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 \nu _ { QPO } = 202.5 _ { -3.8 } ^ { +4.9 } { ~ { } mHz } , a coherence Q \equiv \nu _ { QPO } / \Delta \nu _ { FWHM } \approx 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 \approx 0.7 .
The total integrated power ( rms ) is \approx 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 ( kT _ { in } \sim 0.3 { ~ { } keV } ) and a high energy curvature that can be modeled either by a cut-off power law ( \Gamma \sim 1 ; E _ { cutoff } = 9 { ~ { } keV } ) or as a strongly Comptonized continuum in an optically thick ( \tau \approx 7.3 ) and cool ( kT _ { e } \approx 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 .