Loving ISIS - Confessions of a Former XSPEC User
Parameters:
As discussed on the models page, the model
parameters can be edited via the ISIS edit_par; command (which
will pop up an emacs, vi, jed, etc., window). This is very
convenient. Likewise, there is a set_par() command that allows
parameters to be set on the command line, or in scripts:
isis> model("powerlaw(1)");
isis> list_par;
powerlaw(1)
idx param tie-to freeze value min max
1 powerlaw(1).norm 0 0 1 0 1e+10
2 powerlaw(1).PhoIndex 0 0 1 -2 9
isis> set_par(2,1.7,0,1,3); % set_par(2,1.7,1); would have frozen it
I have wrapped a function around this, newpar(), which will
also take -1 as indicating a frozen parameter. (Bowing to my
XSPEC days...)
Parameters can be tied together, and they can be arbitrary
functions of one another. This allows greater flexibility than the
improved parameter handling of XSPEC 12. The ISIS intrinsic command
set_par_fun() will parse expressions either via
"model_name(id).parameter", or via parameter idx and
expressions using "_par(idx)". The
analysis script shows several examples.
Here is one from the help file that might be useful if different
instruments have systematic offsets from one another:
public define offset ()
{
if (Isis_Active_Dataset == 2)
return 3.0;
else
return 2.5;
}
set_par_fun ("gauss(2).center", "gauss(1).center + offset()");
I.e., Isis_Active_Dataset is used to define an offset();
function which has the value 3.0 for dataset 2, but is 2.5 for all
other datasets. This function is then used to tie together two
gaussian line mid-points.
Here is another example from the analysis script
that ensures that the inner and outer edges of the diskline
model do not overlap one another in the fitting process:
define dummy_fit(lo,hi,par)
{
return 1;
}
add_slang_function("dummy","Delta_R");
model("dummy(1)*constant(Isis_Active_Dataset)*phabs(1)*highecut(1)*(bkn2pow(1)+diskline(1))");
newpar(1,get_par(16)-get_par(15),0,0.1,994);
set_par_fun(16,"_par(1)+_par(15)");
That is, we are setting the outer line radius (_par(16)) to be
the inner line radius (_par(15)) plus an offset
(_par_1). Notice also that in the above example, we were able
to use functions as arguments in other function calls. Important
Note: Use of the "_par(idx)" syntax will not automatically
properly carry forward if the model is redefined, and the parameter
indices change. (Hence the recent update to the more robust
"model_name(id).parameter" syntax above.)
Finally, ISIS of course allows you to save and subsequently reload
models and parameters via the save_par("name"); and
load_par("name"); commands. These are the functional
equivalents of the XSPEC save model name and @name
commands.
Next Up: At long last - fitting!
