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Using two archival XMM-Newton observations, we identify strong X-ray fluxvariations in NGC 6946 X-1 indicating it is the most variable ultraluminousX-ray source (ULX) on mHz time scales known so far. The 1-10 keV lightcurveexhibits variability with a fractional rms amplitude of 60% integrated in thefrequency range of 1-100 mHz. The power spectral density of the source shows aflat-topped spectrum that breaks at about 3 mHz with possible quasi-periodicoscillations (QPOs) near 8.5 mHz. Black hole binaries usually produce strongfast variability in the hard or intermediate state. The energy spectrum of NGC6946 X-1 is dominated by two components, a 0.18~keV thermal disk and a powerlaw with a photon index of ~2.2, which is consistent with the intermediatestate. The characteristic time scales of the X-ray emission suggests that theULX may contain a black hole with a mass on the order of 10^3.

Detection of Strong Short-Term Variability in NGC 6946 X-1