The tool spimodfit is intented to perform ``model fitting'' with SPI data, that is, to fit a linear combination of sky emission models, point sources and instrumental background. The principal use of spimodfit is the fitting of maps of physically-based tracers of -ray emission. Examples are fitting the 1809 keV Al line to free-free or IR emission maps, or fitting diffuse continuum -ray s to gas (HI, CO) maps.
spimodfit also allows for fitting spatial functions (e.g. Gaussian, exponential) where there is no `physical' model available. Fitting components is the best method to get the spectra of diffuse emission when a good spatial (but a poor spectral) model is available.
The analysis of measurements with this tool is complementary to deriving results from deconvolved images (e.g. from methods specifically designed for point sources (e.g. spiros). However, it can be fruitfully used to extract the spectra of point sources.
The algorithm performs fitting of raw data (binned counts for many observations) to multi-component models using the full instrument response information. In addition to parameter estimation, a Bayesian statistical analysis is used to include information on model characteristics and other prior information, so that the uncertainties of the fitted parameters are properly assessed. The prime results of the package are fitted model parameters with their uncertainty; complementing these, the fitted models are given also in the forms of skymaps and spectra.
This program is based on a former model fitting program for SPI, spidiffit developped by A. Strong (MPE, Garching). The main changes in spimodfit (w.r.t spidiffit) are:
For the details of the fitting methods and the statistical issues, we refer the reader to the handbook of spimodfit , from which this section of the handbook is extracted.