Which Milky Way X-ray source was the first widely accepted black hole?

The Milky Way X-ray source that was the first widely accepted black hole is Cygnus X-1.

Cygnus X-1 is an X-ray binary in the constellation Cygnus. Its compact object pulls gas from the stellar wind of a massive O-type supergiant companion, HDE 226868. As the gas falls inward, it forms extremely hot, rapidly changing material that emits powerful X-rays.

The source was discovered during sounding-rocket observations in 1964. In 1972, independent spectroscopic studies revealed that HDE 226868 had a massive unseen companion. The companion’s inferred mass exceeded the plausible limit for a neutron star, making a black hole the strongest explanation. By the end of 1973, the astronomical community generally accepted that interpretation.

Cygnus X-1 later became famous for a 1970s wager between Stephen Hawking and Kip Thorne. Hawking bet against the black-hole interpretation but conceded in 1990 as evidence strengthened. The system is an X-ray binary and microquasar, not a supermassive black hole like Sagittarius A* at the Milky Way’s center.