What was the first X-ray source found in the constellation Cygnus?

• A. Cygnus X-1
• B. Cygnus X-2
• C. Cygnus X-3
• D. Cygnus X-4

Answer: (A)

X-ray sources in a constellation are labeled with the constellation name and a number that grows as new sources are discovered there. The earliest X-ray source detected in the constellation Cygnus is Cygnus X-1. It was found in 1964 by a rocket-borne X-ray detector and stood out as a bright, persistent source in that region of the sky. Because it was the first X-ray source identified in Cygnus, it earned the designation X-1. The other Cygnus X sources were discovered later and given higher numbers, so they are not the first. Cygnus X-1 is also famous for being a strong black hole candidate, highlighted by its X-ray behavior and the mass inferred from its optical companion.