Analysis of a Suzaku Observation of
Diffuse X-Ray Emission
Before class
Download and install LHEASOFT
- Download the software from http://heasarc.gsfc.nasa.gov/lheasoft/download.html
- For step #1, you should consult with an instructor to figure
out the best settings for your machine.
- For step #2, you should click on "All".
- For step #3, you should follow the instructions for your
machine.
Get the spectrum
- Make a directory called suzaku.
- Move to the directory.
- Download (right-click, save as) the source and background spectra and response files, rmf and arf, associated with observation 12 (Low latitude 86-21 (LL21)) from the paper.
First try at fitting the
spectrum
- Start xspec
- Load the data (data spec1.grp)
- Load the response files if they don't load automatically (response spec1.rmf; arf spec1.arf)
- Set up the plot device as xwindows (cpd /xw)
- Set the horizontal axis to be energy (setplot energy)
- Ignore low and high energies (ignore **-0.4; ignore 5.0-**)
- Plot the data on a log scale (plot ldata)
- Try fitting the data with an absorbed powerlaw model (model
wabs*pow; fit)
- Plot the data and the residuals (plot ldata delchi)
- Try fitting the data with an absorbed apec model (model
wabs*apec; fit)
- Try adding a powerlaw to your model (model
wabs*(apec+pow); fit)
- Find the error on the temperature of the apec model (error [parameter #]).
- Fix the absorption column density using the freeze command to
7.24E20 cm-2. Note that xspec uses funny units for
absorption. Try fitting the model again with the column density
frozen.
- Plot the data and the residuals (plot ldata delchi)
- For more information on XSpec commands and models, go to http://heasarc.gsfc.nasa.gov/xanadu/xspec/manual/manual.html.
Fitting the spectrum to find the oxygen line emission
Read the paper "Energy Spectra of the Soft X-Ray Diffuse Emission
in Fourteen Fields Observed with Suzaku" by Tomotaka Yoshino et
al. (2009) from http://adsabs.harvard.edu/abs/2009PASJ...61..805Y.
With your spectrum loaded into xspec, try to reproduce the fits in
tables 3, 4, and 5 for this spectrum. For the model components, you
should use TBabs for absorption, apec for thermal plasma emission,
powerlaw for a powerlaw, bknpower for a broken powerlaw. For the
fits for table 5, you will need to use vapec in order to set the
oxygen line emission to zero and gaussian for oxygen lines.
The last model is something like:
TBabs(vapec + bknpower) + vapec + gaussian + gaussian
The minimum temperature parameter value for vapec is different
than apec, and may be set to a new value using 'newpar'.
For this last model it may be easier to use apec to find a
best-fit, then switch to vapec models for which you substitute
in the best-fit parameter values.
For each model corresponding to each table, record the best fit
parameters and the quality of the fit. You can do that by capturing
the screen output from xspec into a text file. Compare with the
values in the paper. Note that you should run the error command to
find uncertainties. You may need to redo the fitting after running
error. Sometimes a few iterations are needed. Occasionally, XSpec will
get stuck in a local minimum of chi-squared space and you will need to run the
'steppar' command (read more about this in the XSpec manual).
In order to properly compare your normalization values to those in
the paper, you will need to divide your values by 2.215E-5 steradians
(total field of view for this observation) so that the units are consistent.
For the last model, save a plot of your spectrum with your best
fit using the 'hardcopy' command. Do this both for 'plot ldata delchi' and 'plot' with
ufspec, eufspec, or eeufspec - pick the one that you think best
shows the spectrum. If you wish to plot the underlying model
components in your figures (as seen in the reference paper),
use the command 'setplot add' before plotting. You may need to resize the
axes using the 'iplot' command followed by the resizing command for the
y-axis 'r y [min] [max]'.