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

TL;DR

This paper challenges previous claims by showing that an ultra-luminous X-ray source in NGC 6946 can be explained by a 5 solar mass black hole with super-Eddington accretion, rather than a >100 solar mass black hole.

Contribution

It demonstrates that the observed X-ray properties are compatible with a stellar-mass black hole and super-Eddington accretion, revising the interpretation of the source's mass.

Findings

Data consistent with a 5 solar mass black hole

Accretion rate about 60 times the Eddington limit

Similar to a descendant of Cygnus X-1

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 $>100\msun$, and more probably $\sim 400\msun$, even if the source is beamed. I show that the data are instead very compatible with a black hole mass of only $5\msun$, 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.