The Spectral and Temporal Properties of an Ultra-luminous X-ray Source in Ngc

We report variability of the X-ray source, X-7, in NGC 6946, during a 60 ksec Chandra observation when the count rate decreased by a factor of ∼ 1.5 in ∼ 5000 secs. 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 black body emission with an absorbing hydrogen column density equal to the Galactic value of 2.1 × 10 cm. During the variation, the inner disk temperature decreased from ∼ 0.29 to ∼ 0.26 keV while the inner disk radius remained constant at ∼ 6× 10 cm. This translates into a luminosity variation from 3.8 to 2.8 × 10 ergs cmsec and a black hole mass of ∼ 400M⊙. 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 > 2.7×10 cm implying a black hole mass > 180M⊙. Thus, these spectral results provide strong evidence that the mass of the black hole in this source is definitely > 100M⊙ and more probably ∼ 400M⊙. Subject headings: accretion, accretion disks — galaxies: individual (NGC6946) — X-rays: binaries