Thermal Plasma Continuum-Fitting Tutorial

dph Tue Oct 23 2001

When fitting emission lines, it is crucial to have an accurate continuum. There are some portions of the HETGS spectrum which are so crowded in coronal spectra, that the "inter-line" minima may not be very good estimates of the continuum. This is especially relevant to the density sensitive, weak, intersystem lines of the He-like triplets.

The ISIS script, Contin_tutorial.sl walks through a session in which a thermal plasma continuum model from APED is fit to relatively line-free regions in a stellar coronal spectrum. Once an acceptable model is achieved by fitting different wavelength regions with one or more temperature components, the model array is read and saved as a slang static variable for use in a very fast continuum model evaluation, in which that array is scaled by a normalization constant. For demonstration, the fast continuum model is added to a gaussian and folded by the instrument response to simultaneously fit HEG and MEG spectra.

The script is meant to be used in a cut-and-paste fashion, possibly with edits as appropriate for the data loaded (consistent set of PHA, ARF, and RMF files).

The continuum models are written as ISIS user models in the file, aped_contin.sl. The relevant functions are:

• aped_contin_fit( xlo, xhi, par )
• fast_aped_contin_init(nhist)
• fast_aped_contin_fit( xlo, xhi, par )

Line-free (relatively speaking) regions are from a function defined in line_free_regs.sl.

(NOTE: the script uses the function rplot, which was a recent addition to ISIS and is not in the ciao-imported version.)

Several plots are made by the script as shown below. They can be found in the script by searching for Contin_fit_1.*.

A subsequent study will use high signal-to-noise simulations to determine just how accurate the derived continuum is for an a priori known model.

This is the short-wavelengh line-free region and fitted APED continuum.