XMM-Newton Observations of Ultraluminous X-Ray Sources in Nearby Galaxies

We examined X-ray spectral and timing properties of ultraluminous X-ray sources (ULXs) in nearby galaxies in XMM-Newton archival data. There appear to be three distinct classes of spectra. One class shows emission from hot, diffuse plasma. This thermal emission is similar to that seen from recent supernovae; the temperatures are in the range 0.6-0.8 keV, and the luminosities are the lowest in our sample, near 1039 ergs s-1. Three sources have spectra that are strongly curved at high energies and have the highest temperatures in our sample, 1.0-1.4 keV. These spectra are well fitted with a power-law plus multicolor disk blackbody model with the power law dominant at low energies or a Comptonization model. The remainder of the sources are best fitted with a power-law plus multicolor disk blackbody model, as is commonly used to describe the spectra of accreting black holes. These sources have the lowest thermal component temperatures, 0.1-0.4 keV, and extend to the highest luminosities, above 1040 ergs s-1. The temperature of the thermal component is in three distinct ranges for the three source classes. This diversity of spectral shapes and the fact that the sources lie in three distinct temperature ranges suggests that the ULXs are a diverse population. Two ULXs that show state transitions stay within a single class over the course of the transition. However, we cannot conclude with certainty that the classes represent distinct types of objects rather than spectral states of a single population of objects. More monitoring observations of ULXs with XMM-Newton are required. We also searched for timing noise from the sources and report detection of noise above the Poisson level from five sources. In three of the sources, the power density spectrum increases with decreasing frequency as a power law down to the lowest frequencies observed, below 10-4 Hz.