Final answer:
Observations of Hawking radiation is not an argument for Cygnus X-1 being a true black hole since this phenomenon has not been observed due to its very weak signal.
Step-by-step explanation:
Among the options provided in the question, the one that is NOT an argument for Cygnus X-1 being a true black hole is observations of Hawking radiation. While gravitational pull on a companion star implies a very dense, invisible mass; X-ray emissions characteristic of a black hole suggests accretion phenomena; and lack of visible light from Cygnus X-1 indicates an object that does not emit or reflect light, it is important to note that Hawking radiation, a theoretical prediction of black holes emitting radiation due to quantum effects, has not yet been observed in practice due to its expectedly weak signal.
Therefore, the fourth choice does not currently serve as evidence for Cygnus X-1 being a black hole. This is because the technology required to observe such faint emissions has not been developed to the point where we can detect Hawking radiation from astrophysical black holes like Cygnus X-1.