Abstract

We present results from a study of short term variability in 19 archival observations by XMM -Newton of 16 Ultraluminous X-ray Sources (ULXs). Eight observations (six sources) showed intrinsic variability with power spectra in the form of either a power law or broken power law-like continuum and in some cases quasi-periodic oscillations (QPOs) seen in previous studies of these observations. Seven observations (seven sources) yielded upper limits comparable to, or higher than, the values measured from those observations with detectable variations. These represented the seven faintest sources all with f_x < 3×10^-12 erg cm^-2 ~ s^-1.In contrast there are four observations (three sources) that gave upper limits significantly lower than both the values measured from the ULX observations with detectable variations, and the values expected by comparison with luminous Galactic black hole X-ray binaries (BHBs) and Active Galactic Nuclei (AGN). The lack of detectable variability from these bright sources, in the observed frequency bandpass (10^-3 – 1 Hz), is at odds with the expectations based on observations of Galactic BHBs and AGN . We present an analysis of these results in terms of the fluxes and X-ray luminosities from the observations, and discuss solutions for suppression of variability from these sources.