The best case for a stellar black hole.
Observations at X-ray and optical wavelengths
have shown that this is a binary system in which a late G or early K star
revolves, every 6.47 days, around a compact companion with a probable mass
of 8 to 15 solar masses, well above the mass limit at which a collapsed
star must become a black hole. Most of the time the X-ray emission is quiescent,
indicating only a slow trickle of material from the bright star onto the
black hole's accretion disk.