Suppose we observe a source of X- rays that varies substantially in brightness over a period of a few days. What can we conclude?
A)The X- ray source must have a strong, rapidly varying magnetic field.
B)We must be seeing the rapid orbit of two stars in a binary system.
C)The X- ray source must be a quasar.
D)The X- ray source must contain a black hole with an accretion disk.
E)The X- ray source can be no more than a few light- days in diameter.

Question

Quizplus · Accepted Answer

Rapid variations in brightness imply that the source is relatively small, as changes can propagate through it quickly. The maximum size is constrained by the time it takes light to travel across it, hence a few light-days in diameter.

Suppose We Observe a Source of X- Rays That Varies