Black Hole Mass of the Ultraluminous X-Ray Source M82 X-1

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Abstract
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}mHz. The QPO has a centroid at 114.3pm1.5mHz, a coherence Q~3.5 and an amplitude (rms) of 19% in the 2-10keV 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 superimposed. 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~2) and an iron line at ~6.6keV with a width sigma~0.2keV and an equivalent width of ~180eV. 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 25-520Msun including systematic errors that arise due to the uncertainty in the calibration of the photon spectral index versus QPO frequency relation.Comment: Final version, accepted for publication in ApJ Letter