We report the discovery with the EPIC CCD cameras onboard XMM-Newton of a 54 mHz quasiperiodic oscillation ( QPO ) in the > 2 keV X-ray flux from an ultraluminous X-ray source ( ULX ) in the starburst galaxy M82 .
This is the first detection of a QPO in the X-ray flux from an extra-Galactic ULX , and confirms that the source is a compact object .
Based on the QPO strength and previous Chandra observations it appears likely that the QPO is associated with the most luminous object in the central region of M82 , CXOM82 J095550.2+694047 , however , XMM imaging alone is not sufficient to unambiguously confirm this .
The other plausible candidate is CXOM82 J095551.1+694045 , however , the QPO luminosity is comparable to the peak luminosity of this object in Chandra observations , which argues against it being the source of the QPO .
The QPO had a centroid frequency of 54.3 \pm 0.9 mHz , a coherence Q \equiv \nu _ { 0 } / \Delta \nu _ { fwhm } \approx 5 , and an amplitude ( rms ) in the 2 - 10 keV band of 8.5 \% .
Below 0.2 Hz the power spectrum can be described by a power-law with index \approx 1 , and amplitude ( rms ) of 13.5 \% .
The X-ray spectrum requires a curving continuum , with a disk-blackbody ( diskbb ) at T = 3.1 keV providing an acceptable fit .
A broad Fe line centered at 6.55 keV is required in all fits , but the equivalent width ( EW ) is sensitive to the continuum model .
There is no evidence of a reflection component .
The implied bolometric luminosity is \approx 4 - 5 \times 10 ^ { 40 } ergs s ^ { -1 } .
Archival Rossi X-ray Timing Explorer ( RXTE ) pointings at M82 also show evidence for QPOs in the 50 - 100 mHz frequency range .
We discuss the implications of our findings for models of ULXs .