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 is a quasar.
B)The X-ray source contains a black hole with an accretion disk.
C)The X-ray source is no more than a few light-days in diameter.
D)The X-ray source must have a strong,rapidly varying magnetic field.
E)We are seeing the rapid orbit of two stars in a binary system.

Question

Quizplus · Accepted Answer

Rapid variations in brightness over a period of a few days suggest that the source is compact, as the time it takes for changes to occur is related to the size of the emitting region. Light can only travel so far in a given time, so a few light-days in diameter is a reasonable conclusion.

Suppose We Observe a Source of X-Rays That Varies Substantially