1. The problem statement, all variables and given/known data Sgr A* is the radio source at the centre of the galaxy. When it flares at X-ray wavelengths the flux increases by x 40 in 40 minutes. Estimate the size of the emitting region. 3. The attempt at a solution The solution is to consider the distance travelled by the light in 40 minutes, which gives 40*60*3x10^8m or around 5AU. However I really don't understand how that is relevant in terms of finding the size of the emitting region. Any hints? Thanks.