2006ApJ...641L.125D

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 ν_QPO=202.5^+4.9_–3.8mHz, a coherence Q≡ν_QPO/Δν_FWHM~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 ~0.7. The total integrated power (rms) is ~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∼0.3 keV) and a high-energy curvature that can be modeled either by a cutoff power law (Γ∼1; E_cutoff=9 keV) or as a strongly Comptonized continuum in an optically thick (τ~7.3) and cool (kT_e~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.