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)The X- ray source is a quasar.
C)The X- ray source contains a black hole with an accretion disk.
D)The X- ray source is no more than a few light- days in diameter.

Question

Quizplus · Accepted Answer

Rapid variations in brightness suggest that the source is compact, as changes can propagate through it quickly. A few days of variation implies it's no more than a few light-days across, since signals cannot travel faster than the speed of light.

Suppose We Observe a Source of X- Rays That Varies