Estimating the Size of Sgr A*'s Emitting Region

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Homework Help Overview

The problem involves estimating the size of the emitting region of Sgr A*, a radio source at the center of the galaxy, based on its X-ray flares. The original poster attempts to relate the increase in flux over a specific time period to the dimensions of the emitting region.

Discussion Character

  • Exploratory, Assumption checking

Approaches and Questions Raised

  • Participants discuss the relevance of the distance light travels in relation to the size of the emitting region and question how this relates to the overall dimensions. There is consideration of the emitting region as a cloud of emitters and how their simultaneous flashing affects visibility.

Discussion Status

Some participants have offered guidance on interpreting the calculations, suggesting that while the initial calculation provides an upper bound on the radial extent, it does not fully define the size of the region. The discussion includes acknowledgment of assumptions made, such as spherical symmetry and instantaneous flashing, which may not hold true.

Contextual Notes

Participants note that the assumptions regarding the nature of the emitting region and the behavior of the emitters may not be physically accurate, indicating a need for further refinement based on additional observations.

albega
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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.
 
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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?
 
Bandersnatch said:
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?
 
albega said:
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.
 

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