On `The spectral and temporal properties of an Ultra--Luminous X-ray source in NGC 6946', by Senorita Devi et al

A recent paper (arXiv:0804.3463) claims that the presence of a soft X-ray component in an X-ray source in NGC 6946 provides strong evidence that its black hole mass is definitely $>100\text{msun}$, and more probably $\sim 400\text{msun}$, even if the source is beamed. I show that the data are instead very compatible with a black hole mass of only $5\text{msun}$, radiating isotropically (i.e. no beaming) and a mass transfer rate about 60 times the Eddington value, i.e. $1.25 \times 10^{-5} \msun {\rm yr}^{-1}$. Such a system is very similar to the probable descendant of the Galactic X-ray binary Cygnus X–1 once the supergiant companion evolves to fill its Roche lobe.