The X-ray source Cygnus X-1 is a black hole candidate located in a binary star system. The X-ray source is believed to occupy a volume smaller than Earth. This size is deduced from
A) rapid flickering in the X-ray brightness of Cygnus X-1.
B) Cygnus X-1's apparent magnitude and distance.
C) Cygnus X-1's luminosity and spectral class.
D) the shortness of Cygnus X-1's orbital period.

Question

Quizplus · Accepted Answer

The rapid flickering in the X-ray brightness of Cygnus X-1 suggests that the emitting region is very small. This is because the time it takes for the brightness to change is related to the size of the emitting region; shorter times imply a smaller size.

The X-Ray Source Cygnus X-1 Is a Black Hole Candidate