Detection of strong short-term variability in NGC 6946 X-1

Abstract

Using two archival XMM-Newton observations,we identify strong X-ray flux variations in NGC6946 X-1 indicating that it is the most variable ultraluminous X-ray source (ULX) on mHz timescales known so far. The 1-10 keV light curve exhibits variability with a fractional rms amplitude of 60% integrated in the frequency range of 1-100 mHz. The power spectral density of the source shows a flat-topped spectrum that breaks at about 3mHzwith possible quasiperiodic oscillations near 8.5 mHz. Black hole binaries usually produce strong fast variability in the hard or intermediate state. The energy spectrum of NGC 6946 X-1 is dominated by two components, a 0.18 keV thermal disk and a power lawwith a photon index of≈2.2, which is consistent with the intermediate state. The characteristic timescales of the X-ray emission suggest that the ULX may contain a black hole with a mass on the order of 103 solar masses. © 2010. The American Astronomical Society. All rights reserved.