Master thesis - UBC Physics & Astronomy

44 M. Alexandersen : Master thesis
Some time was spent investigating which energy range would be best
to use. Using mainly GRB050724, as it had the most visible of the Swift
haloes, I split the observation up into many different energy ranges, and
viewed the resulting fits images in ds9, in order to determine which range
seemed to give the best visibility of the ring with least background. In the
Swift fits files, the energy column is channels of 10 eV, hence the range
seen in the code above. See Fig. 20 for an example of the images used
to evaluate the optimal energy range. As can be seen, it can be difficult
when there are so few counts to determine whether a halo is better or
worse off from including an energy range, but in the end 0.8 − 2.2 keV
seemed reasonable, and was chosen for use throughout this project.
Fig. 20 shows the XMM spectra of GRB031203 and its halo, from
Vaughan et al. (2004), as well as the effective area of the XMM. This
shows that 0.8 − 2.2 keV corresponds well with the peak of the halo
spectrum. As can be seem from the figure, this peak is not only due to
the effective area being greatest in this range, but also partly because the
halo is brighter. The halo should therefore be most prominent against the
background in the range used in this thesis.
Next, the procedure calls ds9, opening the fits file to be viewed. This
is the first human interaction. In ds9, the user has to mark out all the
background sources in the image with exclusion regions. See Fig. 21 for
an example of exclusion of background sources. The ftoolsreducea procedure
prints thorough instructions to the screen. I have adjusted my ds9
default settings to be the most favourable, in my opinion, so that I did not
have to adjust several settings each time I open a file. These include that
regions drawn on the image should be exclusion regions rather than inclusion
regions and that the regions are saved in WCS co-ordinates, rather
than image co-ordinates. Also included is that the image gets binned in 2
by 2 blocks and smoothed using a tophat smoothing, making background
sources more visible. I also have the default scale set to hist equ and the
colour scheme set to b. These are the settings that I feel make details
most visible. See Fig. 22 for a demonstration of the difference between
an un-binned, un-smoothed image and a binned, smoothed one.