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⁺⁶_-3 mHz. The QPO has a centroid at ν_QPO=114.3+/-1.5 mHz, a coherence Q≡ν_QPO/Δν_FWHM~=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 (Γ~2) and an iron line at ~6.6 keV with a width σ~0.2 keV and an equivalent width of ~180 eV. Using the well-established correlations between the power and energy spectral parameters for XRBs, we estimate a black hole mass for M82 X-1 in the range of ~25-520 M_solar, including systematic errors that arise due to the uncertainty in the calibration of the photon spectral index versus QPO frequency relation.