Image reconstruction and spectral extraction - spiros

A detailed description of spiros is given in the spiros User Manual written by P.H.Connell, [6]. Here we give just some brief explanations and describe only the parameters used by the main script. Note that even for the explained parameters we do not give the full range of possible values.

SPI is a coded-mask instrument, so direct deconvolution is in principle possible, but in practice the response is complex and the data include many pointing directions of the instrument, so `indirect imaging' is essential. Indirect imaging implies `forward-folding': for any candidate image we convolve with the instrument response function and compare the result with the observed data. This gives the basis for any iterative method which seeks to successively improve the agreement of the predicted with the observed data by adjusting the image. One approach is to regard the image as made up of `point sources' and to adjust their positions and fluxes to give a best-fit to the data. This is the basic principle of the spiros method.

The primary purpose of spiros is to use SPI data to locate point or point like sources in the observation field of view and to output a catalog of their parameters along with images, their spectra and any flux variability in time.

spiros has three basic operating modes to choose from:

• IMAGING mode.
  Here the aim is only to locate point or point like sources in the observation field of view and to output a catalog of their locations and width parameters. This step also produces images.

• IMAGING-TIMING
  This step produces images in the case that variable bright sources are in the field of view.

• SPECTRAL and SPECTRAL-TIMING mode.
  In this mode spiros reconstructs spectra of sources in the SPI field of view. Source positions are either taken from the source-location process (IMAGING mode) or from a catalog. The output spectra can be used for model fitting in XSPEC. The SPECTRAL-TIMING mode is not different from the normal SPECTRAL mode except that it considers variable sources as in the timing mode.

• TIMING mode.
  As in SPECTRAL mode a catalog of known sources is used to create a light curve of sources and background in several energy bins (one flux value per pointing).