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We investigate the energetics of the jet and X‐ray corona of Cyg X‐1. We show that the current estimates of the jet power obtained from Hα and [O  iii ] measurements of the optical nebula surrounding the X‐ray source allow one to constrain the bulk velocity of the jet. It is definitely relativistic  ( v  &gt; 0.1 c )  and most probably in the range (0.3–0.8) c . The exact value of the velocity depends on the accretion efficiency. These constraints are obtained independently of, and are consistent with, previous estimates of the jet bulk velocity based on radio measurements. We then show that the X‐ray emission does not originate in the jet. Indeed, the energy budget does not allow the corona to be ejected to infinity at relativistic speed. Rather, either a small fraction of the corona escapes to infinity or the ejection velocity of the corona is vanishingly low. Although the corona could constitute the jet launching region, it cannot be identified with the jet itself. We discuss the consequences for various X‐ray emission models.

Energetics of a black hole: constraints on the jet velocity and the nature of the X‐ray emitting region in Cyg X‐1