On `The spectral and temporal properties of an Ultra--Luminous X-ray source in NGC 6946', by Senorita Devi et al
Abstract
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, and more probably 400, even if the source is beamed. I show that the data are instead very compatible with a black hole mass of only 5, radiating isotropically (i.e. no beaming) and a mass transfer rate about 60 times the Eddington value, i.e. 1.25 × 10-5 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.
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