Interpreting the Results
Now that you have a good fit of the data, you need to step back and
figure out what those results mean. Recall that your goal was to
characterize the accretion in the black hole. By finding an absorption
line, you can figure out at least one element that is near the black
hole. The line that you saw was from iron, Fe^{+25}.
Go back to your prediction from the beginning of this investigation.
Did you predict that if you found a line that it would be redshifted
(moving away from you, and into the black hole) or it would be
blueshifted (moving toward you, away from the black hole)?
To test your prediction, you will need the energy of the line from
your model. This can be found in the Xspec command window – look
for the current value of LineE. If you need
help finding this value, go back to the "Modeling the Spectrum" page to
see how to read the summary of the model. Write this down as
the "shifted" energy of the line, E_{shift}.
The second piece of information you need is the energy of this line
if the source is stationary relative to the observer (you). This is
E_{stationary} = 6.97 keV.
If the energy of the observed light (E_{shift}) is
higher than the energy of the emitted light
(E_{stationary}), then the light is blueshifted, and the matter
is moving toward the observer (you). Conversely, if the energy of the
observed light (E_{shift}) is lower than the energy of
the emitted light (E_{stationary}), then the light is
redshifted, and the matter is moving toward the observer (you). Is the
line you observed redshifted or blueshifted (or neither)?
Finally, you can calculate the velocity of the matter that absorbed
the light using the formula for the Doppler effect. Usually, the
formula looks like this:
In that equation, the v is velocity of the matter that absorbed the
light, and c is the speed of light.
You can rearrange and solve for the velocity, so the formula
looks like this:
Using your values for E_{shift} and E_{stationary},
and the fact that the "c" in the formula is the speed of light (equal to
300,000 km/s), find the velocity (in km/s) of the material responsible for
the iron absorption line seen in your data.
Answer the following questions
 What energy (LineE) did you measure for the absorption line?
 Is the shifted value more or less than the stationary value?
 Is the line you observed redshifted or blueshifted?
 What value do you get for the velocity of the matter emitting the iron line (show your work)?
 Is the source of the absorption line moving toward or away from you?
 Is the source of the absorption line moving toward or away from the black hole?
 Is this what you originally predicted?
