PIF-cleaning of images around strong sources

The image generating algorithm used by default in the JEM-X Scientific Analysis assigns equal weights to all active detector pixels. This allows to assign errors to the source fluxes with reasonable accuracy. For observations where the diffuse background is dominating the count rate this imaging technique appear as the best approach.

However, when bright sources are in the field of view, they significantly increase the global count rate and the situation changes. Due to the presence of the 25% open mask, the counts from a source will affect strongly at most a quarter of the detector pixels, but in those it may strongly dominate the pixel counts. Consequently, it could be advantageous to reduce the weight of pixels illuminated by a bright source when generating images intended for detection of weak sources. Since we are always operating with very few counts in the individual detector pixels the pixel counts cannot be used to determine the relevant weights, instead the weights are derived from the pixel illumination functions (PIF) used in the source fitting procedure. A new "PIF-weighted" image generation algorithm based on the above considerations have been implemented and is available in the new JEM-X Scientific Analysis package.

PIF imaging can be activated by including the string "PIF" in the IMA_skyImagesOut string accessible from the jemx_science_analysis GUI as one of the "hidden" parameters.
(Example: IMA_skyImagesOut="PIF,RECONSTRUCTED,VARIANCE". Abbreviations as e.g. "PIF,RECON,VARIA" can also be used) .

The PIF-imaging technique improves the visibility of weak sources in crowded fields like the Galactic Centre or in the neighborhood of strong sources like the Crab or GRS 1915+105 (Figure ).

Figure: Left: Mosaic from conventional images centered on the strong source GRS 1915+105. Right: Mosaic from PIF images in the same region of the sky.

Note however, that pif-images should not be used with mosaic_spec, as it is not reliable to extract spectra with mosaic_spec () for strong sources from pif images - or from mosaics based on pif images. The fluxes reported in src_ls_res should be used to extract spectra for strong sources, these fluxes are not affected by the pif-imaging option. This is the default in the SPE step. Here, with ``stron sources'', we intend sources that are bright enough to be detected in single science windows.