Black Hole Mass of the Ultraluminous X-ray source M82 X-1
Abstract
We report first clear evidence for the simultaneous presence of a low frequency break and a QPO in the power spectrum of a well known ultralumnious 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 114.3\pm1.5mHz, a coherence Q~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 as a power law with the QPO on top of it. 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 of 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 in the range of ~50-260 solar mass for the ULX M82 X-1. An additional uncertainty of about a factor of 2 in the black hole mass is possible due to the uncertainty in the calibration of the photon index and QPO frequency relation.Keywords
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