What does this tell us about the region that is emitting the X-rays?

Question

A quasar is detected to vary in X-rays by a large amount over a time period of 1 week. What does this tell us about the region that is emitting the X-rays? Why are observations like this important evidence that supermassive black holes power AGN?

Answer 1

This is a homework question.  But I'll give you a hint:

Suppose the size of the x-ray emitting region were a lightyear across.  Is it possible that it its x-ray brightness could vary significantly in a week?