Education Activities To Accompany
Chandra Data Analysis Software
Cas-A
Activity 3: How big is it?
Go back to the image of Cas
A, and zoom it back out so you can see the entire object in the
DS9 window. Place the cursor on the very top of the SNR, and
note the physical Y value of the position. Sweep the cursor down
to the very bottom of the remnant, trying to keep the X value
constant. This is a crude "eyeball" measurement of
the size of the remnant in the sky. Do the same by sweeping across
the remnant, keeping the Y value constant this time.
Note: You will see that in the Photo Album image
of Cas A the scale (size of the image) is given as 6 arc minutes,
or 360 arc seconds which is 720 physical pixels Your result will
be somewhat different depending on where you select the "edge"
of the remnant to be. Moreover, different observations will yield
slightly different results if their exposure times are different.
(If the exposure is longer, for example, you might see fainter
features beyond where you detected the edge to be here). In any
event, the values you derive should be in the vicinity of the
answers given below. Please be aware that since the data can
be binned (i.e. several adjacent pixels can be added and displayed
as one image pixel), you must always use "physical"
pixels when calculating the extent of any object, since these
values will always reflect the native 0.5 arc-sec resolution
of the satellite. This point will is discussed more fully in
the activities chapter for "galaxies and and clusters of
galaxies".
How wide is the remnant
(in pixels)? How high is the remnant?
(You can also answer this by placing a region down, centered
on the pulsar, selecting it, and dragging the handles to make
the size roughly encompass the entire remnant. Then you can go
to the region menu in DS9, click on Get Info...
and read off the radius directly).
Answer: about 700 physical pixels across
What does this correspond
to in arc-seconds on the sky?
Each pixel is 0.5 arc-seconds, so the remnant is about 350 arc-seconds
in diameter (in x-rays).
What is the physical size
of the remnant?
length= theta (arc-sec) x distance / 206265,
so l= 350 x 3000pc / 206265 = 5.1 pc or about 16 ly. across the
diameter
