Fitting CCD Data

Here's a simple example of fitting a single CCD spectrum with an XSPEC model:

        % First, load the data, responses
        % and a background spectrum.

    load_dataset ("data.pi", "rmf.fits", "arf.fits");
    () = define_back (1, "bgd.pi");

        % ISIS supports several alternatives
        % for rebinning data.
        % Here we group by an integer factor:

    group_data(1,8);

        % Import the XSPEC module, and
        % initialize the model

    require("xspec");
    fit_fun ("phabs(1) * mekal(1)");

        % There are several ways to specify the parameters
        % This way is clear and human-readable:

    set_par ("phabs(1).nH", 0.05);
    set_par ("mekal(1).kT", 10.0);
    set_par ("mekal(1).Abundanc", 0.2);
    set_par ("mekal(1).Redshift", 0.2256);

        % Notice the energy range 0.5-7.0 keV in dataset 1,
        % and ignore the rest of the spectrum:

    xnotice_en (1, 0.5, 7.0);

        % Do the fit....

    () = renorm_counts;
    () = fit_counts();

        % .. and generate a plot.

    variable pid = plot_open ("ex_ccd.ps/cps");

    ylog;
    xrange(0.5,7.0);
    errorbars(1);
    rplot_counts (1);

    plot_close (pid);