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Astro 193 Homework 2 28-jan-2015
Due Mon Feb 2, 2015

The goal is to measure the count rates [counts/second] of individual sources
in an X-ray dataset of the Orion Nebula Cluster. The data, in the form of a
table of events in FITS format is at
http://hea-www.harvard.edu/AstroStat/Astro193/orion_evt2.fits

You may view and analyze it with any software tool.

One possible tool is SAOimage ds9, which can be downloaded from
http://ds9.si.edu/site/Home.html

Place the results of each step into a table.

1. Display the image and select sources (at least 2) that you want to analyze.
Make an image showing your selected sources.

2. For each selected source, define a region in which to collect source counts.
Describe the parameters of the region.

3. For each selected source, define a region in which to collect background counts.
Describe the parameters of the region.

4. Measure the counts in each of the regions.
Report the measured counts.

5. Compute the measurement errors for all the quantities as sqrt(N)
Report the errors.

6. Subtract background from the source region by weighting with the areas of extraction.
The background counts scaled to under the source is
(area_source/area_background)*(measured_background)
Report the source counts and the errors. Note that the errors scale the
same way as the quanitities, and are combined by summing their squares and
then taking the square root.

7. Get the exposure time of the observation from the header of the FITS file.
The exposure value is associated with the header keyword EXPOSURE.
Report the exposure time.

8. Compute source count rates (and errors).
Report the results.

8. For extra challenge, do this for 10 or more sources and
plot the count rate distribution, either as a scatter plot or
as histograms in differential or cumulative form.