When we think of an objects postion, 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 number of pixels in the region you selected.  If each pixel is 0.5 arc seconds, approximately how many arc seconds in area is the remanant?

40,000
80,000
160,000

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 Analysis menu and select the Radial Profile Plot menu option.

• In a few seconds, a plot window will display your analysis results.
• Try running this analysis program on annuli in different places on the Cas-A image by moving the annulus. For example, move it to near Physical X,Y = 4506,4576 (in the "empty" part of the image). See how the resulting radial profile is much different from that of the bright area at X,Y = 4158,4398.
• When you have finished, pull down the Region menu and select Delete All to remove the annulus region.

To see the actual distribution of the material in the "snowplowed" interstellar region surrounding Cas-A, we can look at the intensity of x-rays coming from concentric rings of material, projected onto the sky.

1. Select an annulus for your region shape , and center the cursor over the pulsar. Click once to create the region, and again to select it.

2. Go to the Region-->Get info... dialog box and enter an inner radius of "0" and an outer radius of 360, with 10 for the number of annuli. ( Note: you MUST select the region by clicking once anywhere inside it before you go to the "get info" dialog box).

3. Click on "generate" and then "apply". You will notice that 10 concentric rings appear like a bulls-eye surrounding the remnant. What we will find out is how bright the remnant is as a function of how far you are away from the center.

4. Under analysis, click on "radial profile".

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.