When we think of an objects' position, we want to know where it is located and how much space it occupies.  In order to determine the area of an x-ray image, we need to know the number of photons in the area.  To do this, we can make a circular region around the entire object.

Now go to ChandraEd Analysis Tools >> Counts in Region.  A box similar to the one below should appear.

We are interested in the net_counts value in the region you selected. 

How many photons make up the Cas A image?  What about the 3C273? 

What does this tell you about the size of each object?

Radial Profile Plot:

To understand the shape of an x-ray emission we can display a graphical plot of the brightness of the x-ray emission in concentric annuli around a central point.  The brightness of the x-ray emission is the average number of photons per unit area.

If the x-ray emission in the region is from a:

• very strong central source, then the shape of the plot will fall off steeply
• less strong central source, then the plot will have a more gradual downward slope
• diffuse source, then the plot might not even have a recognizable shape.

1.)  Load the image

2.)  Pull down the Region menu on the toolbar and select the shape sub-menu.

3.)  Now choose the Annulus sub-menu option.

4.)  Create an annular region by moving the mouse to the location on the image that you want to study.

5.)  You can adjust the number of annuli that are displayed (2 is the default).

6.)  Go to Region and select the Get Info sub-menu.  A menu similar to the following should appear.

 

7.)  Enter 0 for the inner radius, 360 for the outer radius, and 10 for the number of annuli.

8.)  Click on Generate and then Apply.  The resulting image should be similar to the image below.

9.)  Pull down the Analysis menu and select the Radial Profile Plot menu option.  A graph of the surface brightness (the average
      number of x-ray photons in a unit area) versus average radius should appear. 

What can you say about this graph?  What happens to the surface brightness as the radius increases?  Does this relationship remain the same? 

Try to interpret what this data means.  For an explanation, scroll down.

Your result shows you that the intensity of the remnant steadily increases from the center until about 100 arc-seconds in radius. Thereafter, the remnant gets weaker and weaker. This is what you might expect for a more or less hollow ball, with a dense outer shell. In fact, the exact nature of this emission and its morphology (or shape), is a subject of intense research currently. By looking at profiles like these, astronomers hope to gain some insight into the nature of the original explosion, and better understand the mechanisms by which the shock fronts form and travel through space.