Gaia BH1

Black holes are normally found when they pull matter inward at an accelerated rate which beams out radiation, like the one in Cygnus. That "first Cygnus X-ray source" X-1 is draining its orbiting star. If the companion is far enough away, then it won't be drained; we'd have to look for that (visible) companion orbiting something invisible... which might take years for us to notice, given Kepler. It looks like we've noticed one, 1600 light years away: 4373465352415301632. h/t Science Daily and this release UPDATE 7:20 PM MST and Bad Astronomy and Nyrath.

We're lucky that Gaia found this, its first black hole; score another W for the DR3. Incredibly lucky, some might say...

The 185.6-day period is such that the orbiting star, which is a G class 1 M_☉ like our sun, is estimated at 1 AU from an 11-sol-mass barycentre. The just-under-10 M_☉ black hole, when it was an O star, should have been 20 M_☉. So how did it die without going supergiant in between? Our own dwarf Sun is supposed to puff out to about 1 AU!

I don't think the G star is a capture, since its orbit would be madly eccentric if so (and they'd tell us). Maybe the dying star puffed out most of its hydrogen before going supergiant, such that it only got to, oh, 0.5 AU radius. Maybe the central hole wasn't from one star but from two very heavy stars which merged - a neutron-star devouring a companion whose mass was just enough to erect an event-horizon, but quietly. Although there too I'd have to ask how the G star got here in a circular orbit.

The neutron-star scenario opens a hypothesis: secondary formation. The black hole after implosion left behind it a lot of gas. Neutron-stars (like classic black holes) always form from young stars so still own primordial plasma. This plasma might, post-implosion, even contain the ripped-up remnants of the companion. The debris, then, whirled around the black hole in an accretion-disc perhaps pushed to 1 AU by the gravity-waves and gamma-rays of said implosion. As normal systems form planets like Jupiter, this system - so much more massive - formed a star like our Sun. Computer-modeling might tell us if this is possible.