Cygnus X-1

I first read 'bout Cygnus X-1 in the earliest 1980s. It was the poster-entity for Black Holes. I didn't know at the time but apparently Stephen Hawking, the poster-boy for 'hole research, didn't approve X-1 over the whole 1980s. (He surrendered.)

So James Miller-Jones sicced his team and the VLArray on this monster, between May 29 and June 3, 2016. Almost five years later their results are in. Where Betelgeuse is closer to us, so dimmer; X-1 has proven out further, and stronger. Twenty-one solar masses!

X-1 was found in the first place because it was orbiting, and sucking protons off, a supergiant, sixty sols. Now this team has constrained some Kepler: 0.2 AU semimajor from barycentre, 5.5 day period. Pretty epic. Much mutually-closer than the Sol-Mercury.

Wonder if that system's supernova was visible to our ancestors. Wonder when the still-shining companion will follow it.

Re that Cygnus X-1 is spinning incredibly quickly—very close to the speed of light - that cannot mean orbit. For that you go to "Cygnus X-1 contains a 21-solar mass black hole – implications for massive star winds" (pdf). That spin parameter 0.97+ (1 is c) is inferred from spectral fitting of the X-ray continuum. I actually didn't know you could find black hole spin (which is oblateness) until it merged with some other high-mass entity. Good to find out.