Discovery of X-Ray Quasi-periodic Oscillations from an Ultraluminous X-Ray Source in M82: Evidence against Beaming

We report the discovery with the European Photon Imaging Camera CCDs on board XMM-Newton of a 54 mHz quasi-periodic oscillation (QPO) in the greater than 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 extragalactic ULX and confirms that the source is a compact object. On the basis of 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, CXO M82 J095550.2+694047; however, XMM imaging alone is not sufficient to unambiguously confirm this. The other plausible candidate is CXO M82 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+/-0.9 mHz, a coherence Q≡ν₀/Δν_fwhm~5, and an amplitude (rms) in the 2-10 keV band of 8.5%. Below 0.2 Hz, the power spectrum can be fitted by a power law with index ~1 and amplitude (rms) of 13.5%. The X-ray spectrum requires a curving continuum, with a disk blackbody 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 is sensitive to the continuum model. There is no evidence of a reflection component. The implied bolometric luminosity is ~(4-5)×10⁴⁰ ergs s^-1. Archival Rossi X-Ray Timing Explorer 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.