Quasi-periodic oscillations and strongly comptonized X-ray emission from Holmberg IX X-1

We report the discovery of a 200 mHz quasi-periodic oscillation (QPO) in the X-ray emission from a bright ultraluminous X-ray source (ULX), Holmberg IX X-1, using a long XMM-Newton observation. The QPO has a centroid at v(QPO) = 202.5(-3.8)(+4.9) mHz, a coherence Q v(QPO)/Delta v(FWHM) approximate to 9.3, and an amplitude (rms) of 6% in the 0.2-10 keV band. This is only the second detection of a QPO from a ULX, after M82 X-1, and provides strong evidence against beaming. The power spectrum is well fitted by a power law with an index of approximate to 0.7. The total integrated power (rms) is approximate to 9.4% in the 0.001-1 Hz range. The X-ray spectrum shows clear evidence of a soft X-ray excess component that is well described by a multicolor disk blackbody (kT(in) similar to 0.3 keV) and a high-energy curvature that can be modeled either by a cutoff power law (Gamma similar to 1; E-cutoff = 9 keV) or as a strongly Comptonized continuum in an optically thick (tau approximate to 7.3) and cool (kT(e) approximate to 3 keV) plasma. Both the presence of the QPO and the shape of the X-ray spectrum strongly suggest that the ULX is not in the high/soft or thermally dominated state. A truncated disk and inner optically thick corona may explain the observed X-ray spectrum and the presence of the QPO.