Estimating the Size of Sgr A*'s Emitting Region

[albega]

Homework Statement

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.

The Attempt at a Solution

The solution is to consider the distance traveled 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.

 

[Bandersnatch]

Imagine the region being a cloud of emitters of some radial extent. If all of the emitters begin flashing at the same instant, which ones will you notice first, and when will you finally see all of them?

 

[albega]

Imagine the region being a cloud of emitters of some radial extent. If all of the emitters begin flashing at the same instant, which ones will you notice first, and when will you finally see all of them?

Ahhhhh thanks :)

Also, does the above calculation then give the diameter of the region?

 

[Bandersnatch]

Also, does the above calculation then give the diameter of the region?

It only gives you an upper bound on the radial extent. You can't really extract the tangential dimensions just from the time it takes to reach maximum flux.
You can, though, assume shperical symmetry of the region, and use the radial extent as the diametre.
Neither of the assumptions(shperical symmetry and instantenous flashing of the whole region) is likely to be correct(the second one's physically impossible), but they do give you a ballpark result that later can be narrowed down by future observations.