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 ultra-luminous X-ray source (ULX) Holmberg IX X-1 using a long XMM-Newton observation. The QPO has a centroid at νQPO = 202.5 +4.9 −3.8 mHz, a coherence Q ≡ νQPO/∆νFWHM ≈ 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 an ULX, after M 82 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 for a soft X-ray excess component that is well described by a multicolor disk blackbody (kTin ∼ 0.3 keV) and a high energy curvature that can be modeled either by a cut-off power law (Γ ∼ 1; Ecutoff = 9 keV) or as a strongly Comptonized continuum in an optically thick (τ ≈ 7.3) and cool (kTe ≈ 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. Subject headings: accretion, accretion disks — stars: individual (Holmberg IX X-1; M81 X-9) — X-rays: stars