The Spectral and Temporal Properties of an Ultraluminous X-Ray Source in NGC 6946

We report variability of the X-ray source, X-7, in NGC 6946, during a 60 ks Chandra observation when the count rate decreased by a factor of ~1.5 in ~5000 s. Spectral fitting of the high and low count rate segments of the light curve reveal that the simplest and most probable interpretation is that the X-ray spectra are due to disk blackbody emission with an absorbing hydrogen column density equal to the Galactic value of 2.1×10²¹ cm^-2. During the variation, the inner disk temperature decreased from ~0.29 to ~0.26 keV while the inner disk radius remained constant at ~6×10⁸ cm. This translates into a luminosity variation from 3.8 to 2.8×10³⁹ ergs cm^-2 s^-1 and a black hole mass of ~400 M_solar. More complicated models like assuming intrinsic absorption and/or the addition of a power-law component imply a higher luminosity and a larger black hole mass. Even if the emission is beamed by a factor of ~5, the size of the emitting region would be >2.7×10⁸ cm, implying a black hole mass >180 M_solar. Thus, these spectral results provide strong evidence that the mass of the black hole in this source is definitely >100 M_solar and more probably ~400 M_solar.