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Hopefully when you examined the data and model between 6.0 and 8.0 keV you noticed a "feature" in the data that was not modeled well. This feature is an absorption line. Recall that you were hoping to find an emission or absorption line to identify what matter the black hole was consuming. Finding this absorption feature will help you answer that question.

Plot showing the powerlaw + blackbody model (black line) and the data (red line) between 6.0 and 8.0 keV.

To model this feature, you will use Xspec's notch model. This model has three free parameters:

• LineE, the energy of the central dip of the absorption line
• Width, a measure of the width of the absorption line
• CvrFract, a measure of how much "stuff" around the X-ray source produces the absorption feature (this will be fixed at 1, which basically means that the central X-ray source is completely surrounded by the matter causing the absorption line).

Before we add the notch to your model, look back at your plot at the 6.0-8.0 keV plot. At what energy does that dip appear to occur? What is the range of possible values for notch? (Keep in mind that the x-axis on the plot is logarithmic, not linear.)

It is possible for Xspec to get confused when fitting spectral features if you add too many parameters at once. To help keep Xspec on track, it is best to put limits on some of the free parameters. If you look at the 6.0-8.0 keV plot, you will see that the lowest point of the absorption line is at about 7.0 keV. In addition, you can narrow down the possible energies of that dip between 6.0 and 8.0 keV.

Now you can add a notch component to your model. To do this, type:

into the Xspec Command Window.

When Xspec prompts you for the first model input, LineE, you will give Xspec a guess of the lowest point of the line's energy and give it the range of possible values. When you see the following prompt:

1:notch:LineE>

type:

7.0 0.05 6.0 6.0 8.0 8.0

The image below shows what this will look like in your Xspec Command Window, and gives a brief explanation of each number.

This screen capture shows the Xspec Command window with the input for the notch LineE parameter. The first number is the initial guess for the peak of the emission line. The second number is "Delta", a value used by Xspec to determine how to step through different values of the line energy. Delta should be kept at 0.05. The third and fourth numbers are both the lower end of the range of possible central absorption energies. The fifth and sixth are both the higher end of the range of the possible central absorption energies. (Click image for larger version.)

You should also put in an initial guess for the second parameter, Width, of 0.1. This time you don't need to put in a range, like you did for the LineE parameter. At the following prompt:

2:notch:Width>

type:

0.1

and then hit return.

For the other model component, CvrFract, hit return to accept Xspec's default value.

The model will need to be re-fitted at this point, so that Xspec can find the best values not only for the new notch component, but for the existing absorption and continuum. To fit, type:

fit

into the Xspec command window. If needed, remember to hit return when Xspec pauses with the message:

Number of trials exceeded: continue fitting?