor disk blackbody (MCD). However, the soft excess emission, inferred from all ULXs except source 69, are unlikely to be physically associated with the ULXs as their strengths are comparable to that of the surrounding diffuse emission. The soft excess emission of source 69 is well described either by a two temperature mekal plasma or a single temperature mekal plasma kT~690eV) and an MCD (kT~170eV). The MCD component suggests a cooler accretion disks compared to that in Galactic X-ray binaries and consistent with that expected for intermediate mass black holes (IMBHs). An iron line (EW 700eV) or K absorption edge at 7.1keV is present in the EPIC PN spectrum of source 26. The spectrum of the ULX in NGC5195, source 12, is consistent with a simple power law. 
Gulab C. Dewangan (CMU), Richard E. Griffiths (CMU), Manojendu   Choudhury (NCRA), Takamitsu Miyaji (CMU), & Nicholas J. Sch...