Black Hole Mass of the Ultraluminous X-ray Source M82 X–1 Accepted for Publication in Apj Letters

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 mHz. The QPO has a centroid at νQPO = 114.3±1.5 mHz, a coherence Q ≡ νQPO/∆νFWHM ≃ 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 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 (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 M 82 X-1 in the range of ∼ 25 − 520M⊙ including systematic errors that arise due to the uncertainty in the calibration of the photon spectral index versus QPO frequency relation. Subject headings: accretion, accretion disks — stars: individual (M82 X-1) — X-rays: stars