# 9.2. Powerlaw with a Gaussian line¶

The files powgaus.spo and powgaus.res contain an absorbed powerlaw spectrum with a Gaussian line. From an optical observation of the source we know that this source has a redshift of = 0.0345. There is also Galactic foreground absorption. The source has been observed with the same instrument as exercise 1.

1. Load the spectrum into SPEX, select the proper energy range and rebin the spectrum properly. Set up a model with the right components (a gaussian is added with the command com gaus) and fit the spectrum.

2. You may want to fit first with the line flux set to zero (freeze or fix some parameter of the line, make a fit, and then fine-tune the line by releasing the relevant parameters.

You can find the absorbed and unabsorbed fluxes and luminosity just above the fit statistics. The energy limits can be changed by the command elim. To change the energy range over which the fluxes are calculated, type elim 0.2 10.. This changes the range to 0.2–10 keV.

3. What is the 2–10 keV luminosity of the source? What is the difference between absorbed and unabsorbed flux? Find out which of the columns in the table (listed above the fit statistic values) is absorbed.

4. What is the energy of the centroid of the line? Calculate the equivalent width of the line (by hand). (equivalent width is the ratio , where is the photon flux of the line in unit and is the flux per unit of energy of the continuum at the energy of the line in unit )

5. Calculate the errors on all free parameters. What is the error in the equivalent width you calculated?

Learning goals:

After having done this spectrum, you should know: