Modeling the Spectrum
You are now ready to build a model for the spectrum you just loaded
into Xspec. Xspec has several standard models, and in general, it is
best to start with the simplest model and add on components as
necessary. (If you skipped the general discussions on models: What is a Model in Science? and How Good is the Model?, now might be a good time to look at
those pages.)
In the world of astrophysics, simple phenomena include those that
produce a continuous Xray emission, much like a rainbow, where all
colors are represented. You will start with a simple model that
describes a blackbody. A black body is an idealized object that
absorbs all electromagnetic radiation that falls on it, and it is
characterized by a temperature. Previous studies have shown that
this is a good model for accretion disks.
In the Xspec command window, type:
model wabs * bbody
This model has two components:

wabs models the effects of absorption. Space is not empty.
As light travels across the great distances to reach us, some of it
will be absorbed by matter that lies between us and the source.
This component of your model accounts for that absorption. It is
characterized by the free parameter, nH.

bbody models the continuous emission due to blackbody
radiation. This model component has two free parameters. The first
is kT, which characterizes the temperature of
the Xrayemitting matter. The second is norm, which is a measure of the intensity of the
spectrum.
Xspec will prompt you to enter a value for each of these parameters,
as shown below:
1:wabs:nH>
For these parameters, you can just use the default values in Xspec.
To do that, hit return when Xspec prompts for the values for nH, kT, and norm. Xspec will fill in the values for these
parameters.
Xspec Command Window showing the initial model inputs.
(Click for a larger view.)
Recall that in our simple data
fitting example the first guess was not the best model for the data.
The same is true here – the first guess is probably not the best
fit of the spectrum. Xspec will fit the model to the data attempting to
minimize the Chisquared value as discussed in the data fitting
example.
Have Xspec minimize Chisquared for the current model and determine
the bestfit values for the free parameters by typing:
fit
at the command line interface.
When Xspec has finished fitting, you should see a summary of the fit
above the prompt. This summary has a table that lists the bestfit
values for each of the free parameters in the model. In addition, the
summary includes the Chisquared value for the current fit. These are
shown in the screenshot below. (You may need to resize the Xpsec
command window, or scroll up, to see the entire table.)
Screen shot of the Xspec command window showing the results of
fitting the blackbody radiation model. Arrows indicate how to read
the results.
(Click for a larger view.)
