Black hole mass of the ultraluminous X-ray source M82 X-1

We report the first clear evidence for the simultaneous presence of a low-frequency break and a QPO in the fluctuation power spectrum of a well-known ultraluminous X-ray source (ULX) in M82 using long XMM-Newton observations. The break occurs at a frequency of 34.2(-3)(+6) m Hz. The QPO has a centroid at nu(QPO) = 114.3 +/- 1.5 m Hz, a coherence Q equivalent to nu(QPO/Delta FWHM) similar or equal to 3.5, and an amplitude (rms) of 19% in the 2-10 keV band. The power spectrum is approximately flat below the break frequency and then falls off above the break frequency as a power law with the QPO superposed. This form of the power spectrum is characteristic of the Galactic X-ray binaries (XRBs) in their high or intermediate states. M82 X-1 was likely in an intermediate state during the observation. The EPIC pn spectrum is well described by a model comprising an absorbed power law (Gamma similar to 2) and an iron line at similar to 6.6 keV with a width and an equivalent width of similar to 180 eV. Using the well-established correlations sigma similar to 0.2 keV between the power and energy spectral parameters for XRBs, we estimate a black hole mass for M82 X-1 in the range of similar to 25-520 M-circle dot, including systematic errors that arise due to the uncertainty in the calibration of the photon spectral index versus QPO frequency relation.