North of the microquasar SS 433, an arc has emerged in the soft X-ray band. Faint and diffuse, yet stubbornly tracing contours already familiar from radio observations. Astronomers studied its spectrum and saw gas heated to eleven million kelvin. Gas that could not have been heated by the jets or the accretion disk wind.

This gas is a frozen shock wave. A cosmic snapshot from the past: a shell preserving the moment of a black hole's birth. Twenty to thirty thousand years ago, a massive star exploded as a supernova, and its outer layers are still expanding into space. At the center remains a black hole with a mass of five to fifteen suns, which now pulls in matter from its companion star and ejects narrow plasma jets, the very jets that make SS 433 look like a giant cosmic corkscrew.

Previously, it was unclear whether a black hole in such a system forms via an explosion or through quiet collapse without a flash. Now we have the answer: there was an explosion. This is indicated by the plasma temperature and the silicon abundance, which is two to three times solar. Alternative scenarios, heating by jets or disk wind, yield too low a temperature and luminosity. One explanation remains: a supernova remnant expanding according to the Sedov-Taylor model. The explosion energy is about $10^{51}$ erg. For comparison, that is the energy the Sun radiates over ten billion years.

This discovery links the evolution of the binary system to its explosive past. It explains where the matter for supercritical accretion in young microquasars comes from: some of the material ejected in the explosion falls back onto the black hole. And it provides the first observational point on the "explosion energy versus remnant mass" diagram, a point critically important for calibrating models of black hole formation. Now, looking at the X-ray arc north of SS 433, astronomers see not just hot gas. They see the moment when a star died to give birth to a black hole.